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Recent Advances in the Synthesis of High Explosive Materials

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This review discusses the recent advances in the syntheses of high explosive energetic materials. Syntheses of some relevant modern primary explosives and secondary high explosives, and the sensitivities and properties of these molecules are provided. In addition to the synthesis of such materials, processing improvement and formulating aspects using these ingredients, where applicable, are discussed in detail.
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Review
Recent Advances in the Synthesis of High
Explosive Materials
Jesse J. Sabatini
1,
* and Karl D. Oyler
2,
*
Received: 31 July 2015; Accepted: 22 December 2015; Published: 29 December 2015
Academic Editor: Thomas M. Klapötke
1
US Army Research Laboratory Weapons & Materials Research Directorate (WMRD) Lethality Division,
Energetics Technology Branch, Aberdeen Proving Ground, MD 21005, USA
2
US Army Armaments Research, Development & Engineering Center (ARDEC) Energetics, Warheads, &
Manufacturing Directorate Explosives Development Branch, Picatinny Arsenal, NJ 07806-5000, USA
* Correspondence: jesse.j.sabatini.civ@mail.mil (J.J.S.); karl.d.oyler.civ@mail.mil (K.D.O.);
Tel.: +1-410-278-0235 (J.J.S.); +1-973-724-4784 (K.D.O.)
Abstract:
This review discusses the recent advances in the syntheses of high explosive energetic
materials. Syntheses of some relevant modern primary explosives and secondary high explosives,
and the sensitivities and properties of these molecules are provided. In addition to the synthesis of
such materials, processing improvement and formulating aspects using these ingredients, where
applicable, are discussed in detail.
Keywords: energetic materials; explosives; synthesis; organic chemistry; processing
1. Introduction
There is an ever-increasing need for the development of new energetic materials for explosive
applications. This includes, but is not limited to, the area of primary explosives and secondary high
explosives. Primary explosives (or “primaries” as they are colloquially called) are defined as energetic
materials that possess an exceptionally high initiation sensitivity to impact, friction, electrostatic
discharge, heat, and shock. Primaries are known to reach detonation very quickly after such an initiation
event. The large amount of energy released upon initiation of a primary—typically in the form of heat
or a shockwave—is used to initiate less sensitive energetic materials, including secondary explosives,
propellants, and pyrotechnics. Despite their highly sensitive nature, primaries are, in general, less
powerful than secondary explosive materials [
1
]. In other words, primaries typically possess lower
detonation velocities, detonation pressures, and a lower heat of detonation than a given secondary
explosive. Despite the lower energy of a primary, extreme care is urged when handling these materials.
Primaries are ubiquitous in both military munitions and commercial items, where their main
function is to serve as the key ingredient in initiators such as detonators, blasting caps, and pyrotechnic
percussion primer formulations. By far, the most commonly used primary explosives by the U.S. military
are lead azide (used most often in detonators and blasting caps) and lead styphnate (most often
found in percussion primers). There has been significant interest in recent years to develop new
primary explosive materials, as the lead content of these materials is highly objectionable from both
a toxicological and environmental standpoint [2].
Secondary explosives (colloquially known as “secondaries”) differ from primaries in that they
are much less sensitive to impact, friction, electrostatic discharge, heat, and shock. Instead, they are
intended to be initiated by the heat and shockwave generated from a detonating primary explosive
charge. There is a tremendous need for the development of secondary explosives that have higher
performance, lower sensitivity, and lower toxicity than the currently fielded explosive compounds.
Crystals 2016, 6, 5; doi:10.3390/cryst6010005 www.mdpi.com/journal/crystals
Crystals 2016, 6, 5 2 of 22
Higher performing secondaries are classified as those having higher detonation velocities, higher
detonation pressures, higher crystalline densities, and a higher heat of detonation. Secondary explosives
with a higher performance typically correlate to an increase in brisance, which is defined as the
fragmentation ability or shattering effect of an explosive charge within a specific vicinity. As a rule of
thumb, explosives with higher crystal densities and higher enthalpies of formation (
f
H
0
) directly
correlate to higher detonation pressures, detonation velocities, and overall greater energy release.
Secondaries possessing a lower sensitivity are crucial for the development of insensitive munitions
(IM). In the IM sub-area, the development of explosive materials with higher thermal stability and
secondary explosive-based formulations that have a low sensitivity to bullet/fragment impact are
critical. Minimizing sympathetic detonation—the unintended detonation of a nearby explosive charge
which has been caused by a deliberate explosion of another explosive charge, as well as fast- and slow
cook-off (the exploding of a munition due to excessive heat in the vicinity)—is key to those interested
in IM-based materials. The ideal secondary explosive will have a low toxicity profile of the explosive
itself, its degradation products via natural degradation pathways, as well as its detonation products.
Although the field of explosives chemistry has existed for well over a century and produced
a wealth of published compounds and their performance data, general literature reviews on explosive
compounds [
3
5
] are relatively small in number and infrequently published. To supplement this list,
herein we seek to provide an overview of the more significant recent advances in the area of explosive
molecules. This summary is by no means exhaustive, given the wide scope of the explosives field, and
the topics covered will focus largely on relatively recently developed compounds which are thought to
have the highest likelihood of seeing practical application in the future.
2. Primary Explosives
2.1. Legacy Primary Explosives
While primary explosives have historically consisted of materials that exhibit reasonable explosive
power and a high sensitivity, these legacy materials also possess a high density as a result of the
presence of heavy metals within their respective structures. The structures of some historically used
primary explosives are given in Figure 1. It is commonly agreed that the highly toxic mercury fulminate
(1), adopted for use in some of the first percussion primer formulations in the early 1800s [
6
] and
later by Alfred Nobel for some of the first blasting caps in 1867 [
7
], is the first primary explosive to
see practical application. The high costs associated with mercury, the high sensitivity of mercury
fulminate, and its ability to lose performance under high loading pressures paved the way for lead
azide (2) to replace the compound starting in the early 1900s [
8
]. Today, lead azide (including its
less sensitive dextrin-coated form) finds wide-ranging application in energetic materials as the main
primary explosive ingredient in blasting caps and detonators.
A few years after lead azide began to find applications is the energetics field, the first practical
use of lead styphnate was described in a British patent [
9
]. Lead styphnate comes in two forms;
normal lead styphnate (lead(II) 2,4,6-trinitro-benzene-1,3-diolate hydrate) (3) and basic lead styphnate
(di(lead(II) hydroxide)-2,4,6-trinitro-benzene-1,3-diolate) (4). Although less powerful than lead azide,
lead styphnate is easier to initiate. For this reason, basic lead styphnate is present in some lead
azide-based stab detonator formulations such as NOL-130. The use of basic lead styphnate instead
of normal lead styphnate in stab detonator mixtures assists in preventing the decomposition of lead
azide via a hydrolytic pathway. Normal lead styphnate is commonly encountered as the main primary
explosive ingredient in primer applications.
Crystals 2016, 6, 5 3 of 22
Crystals 2016, 6, 5
3
Figure 1. Chemical structure of the primary explosives; mercury fulminate (1), lead azide (2) and
normal lead styphnate (3) and basic lead styphnate (4).
2.2. Why “Green” Primary Explosives?
Over the past two decades, there has been a significant interest amongst the US Department of
Defense (DoD) community, in particular, to address environmental issues associated with military
explosives, propellants, and pyrotechnics. Today, lead azide and lead styphnate comprise the vast
majority of military and civilian primary explosive use. The obvious issue with these two materials
is the presence of lead, which is an established heavy metal toxin. Until the 1960s, these materials
were not believed to significantly impact health [10]. Over the past five decades, however, the view
of lead-based primary explosives has changed significantly. Ingestion and/or inhalation of lead is
now known to affect the central nervous system, the renal system, the blood, and has been a
contributing factor in mental impairment and behavioral issues [11]. Lead has become one of the most
regulated chemical substances on an international scale. It is classified as a toxic pollutant in the USA,
and efforts have been underway for decades to remove lead from Federal facilities, including DoD
facilities [12]. In 2012, the National Research Council recommended that the DoD review its 30 years
old policy on protecting workers from lead exposure in firing ranges [13]. In Europe, the Registration,
Evaluation, Authorization, and Restriction of Chemicals (REACH) program has called for the specific
regulation of lead azide and lead styphnate. Therefore, a clear need exists for the successful
development and replacement of these two aforementioned primary explosives.
Aside from the environmental concerns associated with lead azide, a further complication exists
in that the material is known to slowly decompose under ambient conditions (Scheme 1). Lead azide
reacts with CO
2 and H2O to produce basic lead carbonate and highly toxic hydrazoic acid (HN3). HN3
can react with exposed copper metal surface in pipes and wires, thus generating the highly sensitive
and deadly copper azide (Cu(N
3)2). Several fatal military accidents over the years have been
associated with the accidental generation of copper azide [14].
Scheme 1. Decomposition reaction of lead azide.
2.3. “Green” Primary Explosive Candidates
2.3.1. Metal-Free Primary Explosives
2.3.1.1. Cyanuric Triazide (CTA)
CTA (6), prepared in a single, efficient, and eco-friendly step by treatment of cyanuric trichloride
(5) with sodium azide (Scheme 2), has more explosive power than lead azide, making it a suitable
replacement for lead azide in large-sized detonators. Unfortunately, there are several complications
with the material, in particular its tendency to undergo sublimation at elevated temperatures [15].
Further, the earlier syntheses of CTA reported an extreme sensitivity of the material to ignition
stimuli [16]. The crystalline material from the earlier syntheses exhibited a needle-like morphology,
which broke easily, thus increasing the risk of accidental detonation.
Figure 1.
Chemical structure of the primary explosives; mercury fulminate (1), lead azide (2) and
normal lead styphnate (3) and basic lead styphnate (4).
2.2. Why “Green” Primary Explosives?
Over the past two decades, there has been a significant interest amongst the US Department of
Defense (DoD) community, in particular, to address environmental issues associated with military
explosives, propellants, and pyrotechnics. Today, lead azide and lead styphnate comprise the vast
majority of military and civilian primary explosive use. The obvious issue with these two materials
is the presence of lead, which is an established heavy metal toxin. Until the 1960s, these materials
were not believed to significantly impact health [
10
]. Over the past five decades, however, the view of
lead-based primary explosives has changed significantly. Ingestion and/or inhalation of lead is now
known to affect the central nervous system, the renal system, the blood, and has been a contributing
factor in mental impairment and behavioral issues [
11
]. Lead has become one of the most regulated
chemical substances on an international scale. It is classified as a toxic pollutant in the USA, and efforts
have been underway for decades to remove lead from Federal facilities, including DoD facilities [
12
].
In 2012, the National Research Council recommended that the DoD review its 30 years old policy on
protecting workers from lead exposure in firing ranges [
13
]. In Europe, the Registration, Evaluation,
Authorization, and Restriction of Chemicals (REACH) program has called for the specific regulation
of lead azide and lead styphnate. Therefore, a clear need exists for the successful development and
replacement of these two aforementioned primary explosives.
Aside from the environmental concerns associated with lead azide, a further complication exists
in that the material is known to slowly decompose under ambient conditions (Scheme 1). Lead azide
reacts with CO
2
and H
2
O to produce basic lead carbonate and highly toxic hydrazoic acid (HN
3
).
HN
3
can react with exposed copper metal surface in pipes and wires, thus generating the highly
sensitive and deadly copper azide (Cu(N
3
)
2
). Several fatal military accidents over the years have been
associated with the accidental generation of copper azide [14].
Crystals 2016, 6, 5
3
Figure 1. Chemical structure of the primary explosives; mercury fulminate (1), lead azide (2) and
normal lead styphnate (3) and basic lead styphnate (4).
2.2. Why “Green” Primary Explosives?
Over the past two decades, there has been a significant interest amongst the US Department of
Defense (DoD) community, in particular, to address environmental issues associated with military
explosives, propellants, and pyrotechnics. Today, lead azide and lead styphnate comprise the vast
majority of military and civilian primary explosive use. The obvious issue with these two materials
is the presence of lead, which is an established heavy metal toxin. Until the 1960s, these materials
were not believed to significantly impact health [10]. Over the past five decades, however, the view
of lead-based primary explosives has changed significantly. Ingestion and/or inhalation of lead is
now known to affect the central nervous system, the renal system, the blood, and has been a
contributing factor in mental impairment and behavioral issues [11]. Lead has become one of the most
regulated chemical substances on an international scale. It is classified as a toxic pollutant in the USA,
and efforts have been underway for decades to remove lead from Federal facilities, including DoD
facilities [12]. In 2012, the National Research Council recommended that the DoD review its 30 years
old policy on protecting workers from lead exposure in firing ranges [13]. In Europe, the Registration,
Evaluation, Authorization, and Restriction of Chemicals (REACH) program has called for the specific
regulation of lead azide and lead styphnate. Therefore, a clear need exists for the successful
development and replacement of these two aforementioned primary explosives.
Aside from the environmental concerns associated with lead azide, a further complication exists
in that the material is known to slowly decompose under ambient conditions (Scheme 1). Lead azide
reacts with CO
2 and H2O to produce basic lead carbonate and highly toxic hydrazoic acid (HN3). HN3
can react with exposed copper metal surface in pipes and wires, thus generating the highly sensitive
and deadly copper azide (Cu(N
3)2). Several fatal military accidents over the years have been
associated with the accidental generation of copper azide [14].
Scheme 1. Decomposition reaction of lead azide.
2.3. “Green” Primary Explosive Candidates
2.3.1. Metal-Free Primary Explosives
2.3.1.1. Cyanuric Triazide (CTA)
CTA (6), prepared in a single, efficient, and eco-friendly step by treatment of cyanuric trichloride
(5) with sodium azide (Scheme 2), has more explosive power than lead azide, making it a suitable
replacement for lead azide in large-sized detonators. Unfortunately, there are several complications
with the material, in particular its tendency to undergo sublimation at elevated temperatures [15].
Further, the earlier syntheses of CTA reported an extreme sensitivity of the material to ignition
stimuli [16]. The crystalline material from the earlier syntheses exhibited a needle-like morphology,
which broke easily, thus increasing the risk of accidental detonation.
Scheme 1. Decomposition reaction of lead azide.
2.3. “Green” Primary Explosive Candidates
2.3.1. Metal-Free Primary Explosives
2.3.1.1. Cyanuric Triazide (CTA)
CTA (6), prepared in a single, efficient, and eco-friendly step by treatment of cyanuric trichloride
(5) with sodium azide (Scheme 2), has more explosive power than lead azide, making it a suitable
replacement for lead azide in large-sized detonators. Unfortunately, there are several complications
with the material, in particular its tendency to undergo sublimation at elevated temperatures [
15
].
Further, the earlier syntheses of CTA reported an extreme sensitivity of the material to ignition
Crystals 2016, 6, 5 4 of 22
stimuli [
16
]. The crystalline material from the earlier syntheses exhibited a needle-like morphology,
which broke easily, thus increasing the risk of accidental detonation.
Crystals 2016, 6, 5
4
Scheme 2. Synthesis of cyanuric triazide (CTA, 6).
Although CTA was first patented in 1921 [17], a newer synthetic procedure for the material
developed by ARDEC in 2009 resulted in a reduced particle size of the material [18]. As a result of
the new synthesis procedure, CTA has been further investigated by researchers at ARDEC, and has
found successful use as a replacement for both lead azide and lead styphnate in the NOL-130 stab
detonator mix, as well as some more recent work as a potential lead styphnate replacement in
percussion primer formulations (NB: The lead-based NOL-130 formulation consists of 40% lead
styphnate, 20% lead azide, 20% barium nitrate, 15% antimony trisulfide, and 5% tetrazene). The
sublimation of CTA, and thus its looming safety issues, however, remains an Achilles heel for this
material if it is to ever gain serious traction in having widespread serious military and civilian
applications.
2.3.1.2. 2-Diazo-4,6-Dinitrophenol (DDNP)
The synthesis of DDNP (9) is described in Scheme 3. Treatment of picric acid (7) with a controlled
simultaneous addition of aqueous sodium sulfide yields sodium picramate (8), which is then
converted to DDNP via the Griess diazotization. DDNP has found explosive use since the early 20th
century [1]. Although old enough to be considered a legacy primary explosive, it tends to fall into a
different category because of its heavy metal-free nature.
Scheme 3. Synthesis of DDNP (9).
At this time, DDNP is generally considered to be an unsuitable candidate for demanding
military primer applications. The material is reputed to suffer from reliability issues in extreme cold
weather climates. DDNP’s low sensitivity to friction, its low flame temperature, and its
incompatibility with lead azide also plague this compound [19]. Due to different requirements
necessary for the production of commercial primer formulations, DDNP has found recent usage and
interest from the civilian sectors as a “green” replacement for lead styphnate in primer applications
[20,21].
2.3.1.3. Peroxide-Based Explosives
Although primary explosives are designed with the intent to be sensitive to various ignition
stimuli, it has always been believed that peroxide-based explosives are too sensitive to garner
practical use for even primary explosive applications. Chemically, the high degree of instability of
the oxygen-oxygen bond is the culprit behind this sensitivity. Two peroxide-based materials that
have been studied extensively are triacetone triperoxide (TATP) and hexamethylenetriperoxide
diamine (HMTD). Their syntheses are remarkably simple and are performed from cheap,
commercially available starting materials, as summarized in Scheme 4. TATP (10) and HMTD (11)
are synthesized from acetone and hexamine, respectively, in the presence of acidic water and
Scheme 2. Synthesis of cyanuric triazide (CTA, 6).
Although CTA was first patented in 1921 [
17
], a newer synthetic procedure for the material
developed by ARDEC in 2009 resulted in a reduced particle size of the material [
18
]. As a result of the
new synthesis procedure, CTA has been further investigated by researchers at ARDEC, and has found
successful use as a replacement for both lead azide and lead styphnate in the NOL-130 stab detonator
mix, as well as some more recent work as a potential lead styphnate replacement in percussion primer
formulations (NB: The lead-based NOL-130 formulation consists of 40% lead styphnate, 20% lead
azide, 20% barium nitrate, 15% antimony trisulfide, and 5% tetrazene). The sublimation of CTA, and
thus its looming safety issues, however, remains an Achilles heel for this material if it is to ever gain
serious traction in having widespread serious military and civilian applications.
2.3.1.2. 2-Diazo-4,6-Dinitrophenol (DDNP)
The synthesis of DDNP (9) is described in Scheme 3. Treatment of picric acid (7) with a controlled
simultaneous addition of aqueous sodium sulfide yields sodium picramate (8), which is then converted
to DDNP via the Griess diazotization. DDNP has found explosive use since the early 20th century [
1
].
Although old enough to be considered a legacy primary explosive, it tends to fall into a different
category because of its heavy metal-free nature.
Crystals 2016, 6, 5
4
Scheme 2. Synthesis of cyanuric triazide (CTA, 6).
Although CTA was first patented in 1921 [17], a newer synthetic procedure for the material
developed by ARDEC in 2009 resulted in a reduced particle size of the material [18]. As a result of
the new synthesis procedure, CTA has been further investigated by researchers at ARDEC, and has
found successful use as a replacement for both lead azide and lead styphnate in the NOL-130 stab
detonator mix, as well as some more recent work as a potential lead styphnate replacement in
percussion primer formulations (NB: The lead-based NOL-130 formulation consists of 40% lead
styphnate, 20% lead azide, 20% barium nitrate, 15% antimony trisulfide, and 5% tetrazene). The
sublimation of CTA, and thus its looming safety issues, however, remains an Achilles heel for this
material if it is to ever gain serious traction in having widespread serious military and civilian
applications.
2.3.1.2. 2-Diazo-4,6-Dinitrophenol (DDNP)
The synthesis of DDNP (9) is described in Scheme 3. Treatment of picric acid (7) with a controlled
simultaneous addition of aqueous sodium sulfide yields sodium picramate (8), which is then
converted to DDNP via the Griess diazotization. DDNP has found explosive use since the early 20th
century [1]. Although old enough to be considered a legacy primary explosive, it tends to fall into a
different category because of its heavy metal-free nature.
Scheme 3. Synthesis of DDNP (9).
At this time, DDNP is generally considered to be an unsuitable candidate for demanding
military primer applications. The material is reputed to suffer from reliability issues in extreme cold
weather climates. DDNP’s low sensitivity to friction, its low flame temperature, and its
incompatibility with lead azide also plague this compound [19]. Due to different requirements
necessary for the production of commercial primer formulations, DDNP has found recent usage and
interest from the civilian sectors as a “green” replacement for lead styphnate in primer applications
[20,21].
2.3.1.3. Peroxide-Based Explosives
Although primary explosives are designed with the intent to be sensitive to various ignition
stimuli, it has always been believed that peroxide-based explosives are too sensitive to garner
practical use for even primary explosive applications. Chemically, the high degree of instability of
the oxygen-oxygen bond is the culprit behind this sensitivity. Two peroxide-based materials that
have been studied extensively are triacetone triperoxide (TATP) and hexamethylenetriperoxide
diamine (HMTD). Their syntheses are remarkably simple and are performed from cheap,
commercially available starting materials, as summarized in Scheme 4. TATP (10) and HMTD (11)
are synthesized from acetone and hexamine, respectively, in the presence of acidic water and
Scheme 3. Synthesis of DDNP (9).
At this time, DDNP is generally considered to be an unsuitable candidate for demanding military
primer applications. The material is reputed to suffer from reliability issues in extreme cold weather
climates. DDNP’s low sensitivity to friction, its low flame temperature, and its incompatibility with
lead azide also plague this compound [
19
]. Due to different requirements necessary for the production
of commercial primer formulations, DDNP has found recent usage and interest from the civilian
sectors as a “green” replacement for lead styphnate in primer applications [20,21].
2.3.1.3. Peroxide-Based Explosives
Although primary explosives are designed with the intent to be sensitive to various ignition
stimuli, it has always been believed that peroxide-based explosives are too sensitive to garner
practical use for even primary explosive applications. Chemically, the high degree of instability
of the oxygen-oxygen bond is the culprit behind this sensitivity. Two peroxide-based materials
that have been studied extensively are triacetone triperoxide (TATP) and hexamethylenetriperoxide
diamine (HMTD). Their syntheses are remarkably simple and are performed from cheap, commercially
Crystals 2016, 6, 5 5 of 22
available starting materials, as summarized in Scheme 4. TATP (10) and HMTD (11) are synthesized
from acetone and hexamine, respectively, in the presence of acidic water and hydrogen peroxide.
The historically negative safety profile of peroxide-based primary explosives, coupled with the blatant
terroristic use of materials such as TATP and HMTD have discouraged the investigation and use of
organic peroxides as a viable energetic material for practical use.
Crystals 2016, 6, 5
5
hydrogen peroxide. The historically negative safety profile of peroxide-based primary explosives,
coupled with the blatant terroristic use of materials such as TATP and HMTD have discouraged the
investigation and use of organic peroxides as a viable energetic material for practical use.
Scheme 4. Synthesis of TATP (10) and HMTD (11).
Work spearheaded by Winter in 2015, however, has shown that organic peroxides can be
designed that exhibit high performance, yet reasonable sensitivities [22]. As summarized in Scheme
5, a given diketone or dialdehyde was reacted with hydrogen peroxide in the presence of a catalytic
amount of iodine to afford the bis-geminal hydroperoxy derivatives 12–15. Despite the presence of
normally labile peroxy groups, it is believed that the lability is suppressed due to the presence of
intermolecular O–H…O hydrogen bonds, as well as several O…O and C…H close contacts which
was determined to occur in the crystalline lattice of 15 upon confirmation of the structure by single
crystal X-ray diffraction.
Scheme 5. Synthesis of stable peroxide-based primary explosives.
The sensitivities and performance of geminal peroxides 12–15, compared to TATP, are
summarized in Table 1. Remarkably, geminal peroxy compounds 12–15 are more insensitive to
impact and friction than TATP, with comparable or better ESD sensitivities. This phenomenon occurs
despite the better oxygen balances, higher crystalline densities, higher detonation velocities, and total
energy of detonation values compared to TATP. The Winter approach is unique because it describes
the first known examples of stabilized peroxide-based primary explosives. Although the thermal
stabilities of this class of compounds could use some improvement, the area of peroxide-based
primary explosives, in light of this discovery, ought to be investigated further for possible
application.
Scheme 4. Synthesis of TATP (10) and HMTD (11).
Work spearheaded by Winter in 2015, however, has shown that organic peroxides can be designed
that exhibit high performance, yet reasonable sensitivities [
22
]. As summarized in Scheme 5, a given
diketone or dialdehyde was reacted with hydrogen peroxide in the presence of a catalytic amount of
iodine to afford the bis-geminal hydroperoxy derivatives 12–15. Despite the presence of normally
labile peroxy groups, it is believed that the lability is suppressed due to the presence of intermolecular
O–H . . . O hydrogen bonds, as well as several O . . . O and C . . . H close contacts which was determined
to occur in the crystalline lattice of 15 upon confirmation of the structure by single crystal X-ray diffraction.
Scheme 5. Synthesis of stable peroxide-based primary explosives.
The sensitivities and performance of geminal peroxides 12–15, compared to TATP, are summarized
in Table 1. Remarkably, geminal peroxy compounds 12–15 are more insensitive to impact and
friction than TATP, with comparable or better ESD sensitivities. This phenomenon occurs despite the
better oxygen balances, higher crystalline densities, higher detonation velocities, and total energy of
detonation values compared to TATP. The Winter approach is unique because it describes the first
known examples of stabilized peroxide-based primary explosives. Although the thermal stabilities of
this class of compounds could use some improvement, the area of peroxide-based primary explosives,
in light of this discovery, ought to be investigated further for possible application.
Crystals 2016, 6, 5 6 of 22
Table 1. Sensitivities and performance of geminal peroxides 12–15 and TATP.
Data Category 12 13 14 15 TATP
IS [J]
a
2 1 2 3 0.3
FS [N]
b
5 5 5 <5 0.1
ESD
c
[J]
c
0.2 0.5 0.1 0.25 0.16
T
dec
[
˝
C]
d
117 98 100 105 150–160
CO
2
[%]
e
´126.2 ´114.18 ´100.76 ´88.83 ´151.19
ρ [g/cc]
f
1.35 1.375 1.6 1.6 1.18
P
cj
[kbar]
g
117 126 195 195
V
det
[m/s]
h
6150 6250 6428 7130 5300
f
H
0
[kJ/mol]
i
´703.6 ´660.8 ´418.2 ´418.2 ´583.8
ex
U
0
[kJ/kg]
j
´4636 ´4875 ´5083 ´5498 ´2745
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
dec
= Temperature
of decomposition;
e
CO
2
= CO
2
oxygen balance;
f
ρ
= crystalline density;
g
P
cj
= detonation pressure;
h
V
det
= detonation velocity;
i
f
H
0
= molar enthalpy of formation;
j
ex
U
0
= total energy of detonation.
2.3.1.4. Tetrazene and MTX-1
1-Amino-1-(1H-tetrazol-5-yl)-azo-guanidine hydrate, known colloquially as tetrazene (16), is
a well-known sensitizer in percussion primer compositions (Figure 2) [
8
]. Without its presence in
primers, initiation is known to be dreadfully unreliable, or fails to function altogether. Despite its
reliability and widespread use in primers, tetrazene suffers from a high degree of hydrolytic and
thermal instability. It has been found to rapidly decompose at temperatures of ca. 90
˝
C, and this
temperature is commonly encountered during handling and storage of the material. Addition of
boiling water to tetrazene is also known to rapidly decompose the material.
Crystals 2016, 6, 5
6
Table 1. Sensitivities and performance of geminal peroxides 12–15 and TATP.
Data Category 12 13 14 15 TATP
IS [J]
a
2 1 2 3 0.3
FS [N]
b
5 5 5 <5 0.1
ESD
c
[J]
c
0.2 0.5 0.1 0.25 0.16
T
dec [°C]
d
117 98 100 105 150–160
Ω

[%]
e
126.2 114.18 100.76 88.83 151.19
ρ [g/cc]
f
1.35 1.375 1.6 1.6 1.18
P
cj [kbar]
g
117 126 195 195
V
det [m/s]
h
6150 6250 6428 7130 5300
Δ
fH
0
[kJ/mol]
i
703.6 660.8 418.2 418.2 583.8
Δ
exU
0
[kJ/kg]
j
4636 4875 5083 5498 2745
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tdec = Temperature
of decomposition;
e
Ω

= CO2 oxygen balance;
f
ρ = crystalline density;
g
Pcj = detonation pressure;
h
V
det = detonation velocity;
i
ΔfH
0
= molar enthalpy of formation;
j
ΔexU
0
= total energy of detonation.
2.3.1.4. Tetrazene and MTX-1
1-Amino-1-(1H-tetrazol-5-yl)-azo-guanidine hydrate, known colloquially as tetrazene (16), is a
well-known sensitizer in percussion primer compositions (Figure 2) [8]. Without its presence in
primers, initiation is known to be dreadfully unreliable, or fails to function altogether. Despite its
reliability and widespread use in primers, tetrazene suffers from a high degree of hydrolytic and
thermal instability. It has been found to rapidly decompose at temperatures of ca. 90 °C, and this
temperature is commonly encountered during handling and storage of the material. Addition of
boiling water to tetrazene is also known to rapidly decompose the material.
Figure 2. Molecular structure of tetrazene (16) and MTX-1 (17).
In light of these concerns, Fronabarger and Williams reacted tetrazene in aqueous solution to
produce MTX-1, which has promise for serving as a more thermally stable replacement [23,24]. Initial
studies have shown that while tetrazene decomposes exothermically at 144 °C, MTX-1 undergoes
exothermic decomposition at 214 °C. Holding tetrazene at 90 °C for 120 h results in 36% weight loss,
while MTX-1 experiences just 4% weight loss after the same time periods. Exposure of MTX-1 to water
for 120 days has virtually no effect, as MTX-1 can be re-isolated in nearly quantitative yield. It has
been stated that MTX-1 has comparable impact, friction and ESD sensitivities to tetrazene, thus
adding to its appeal as a potential replacement to this traditionally used organic sensitizing agent in
primer compositions.
2.3.2. Metal-Organic Primary Explosives
2.3.2.1. Potassium 4,6-Dinitrobenzofuroxan (KDNBF)
KDNBF (21) is synthesized in a three-step procedure, as summarized in Scheme 6 [25]. Exposure
of o-nitroaniline to NaOCl and base affords benzofuroxan 19, which is subjected to mixed acid
nitration to yield dinitrobenzofuroxan 20. Treatment of dinitrobenzofuroxan with potassium
bicarbonate furnishes KDNBF.
Figure 2. Molecular structure of tetrazene (16) and MTX-1 (17).
In light of these concerns, Fronabarger and Williams reacted tetrazene in aqueous solution
to produce MTX-1, which has promise for serving as a more thermally stable replacement [
23
,
24
].
Initial studies have shown that while tetrazene decomposes exothermically at 144
˝
C, MTX-1 undergoes
exothermic decomposition at 214
˝
C. Holding tetrazene at 90
˝
C for 120 h results in 36% weight loss,
while MTX-1 experiences just 4% weight loss after the same time periods. Exposure of MTX-1 to
water for 120 days has virtually no effect, as MTX-1 can be re-isolated in nearly quantitative yield.
It has been stated that MTX-1 has comparable impact, friction and ESD sensitivities to tetrazene, thus
adding to its appeal as a potential replacement to this traditionally used organic sensitizing agent in
primer compositions.
2.3.2. Metal-Organic Primary Explosives
2.3.2.1. Potassium 4,6-Dinitrobenzofuroxan (KDNBF)
KDNBF (21) is synthesized in a three-step procedure, as summarized in Scheme 6 [
25
]. Exposure of
o-nitroaniline to NaOCl and base affords benzofuroxan 19, which is subjected to mixed acid nitration
to yield dinitrobenzofuroxan 20. Treatment of dinitrobenzofuroxan with potassium bicarbonate
furnishes KDNBF.
Crystals 2016, 6, 5 7 of 22
Crystals 2016, 6, 5
7
Scheme 6. Synthesis of KDNBF.
Potassium is generally considered an innocuous and much safer alternative to lead. Thus,
KDNBF, much like DDNP, has found use as a suitable “green” primary explosive alternative to lead
styphnate in civilian primer compositions. While lead styphnate has a decomposition temperature of
280–290 °C, KDNBF’s decomposition temperature is much lower (217 °C). Also like DDNP, KDNBF
has not been adopted for widespread military use; the lower thermal stability of KDNBF may be one
of the reasons as to why this material has not been used as a universal alternative to lead styphnate
in primary explosive compositions.
2.3.2.2. Potassium 4,6-Dinitro-7-Hydroxybenzofuroxan (KDNP)
Scheme 7 details the synthesis of KDNP (25) [26]. Mixed acid nitration of 3-bromoanisole
delivered trinitro derivative 23. This material was treated with potassium azide in refluxing methanol
to effect concomitant substitution at C–3 and cleavage of the methyl ether, thus yielding azide 24.
Treatment of this azide with diethylcarbonate in heat produces KDNP (25). The approach detailed in
Scheme 6 is an improvement over previous syntheses that required the use of water, which was
retained in the product. This led to the formation of needles that were capable of breaking, and thus
dramatically increased the sensitivity of the material. The new synthetic procedure allowed for a free-
flowing powder to be obtained, and particle size could be varied by simple variations in temperature
and the addition rate of the solvent during recrystallization.
Scheme 7. Synthesis of KDNP (25).
Though originally synthesized over 30 years ago, KDNP has drawn renewed interest as a lead
styphnate replacement, due to its comparable explosive performance in military-relevant primers
and its thermal stability, as provided in Table 2 [22,26]. Note the structural similarities that exist
between KDNBF and KDNP; though KDNP possess a single hydrogen atom less than KDNBF, the
absence of this hydrogen allows for the restoration of aromaticity in the benzofuroxan ring system.
This phenomenon likely serves to explain the increased thermal stability of KDNP compared to
KDNBF.
Scheme 6. Synthesis of KDNBF.
Potassium is generally considered an innocuous and much safer alternative to lead. Thus, KDNBF,
much like DDNP, has found use as a suitable “green” primary explosive alternative to lead styphnate
in civilian primer compositions. While lead styphnate has a decomposition temperature of 280–290
˝
C,
KDNBF’s decomposition temperature is much lower (217
˝
C). Also like DDNP, KDNBF has not been
adopted for widespread military use; the lower thermal stability of KDNBF may be one of the reasons
as to why this material has not been used as a universal alternative to lead styphnate in primary
explosive compositions.
2.3.2.2. Potassium 4,6-Dinitro-7-Hydroxybenzofuroxan (KDNP)
Scheme 7 details the synthesis of KDNP (25) [
26
]. Mixed acid nitration of 3-bromoanisole delivered
trinitro derivative 23. This material was treated with potassium azide in refluxing methanol to effect
concomitant substitution at C–3 and cleavage of the methyl ether, thus yielding azide 24. Treatment of
this azide with diethylcarbonate in heat produces KDNP (25). The approach detailed in Scheme 6 is
an improvement over previous syntheses that required the use of water, which was retained in the
product. This led to the formation of needles that were capable of breaking, and thus dramatically
increased the sensitivity of the material. The new synthetic procedure allowed for a free-flowing
powder to be obtained, and particle size could be varied by simple variations in temperature and the
addition rate of the solvent during recrystallization.
Crystals 2016, 6, 5
7
Scheme 6. Synthesis of KDNBF.
Potassium is generally considered an innocuous and much safer alternative to lead. Thus,
KDNBF, much like DDNP, has found use as a suitable “green” primary explosive alternative to lead
styphnate in civilian primer compositions. While lead styphnate has a decomposition temperature of
280–290 °C, KDNBF’s decomposition temperature is much lower (217 °C). Also like DDNP, KDNBF
has not been adopted for widespread military use; the lower thermal stability of KDNBF may be one
of the reasons as to why this material has not been used as a universal alternative to lead styphnate
in primary explosive compositions.
2.3.2.2. Potassium 4,6-Dinitro-7-Hydroxybenzofuroxan (KDNP)
Scheme 7 details the synthesis of KDNP (25) [26]. Mixed acid nitration of 3-bromoanisole
delivered trinitro derivative 23. This material was treated with potassium azide in refluxing methanol
to effect concomitant substitution at C–3 and cleavage of the methyl ether, thus yielding azide 24.
Treatment of this azide with diethylcarbonate in heat produces KDNP (25). The approach detailed in
Scheme 6 is an improvement over previous syntheses that required the use of water, which was
retained in the product. This led to the formation of needles that were capable of breaking, and thus
dramatically increased the sensitivity of the material. The new synthetic procedure allowed for a free-
flowing powder to be obtained, and particle size could be varied by simple variations in temperature
and the addition rate of the solvent during recrystallization.
Scheme 7. Synthesis of KDNP (25).
Though originally synthesized over 30 years ago, KDNP has drawn renewed interest as a lead
styphnate replacement, due to its comparable explosive performance in military-relevant primers
and its thermal stability, as provided in Table 2 [22,26]. Note the structural similarities that exist
between KDNBF and KDNP; though KDNP possess a single hydrogen atom less than KDNBF, the
absence of this hydrogen allows for the restoration of aromaticity in the benzofuroxan ring system.
This phenomenon likely serves to explain the increased thermal stability of KDNP compared to
KDNBF.
Scheme 7. Synthesis of KDNP (25).
Though originally synthesized over 30 years ago, KDNP has drawn renewed interest as a lead
styphnate replacement, due to its comparable explosive performance in military-relevant primers
and its thermal stability, as provided in Table 2 [
22
,
26
]. Note the structural similarities that exist
between KDNBF and KDNP; though KDNP possess a single hydrogen atom less than KDNBF, the
absence of this hydrogen allows for the restoration of aromaticity in the benzofuroxan ring system.
This phenomenon likely serves to explain the increased thermal stability of KDNP compared to KDNBF.
Crystals 2016, 6, 5 8 of 22
Table 2. Sensitivities and performance of KDNP (25).
Data Category KDNP (25)
IS [J]
a
0.047
FS [N]
b
9.81
ESD
c
[µJ]
c
>675
T
dec
[
˝
C]
d
285
CO
2
[%]
e
´34.3
ρ [g/cc]
f
1.982
f
H
0
[kJ/mol]
g
´197.07
ex
U
0
[kJ/kg]
h
3280
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
dec
= Temperature of
decomposition;
e
CO
2
= CO
2
oxygen balance;
f
ρ = crystalline density;
g
f
H
0
= molar enthalpy of formation;
h
ex
U
0
= total energy of detonation.
2.3.2.3. Potassium 1,1'-dinitramino-5,5'-Bis(tetrazolate) (K
2
DNABT)
In 2014, Klapötke reported the synthesis and performance of K
2
DNABT (31) [
27
]. This material is
prepared in a seven step sequence, starting from dimethyl carbonate (Scheme 8). Desymmetrization of
dimethyl carbonate, followed by condensation with glyoxal afforded bis(hydrazone) 27. Treatment with
N-chlorosuccinimide (NCS), followed by displacement of the chlorines with azide, afforded diazido
bis(hydrazone) intermediate (29). Acid-induced cyclization to give bis(tetrazole) (30), nitration of
this species with dinitrogen pentoxide, and formation of the resultant dipotassium salt afforded K
2
DNABT.
Crystals 2016, 6, 5
8
Table 2. Sensitivities and performance of KDNP (25).
Data Category KDNP (25)
IS [J]
a
0.047
FS [N]
b
9.81
ESD
c
[µJ]
c
>675
T
dec [°C]
d
285
Ω

[%]
e
34.3
ρ [g/cc]
f
1.982
Δ
fH
0
[kJ/mol]
g
197.07
Δ
exU
0
[kJ/kg]
h
3280
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tdec = Temperature
of decomposition;
e
Ω

= CO2 oxygen balance;
f
ρ = crystalline density;
g
ΔfH
0
= molar enthalpy of
formation;
h
ΔexU
0
= total energy of detonation.
2.3.2.3. Potassium 1,1'-dinitramino-5,5'-Bis(tetrazolate) (K2DNABT)
In 2014, Klapötke reported the synthesis and performance of K
2DNABT (31) [27]. This material
is prepared in a seven step sequence, starting from dimethyl carbonate (Scheme 8). Desymmetrization
of dimethyl carbonate, followed by condensation with glyoxal afforded bis(hydrazone) 27. Treatment
with N-chlorosuccinimide (NCS), followed by displacement of the chlorines with azide, afforded
diazido bis(hydrazone) intermediate (29). Acid-induced cyclization to give bis(tetrazole) (30),
nitration of this species with dinitrogen pentoxide, and formation of the resultant dipotassium salt
afforded K
2DNABT.
Scheme 8. Synthesis of K2DNABT (31).
K2DNABT was prepared based on the premise that it could serve as a potential replacement for
lead azide in detonator applications; it is particularly intriguing in that it incorporates two 1-
nitraminotetrazole moieties, each consisting of six consecutive nitrogen atoms which contribute
greatly to the high explosive output of the molecule. Compared to lead azide, K
2DNABT was found
to have comparable sensitivities to impact, friction and ESD, a significantly larger detonation velocity,
and a similar detonation pressure (Table 3). K
2DNABT was subjected to 100 °C for 48 h and was found
to undergo no mass loss or decomposition at this temperature. Undoubtedly, this material holds
promise and should be investigated further as a potential alternative to lead azide. However, its
current synthesis does suffer from a lengthy synthesis route, and low yields were encountered in
some of the synthetic transformations; most notably, the conversion of dichloro bis(hydrazone) (28)
to diazido bis(hydrazone) (29), which proceeded in only 38% yield.
Scheme 8. Synthesis of K
2
DNABT (31).
K
2
DNABT was prepared based on the premise that it could serve as a potential replacement for lead
azide in detonator applications; it is particularly intriguing in that it incorporates two 1-nitraminotetrazole
moieties, each consisting of six consecutive nitrogen atoms which contribute greatly to the high
explosive output of the molecule. Compared to lead azide, K
2
DNABT was found to have comparable
sensitivities to impact, friction and ESD, a significantly larger detonation velocity, and a similar
detonation pressure (Table 3). K
2
DNABT was subjected to 100
˝
C for 48 h and was found to undergo
no mass loss or decomposition at this temperature. Undoubtedly, this material holds promise
and should be investigated further as a potential alternative to lead azide. However, its current
synthesis does suffer from a lengthy synthesis route, and low yields were encountered in some of the
synthetic transformations; most notably, the conversion of dichloro bis(hydrazone) (28) to diazido
bis(hydrazone) (29), which proceeded in only 38% yield.
Crystals 2016, 6, 5 9 of 22
Table 3. Sensitivities and performance of K
2
DNABT as compared to Pb(N
3
)
2
.
Data Category K
2
DNABT (31) Pb(N
3
)
2
IS [J]
a
1 2.5–4
FS [N]
b
ď1 0.1–1
ESD
c
[J]
c
0.003 <0.005
T
dec
[
˝
C]
d
200 315
CO
2
[%]
e
´4.8 ´11
ρ [g/cc]
f
2.11 4.8
P
cj
[kbar]
g
5920 8330
V
det
[m/s]
h
33.8 31.7
f
H
0
[kJ/mol]
i
326.4 450.1
ex
U
0
[kJ/kg]
j
1036.1 1569
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
dec
= Temperature
of decomposition;
e
CO
2
= CO
2
oxygen balance;
f
ρ
= crystalline density;
g
P
cj
= detonation pressure;
h
V
det
= detonation velocity;
i
f
H
0
= molar enthalpy of formation;
j
ex
U
0
= total energy of detonation.
2.3.2.4. 1,5-(dinitramino) Tetrazole Dipotassium Salt
In 2015, Klapötke went on to synthesize the similarly structured dipotassium salt of
1,5-dinitramino tetrazole (33), as described in Scheme 9 [
24
,
28
]. Treatment of methyl carbazate 26 with
cyanogen azide produced tetrazole (32), which was subjected to nitration, followed by basic conditions
to give the dipotassium salt of 1,5-Dinitramino tetrazole.
Crystals 2016, 6, 5
9
Table 3. Sensitivities and performance of K
2DNABT as compared to Pb(N3)2.
Data Category K2DNABT (31) Pb(N3)2
IS [J]
a
1 2.5–4
FS [N]
b
1 0.1–1
ESD
c
[J]
c
0.003 <0.005
T
dec [°C]
d
200 315
Ω

[%]
e
4.8 11
ρ [g/cc]
f
2.11 4.8
P
cj [kbar]
g
5920 8330
V
det [m/s]
h
33.8 31.7
Δ
fH
0
[kJ/mol]
i
326.4 450.1
Δ
exU
0
[kJ/kg]
j
1036.1 1569
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tdec = Temperature
of decomposition;
e
Ω

= CO2 oxygen balance;
f
ρ = crystalline density;
g
Pcj = detonation pressure;
h
Vdet = detonation velocity;
i
ΔfH
0
= molar enthalpy of formation;
j
ΔexU
0
= total energy of detonation.
2.3.2.4. 1,5-(dinitramino) Tetrazole Dipotassium Salt
In 2015, Klapötke went on to synthesize the similarly structured dipotassium salt of 1,5-
dinitramino tetrazole (33), as described in Scheme 9 [24,28]. Treatment of methyl carbazate 26 with
cyanogen azide produced tetrazole (32), which was subjected to nitration, followed by basic
conditions to give the dipotassium salt of 1,5-Dinitramino tetrazole.
Scheme 9. Synthesis of the dipotassium salt of 1,5-Dinitramino tetrazole (33).
Tetrazene and its organic-based replacement, MTX-1, was discussed in Section 2.3.1.2. Klapötke
has reasoned that given the high density, detonation velocity, detonation pressure, and impact and
friction sensitivity of (33) (Table 4), this compound could also serve as a potential replacement for
tetrazene as a sensitizing agent in primary explosive formulations. The shockwave produced by 50
mg of (33) was found to easily detonate the secondary explosive RDX.
Table 4. Sensitivities and performance of the dipotassium salt of 1,5-Dinitramino tetrazole (33) and
CL-20.
Data Category 33 CL-20
IS [J]
a
1 4
FS [N]
b
5 48
T
dec [°C]
c
240 315
Ω

[%]
d
12.03 10.95
ρ [g/cc]
e
2.177 2.04
P
cj [kbar]
f
522 444
V
det [m/s]
g
10011 9730
Δ
fH
0
[kJ/mol]
h
112.4 365.4
Δ
exU
0
[kJ/kg]
i
3938 6168
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
Tdec = Temperature of decomposition;
d
Ω

= CO2
oxygen balance;
e
ρ = crystalline density;
f
Pcj = detonation pressure;
g
Vdet = detonation velocity;
h
ΔfH
0
= molar enthalpy of formation;
i
ΔexU
0
= total energy of detonation.
Scheme 9. Synthesis of the dipotassium salt of 1,5-Dinitramino tetrazole (33).
Tetrazene and its organic-based replacement, MTX-1, was discussed in Section 2.3.1. Klapötke has
reasoned that given the high density, detonation velocity, detonation pressure, and impact and friction
sensitivity of (33) (Table 4), this compound could also serve as a potential replacement for tetrazene as
a sensitizing agent in primary explosive formulations. The shockwave produced by 50 mg of (33) was
found to easily detonate the secondary explosive RDX.
Table 4. Sensitivities and performance of the dipotassium salt of 1,5-Dinitramino tetrazole (33) and CL-20.
Data Category 33 CL-20
IS [J]
a
1 4
FS [N]
b
ď5 48
T
dec
[
˝
C]
c
240 315
CO
2
[%]
d
´12.03 ´10.95
ρ [g/cc]
e
2.177 2.04
P
cj
[kbar]
f
522 444
V
det
[m/s]
g
10011 9730
f
H
0
[kJ/mol]
h
´112.4 365.4
ex
U
0
[kJ/kg]
i
3938 6168
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
T
dec
= Temperature of decomposition;
d
CO
2
= CO
2
oxygen balance;
e
ρ
= crystalline density;
f
P
cj
= detonation pressure;
g
V
det
= detonation velocity;
h
f
H
0
= molar enthalpy of formation;
i
ex
U
0
= total energy of detonation.
Crystals 2016, 6, 5 10 of 22
2.3.2.5. Copper(I) 5-Nitrotetrazolate (DBX-1)
DBX-1 has been a compound of intense investigation recently, due to its similarities to lead azide
with respect to its initiating ability and impact, friction and ESD sensitivity, as well as high thermal
stability [
29
]. Unlike the blue copper(II) nitrotetrazole octahedral complex reported by Huynh [
30
],
DBX-1 is the copper(I) salt that manifests as orange-red crystals and adopts the dimeric structure (as
determined by single-crystal X-ray) shown in Scheme 10. More importantly, it can easily be synthesized
as a free-flowing powder (essential for practical applications) which was never observed for the Huynh
compounds. Further, DBX-1 has been successfully demonstrated to be a “drop-in” replacement for lead
azide (meaning it can be substituted as a direct, volume-for-volume replacement with no hardware
changes) in some existing detonator designs. It is also being investigated as a replacement for lead
styphnate in certain applications as well. DBX-1 has been shown to be more resistant to oxidation than
lead azide when subjected to thermal cycling conditions (i.e., high temperature and high humidity).
DBX-1 is known to slowly decompose when stored in water, and shows incompatibility when in the
presence of periodate-based oxidizers [
31
]. Nonetheless, due to the aforementioned positive qualities,
DBX-1 is being sought as a replacement for lead-based primary explosives.
The traditional synthetic procedure for making DBX-1 is described in Scheme 10. Exposure of
5-aminotetrazole to a modified Sandmeyer reaction affords copper acid salt 34, which is converted to
the sodium salt of 5-nitrotetrazole (NaNT, 35) upon exposure to NaOH as originally described by von
Herz [
32
] and later modified by Gilligan [
33
]. Subsequently, CuCl
2
is treated with sodium ascorbate to
generate the copper(I) species in situ, which reacts with NaNT to yield DBX-1 (36). The performance
and sensitivities for this compound are detailed in Table 5.
Table 5. Performance and sensitivities of DBX-1 (36).
Data Category DBX-1 (36)
IS [J]
a
0.036
FS [N]
b
>0.098
ESD
c
[µJ]
c
12
T
dec
[
˝
C]
d
337
CO
2
[%]
e
´9.0
ρ [g/cc]
f
2.584
f
H
0
[kJ/mol]
g
280.9
ex
U
0
[kJ/kg]
h
3816.6
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
dec
= Temperature of
decomposition;
e
CO
2
= CO
2
oxygen balance;
f
ρ = crystalline density;
g
f
H
0
= molar enthalpy of formation;
h
ex
U
0
= total energy of detonation.
Crystals 2016, 6, 5
10
2.3.2.5. Copper(I) 5-Nitrotetrazolate (DBX-1)
DBX-1 has been a compound of intense investigation recently, due to its similarities to lead azide
with respect to its initiating ability and impact, friction and ESD sensitivity, as well as high thermal
stability [29]. Unlike the blue copper(II) nitrotetrazole octahedral complex reported by Huynh [30],
DBX-1 is the copper(I) salt that manifests as orange-red crystals and adopts the dimeric structure (as
determined by single-crystal X-ray) shown in Scheme 10. More importantly, it can easily be
synthesized as a free-flowing powder (essential for practical applications) which was never observed
for the Huynh compounds. Further, DBX-1 has been successfully demonstrated to be a “drop-in”
replacement for lead azide (meaning it can be substituted as a direct, volume-for-volume replacement
with no hardware changes) in some existing detonator designs. It is also being investigated as a
replacement for lead styphnate in certain applications as well. DBX-1 has been shown to be more
resistant to oxidation than lead azide when subjected to thermal cycling conditions (i.e., high
temperature and high humidity). DBX-1 is known to slowly decompose when stored in water, and
shows incompatibility when in the presence of periodate-based oxidizers [31]. Nonetheless, due to
the aforementioned positive qualities, DBX-1 is being sought as a replacement for lead-based primary
explosives.
The traditional synthetic procedure for making DBX-1 is described in Scheme 10. Exposure of 5-
aminotetrazole to a modified Sandmeyer reaction affords copper acid salt 34, which is converted to
the sodium salt of 5-nitrotetrazole (NaNT, 35) upon exposure to NaOH as originally described by
von Herz [32] and later modified by Gilligan [33]. Subsequently, CuCl
2 is treated with sodium
ascorbate to generate the copper(I) species in situ, which reacts with NaNT to yield DBX-1 (36). The
performance and sensitivities for this compound are detailed in Table 5.
Table 5. Performance and sensitivities of DBX-1 (36).
Data Category DBX-1 (36)
IS [J]
a
0.036
FS [N]
b
>0.098
ESD
c
[µJ]
c
12
T
dec [°C]
d
337
Ω

[%]
e
9.0
ρ [g/cc]
f
2.584
Δ
fH
0
[kJ/mol]
g
280.9
Δ
exU
0
[kJ/kg]
h
3816.6
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tdec = Temperature
of decomposition;
e
Ω

= CO2 oxygen balance;
f
ρ = crystalline density;
g
ΔfH
0
= molar enthalpy of
formation;
h
ΔexU
0
= total energy of detonation.
Scheme 10. Traditional synthetic pathway of DBX-1.
Scheme 10. Traditional synthetic pathway of DBX-1.
Crystals 2016, 6, 5 11 of 22
There are several concerns with this traditional synthesis as described. Since this route has been
discovered, microdetonations have been reported during the reaction; a presumed result of generating
the highly dangerous 5-diazotetrazole in the modified Sandmeyer reaction. On a laboratory-scale,
some improvements have been made over time including the addition of a copper(II) species
such as CuSO
4
¨
5H
2
O to suppress the microdetonation events by converting 5-diazotetrazole to the
innocuous 5-hydroxytetrazole. Furthermore, employing an excess of sodium nitrite in the reaction
mixture enhances the safety of the filtration step to isolate copper acid salt (34). Unfortunately, the
microdetonation and dangerous filtration of copper acid salt (34) commonly resurface when NaNT
is synthesized on a production-sized scale. Therefore, the inability to manufacture NaNT, and thus,
DBX-1 on a production-sized scale has been a major drawback that has limited practical application
for the material.
Fortunately, pioneering work at Nalas Engineering by Salan and co-workers have solved this
looming issue [
34
]. Throughout the course of their investigation, it was discovered that synthesizing
NaNT by the traditional method shown in Scheme 10 does not result in a pure product. NaNT is
obtained through this method with ca. 5% 5-aminotetrazole as an impurity. Furthermore, in
synthesizing copper acid salt (34), it was discovered that use of HNO
3
in the modified Sandmeyer
protocol led to the depositing of the explosive 5-aminotetrazole nitrate on the feeding lines.
To mitigate these issues, HNO
3
was replaced with H
2
SO
4
in the modified Sandmeyer reaction
(Scheme 11).
Crystals 2016, 6, 5
11
There are several concerns with this traditional synthesis as described. Since this route has been
discovered, microdetonations have been reported during the reaction; a presumed result of
generating the highly dangerous 5-diazotetrazole in the modified Sandmeyer reaction. On a
laboratory-scale, some improvements have been made over time including the addition of a
copper(II) species such as CuSO
4·5H2O to suppress the microdetonation events by converting 5-
diazotetrazole to the innocuous 5-hydroxytetrazole. Furthermore, employing an excess of sodium
nitrite in the reaction mixture enhances the safety of the filtration step to isolate copper acid salt (34).
Unfortunately, the microdetonation and dangerous filtration of copper acid salt (34) commonly
resurface when NaNT is synthesized on a production-sized scale. Therefore, the inability to
manufacture NaNT, and thus, DBX-1 on a production-sized scale has been a major drawback that has
limited practical application for the material.
Fortunately, pioneering work at Nalas Engineering by Salan and co-workers have solved this
looming issue [34]. Throughout the course of their investigation, it was discovered that synthesizing
NaNT by the traditional method shown in Scheme 10 does not result in a pure product. NaNT is
obtained through this method with ca. 5% 5-aminotetrazole as an impurity. Furthermore, in
synthesizing copper acid salt (34), it was discovered that use of HNO
3 in the modified Sandmeyer
protocol led to the depositing of the explosive 5-aminotetrazole nitrate on the feeding lines.
To mitigate these issues, HNO
3 was replaced with H2SO4 in the modified Sandmeyer reaction
(Scheme 11).
Scheme 11. Improved synthesis of DBX-1 as performed by Nalas Engineering.
In addition to eliminating the possibility of forming 5-aminotetrazole nitrate, 5-aminotetrazole
was found to be at least twice as soluble in sulfuric acid as in nitric acid. The addition of diatomaceous
earth to the reaction mixture during the modified Sandmeyer protocol resulted in copper acid salt
(34) being retained in the solid phase during washing/purification. Treatment of diatomaceous
earth/copper acid salt 34 with NaOH resulted in the formation of NaNT, which could be isolated in
relatively pure solution by keeping the mother liquor following filtration. In converting NaNT
solution to DBX-1, it was found that doping the reaction mixture with 1–25 wt % of authentic DBX-1
was extremely beneficial in minimizing the induction period of forming DBX-1, and led to an
increased likelihood that a batch of NaNT would be converted to DBX-1. Throughout the process,
handling of any sensitive intermediates in making DBX-1 is avoided, thus increasing the safety of the
operators involved. In light of these significant improvements, it appears that DBX-1 will garner
enthusiastic interest as a lead-free primary explosive in detonator and primer applications.
Scheme 11. Improved synthesis of DBX-1 as performed by Nalas Engineering.
In addition to eliminating the possibility of forming 5-aminotetrazole nitrate, 5-aminotetrazole
was found to be at least twice as soluble in sulfuric acid as in nitric acid. The addition of diatomaceous
earth to the reaction mixture during the modified Sandmeyer protocol resulted in copper acid salt
(34) being retained in the solid phase during washing/purification. Treatment of diatomaceous
earth/copper acid salt 34 with NaOH resulted in the formation of NaNT, which could be isolated
in relatively pure solution by keeping the mother liquor following filtration. In converting NaNT
solution to DBX-1, it was found that doping the reaction mixture with 1–25 wt % of authentic DBX-1
was extremely beneficial in minimizing the induction period of forming DBX-1, and led to an increased
likelihood that a batch of NaNT would be converted to DBX-1. Throughout the process, handling of
any sensitive intermediates in making DBX-1 is avoided, thus increasing the safety of the operators
involved. In light of these significant improvements, it appears that DBX-1 will garner enthusiastic
interest as a lead-free primary explosive in detonator and primer applications.
Crystals 2016, 6, 5 12 of 22
3. Secondary Explosives
3.1. Legacy Secondary Explosives and Why Interest Exists in Developing Alternatives
Unlike primary explosives, secondary explosives are usually harder to initiate, but typically have higher
performance, such as higher detonation velocities and detonation pressures. Figure 3 depicts some examples
of legacy secondary explosives that have been adopted for use (in some cases, widespread use) for
military and/or civilian applications. CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, 40)
has a particularly high performance, but it has failed to find many applications in explosive
formulations due to the costs associated in making this material, which involves palladium catalysis.
There has been interest from the environmental community to discovery metal-free syntheses of
CL-20 [35], but no successful alternative has been demonstrated to date.
2,4,6-Trinitrotoluene (TNT, 37) is the most commonly known secondary explosive in existence
today amongst the general public, and its use has historically been prevalent in military and civilian
sectors. It has found widespread use in the USA since World War I, and is a common ingredient
in explosive mortar and artillery shells, and bombs, for example. The melting point of TNT (80
˝
C)
is significantly below its explosive decomposition temperature, which makes the material useful as
a melt-castable explosive. The insensitive nature of TNT is beneficial because it minimizes the risk
of accidental detonation. Unfortunately, TNT has a negative toxicity profile by today’s increasingly
stringent environmental and human health safety standards. Pink and red wastewater—the former
produced during the washing of equipment following munitions filling, and the latter produced during
the purification process of the crude material—are associated with TNT. Long-term exposure to TNT
has been linked to adverse health effects, and so a replacement for TNT has been sought, particularly
amongst the insensitive munitions community.
Crystals 2016, 6, 5
12
3. Secondary Explosives
3.1. Legacy Secondary Explosives and Why Interest Exists in Developing Alternatives
Unlike primary explosives, secondary explosives are usually harder to initiate, but typically
have higher performance, such as higher detonation velocities and detonation pressures. Figure 3
depicts some examples of legacy secondary explosives that have been adopted for use (in some cases,
widespread use) for military and/or civilian applications. CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-
hexaazaisowurtzitane, 40) has a particularly high performance, but it has failed to find many
applications in explosive formulations due to the costs associated in making this material, which
involves palladium catalysis. There has been interest from the environmental community to
discovery metal-free syntheses of CL-20 [35], but no successful alternative has been demonstrated to
date.
2,4,6-Trinitrotoluene (TNT, 37) is the most commonly known secondary explosive in existence
today amongst the general public, and its use has historically been prevalent in military and civilian
sectors. It has found widespread use in the USA since World War I, and is a common ingredient in
explosive mortar and artillery shells, and bombs, for example. The melting point of TNT (80 °C) is
significantly below its explosive decomposition temperature, which makes the material useful as a
melt-castable explosive. The insensitive nature of TNT is beneficial because it minimizes the risk of
accidental detonation. Unfortunately, TNT has a negative toxicity profile by today’s increasingly
stringent environmental and human health safety standards. Pink and red wastewater—the former
produced during the washing of equipment following munitions filling, and the latter produced
during the purification process of the crude material—are associated with TNT. Long-term exposure
to TNT has been linked to adverse health effects, and so a replacement for TNT has been sought,
particularly amongst the insensitive munitions community.
Figure 3. Examples of legacy secondary explosives TNT (37), RDX (38), HMX (39), and CL-20 (40).
Royal Demolition Explosive (RDX, 1,3,5-trinitro-1,3,5-triazacyclohexane, 38), which is
inexpensively prepared from hexamine, is produced efficiently by a method known as the Bachmann
process. This process also produces HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, 39) as a by-
product [36]. RDX is manufactured on massive scale, and has been used commonly since World War
II. RDX is significantly higher in explosive performance than TNT. The material is rather insensitive
at room temperature, and is often the “gold standard” used when determining acceptable
sensitivities when new secondary explosives are being designed. Like TNT, RDX has come under
increasing pressure from the EPA for regulation due to concerns that the molecule does not bind to
soil effectively, and thus may contaminate groundwater supplies. The EPA has classified RDX as a
possible human carcinogen based on the presence of liver tumors found in mice that ingested food
contaminated with RDX over a one to two year period [37]. On environmental grounds, replacements
for RDX are being sought. While satisfying the stringent environmental regulations, however, it is
also a main objective in energetics synthesis labs to develop new secondary explosives that have as
much power as possible, while having as little sensitivity as possible.
3.2. TNT-Free Formulations
TNT-based explosive fills have historically been ubiquitous in large caliber mortar and artillery
munitions. TNT, as discussed previously, is a melt-castable material with a low sensitivity to impact,
Figure 3. Examples of legacy secondary explosives TNT (37), RDX (38), HMX (39), and CL-20 (40).
Royal Demolition Explosive (RDX, 1,3,5-trinitro-1,3,5-triazacyclohexane, 38), which is inexpensively
prepared from hexamine, is produced efficiently by a method known as the Bachmann process.
This process also produces HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, 39) as a by-product [36].
RDX is manufactured on massive scale, and has been used commonly since World War II. RDX is
significantly higher in explosive performance than TNT. The material is rather insensitive at room
temperature, and is often the “gold standard” used when determining acceptable sensitivities when
new secondary explosives are being designed. Like TNT, RDX has come under increasing pressure
from the EPA for regulation due to concerns that the molecule does not bind to soil effectively, and
thus may contaminate groundwater supplies. The EPA has classified RDX as a possible human
carcinogen based on the presence of liver tumors found in mice that ingested food contaminated with
RDX over a one to two year period [
37
]. On environmental grounds, replacements for RDX are being
sought. While satisfying the stringent environmental regulations, however, it is also a main objective
in energetics synthesis labs to develop new secondary explosives that have as much power as possible,
while having as little sensitivity as possible.
3.2. TNT-Free Formulations
TNT-based explosive fills have historically been ubiquitous in large caliber mortar and artillery
munitions. TNT, as discussed previously, is a melt-castable material with a low sensitivity to impact,
Crystals 2016, 6, 5 13 of 22
friction and ESD. However, it is not the ideal explosive for insensitive munition applications. TNT is
known to exhibit violent behavior when subjected to bullet impact, fire, or when TNT-based artillery
and mortar munitions are hit with an improvised explosive device (IED) or rocket-propelled grenade
(RPG) [
38
], putting those in close proximity of an exploding TNT-based round at risk of being seriously
injured or killed. This, coupled with the environmental drivers to replace TNT, have led to the
development of safer alternatives.
The US Army successfully developed insensitive munitions explosive 101 (IMX-101), which has
been fielded to replace the 155 mm M795 artillery round, with further applications in 105 mm and
120 mm munitions. IMX-101 consists of a melt-castable mixture of 2,4-dinitroanisole (DNAN, 41) and
nitrotriazolone (NTO, 42) (Figure 4). Unlike TNT-based explosive fills, IMX-101 undergoes only a rapid
deflagration rather than a detonation when subjected to fast cook-off, slow cook-off, fragment impact,
bullet impact, and sympathetic detonation. TNT failed to pass in all of these categories. Although TNT
costs $6 per pound, and the DNAN/NTO formulation costs $8 per pound, the marginal increase in
cost is trivial considering how many lives are likely to be saved in future combat situations [35].
Crystals 2016, 6, 5
13
friction and ESD. However, it is not the ideal explosive for insensitive munition applications. TNT is
known to exhibit violent behavior when subjected to bullet impact, fire, or when TNT-based artillery
and mortar munitions are hit with an improvised explosive device (IED) or rocket-propelled grenade
(RPG) [38], putting those in close proximity of an exploding TNT-based round at risk of being
seriously injured or killed. This, coupled with the environmental drivers to replace TNT, have led to
the development of safer alternatives.
The US Army successfully developed insensitive munitions explosive 101 (IMX-101), which has
been fielded to replace the 155 mm M795 artillery round, with further applications in 105 mm and
120 mm munitions. IMX-101 consists of a melt-castable mixture of 2,4-dinitroanisole (DNAN, 41) and
nitrotriazolone (NTO, 42) (Figure 4). Unlike TNT-based explosive fills, IMX-101 undergoes only a
rapid deflagration rather than a detonation when subjected to fast cook-off, slow cook-off, fragment
impact, bullet impact, and sympathetic detonation. TNT failed to pass in all of these categories.
Although TNT costs $6 per pound, and the DNAN/NTO formulation costs $8 per pound, the
marginal increase in cost is trivial considering how many lives are likely to be saved in future combat
situations [35].
Figure 4. Chemical structures of DNAN (41) and NTO (42).
A well-known TNT-based formulation, which commonly finds use in 60 mm, 81 mm, 120 mm
and numerous submunitions is the composition B (aka Comp B) formulation. Comp B consists of a
melt-castable mixture of TNT and RDX. Comp B is also in the process of being replaced in the
aforementioned munitions by insensitive munition explosive 104 (IMX-104), which consists of a melt-
castable mixture of DNAN, NTO and RDX. At the time, the IMX-104 formulation has been qualified
for use, and it is being evaluated in the aforementioned munitions [39].
3.3. Potential RDX Replacements
3.3.1. TKX-50 (Dihydroxylammonium 5,5'-Bis(tetrazolate) diolate)
In 2012, Klapötke reported the synthesis of TKX-50 (47), as summarized in Scheme 12 [36,40].
Condensation of glyoxal with hydroxylamine hydrochloride affords glyoxime (43), which is
converted to dichlorglyoxime (44) in the presence of chlorine gas in ethanol at cold temperatures.
Scheme 12. Synthesis of TKX-50 (47).
Figure 4. Chemical structures of DNAN (41) and NTO (42).
A well-known TNT-based formulation, which commonly finds use in 60 mm, 81 mm, 120 mm
and numerous submunitions is the composition B (aka Comp B) formulation. Comp B consists
of a melt-castable mixture of TNT and RDX. Comp B is also in the process of being replaced in
the aforementioned munitions by insensitive munition explosive 104 (IMX-104), which consists of
a melt-castable mixture of DNAN, NTO and RDX. At the time, the IMX-104 formulation has been
qualified for use, and it is being evaluated in the aforementioned munitions [39].
3.3. Potential RDX Replacements
3.3.1. TKX-50 (Dihydroxylammonium 5,5'-Bis(tetrazolate) diolate)
In 2012, Klapötke reported the synthesis of TKX-50 (47), as summarized in Scheme 12 [
36
,
40
].
Condensation of glyoxal with hydroxylamine hydrochloride affords glyoxime (43), which is converted
to dichlorglyoxime (44) in the presence of chlorine gas in ethanol at cold temperatures.
Crystals 2016, 6, 5
13
friction and ESD. However, it is not the ideal explosive for insensitive munition applications. TNT is
known to exhibit violent behavior when subjected to bullet impact, fire, or when TNT-based artillery
and mortar munitions are hit with an improvised explosive device (IED) or rocket-propelled grenade
(RPG) [38], putting those in close proximity of an exploding TNT-based round at risk of being
seriously injured or killed. This, coupled with the environmental drivers to replace TNT, have led to
the development of safer alternatives.
The US Army successfully developed insensitive munitions explosive 101 (IMX-101), which has
been fielded to replace the 155 mm M795 artillery round, with further applications in 105 mm and
120 mm munitions. IMX-101 consists of a melt-castable mixture of 2,4-dinitroanisole (DNAN, 41) and
nitrotriazolone (NTO, 42) (Figure 4). Unlike TNT-based explosive fills, IMX-101 undergoes only a
rapid deflagration rather than a detonation when subjected to fast cook-off, slow cook-off, fragment
impact, bullet impact, and sympathetic detonation. TNT failed to pass in all of these categories.
Although TNT costs $6 per pound, and the DNAN/NTO formulation costs $8 per pound, the
marginal increase in cost is trivial considering how many lives are likely to be saved in future combat
situations [35].
Figure 4. Chemical structures of DNAN (41) and NTO (42).
A well-known TNT-based formulation, which commonly finds use in 60 mm, 81 mm, 120 mm
and numerous submunitions is the composition B (aka Comp B) formulation. Comp B consists of a
melt-castable mixture of TNT and RDX. Comp B is also in the process of being replaced in the
aforementioned munitions by insensitive munition explosive 104 (IMX-104), which consists of a melt-
castable mixture of DNAN, NTO and RDX. At the time, the IMX-104 formulation has been qualified
for use, and it is being evaluated in the aforementioned munitions [39].
3.3. Potential RDX Replacements
3.3.1. TKX-50 (Dihydroxylammonium 5,5'-Bis(tetrazolate) diolate)
In 2012, Klapötke reported the synthesis of TKX-50 (47), as summarized in Scheme 12 [36,40].
Condensation of glyoxal with hydroxylamine hydrochloride affords glyoxime (43), which is
converted to dichlorglyoxime (44) in the presence of chlorine gas in ethanol at cold temperatures.
Scheme 12. Synthesis of TKX-50 (47).
Scheme 12. Synthesis of TKX-50 (47).
Crystals 2016, 6, 5 14 of 22
Treatment of (44) with sodium azide produces the highly sensitive diazidoglyoxime intermediate
(45), which is treated with HCl under anhydrous conditions to effect cyclization, thus yielding
bis-adjacent tetrazole (46). Exposure of this latter intermediate to dihydroxylammonium chloride
results in the formation of TKX-50.
Compared to RDX, TKX-50 was found to have a lower impact sensitivity, with comparable
friction and ESD sensitivities. TKX-50 was found to have a higher temperature of decomposition,
density, detonation pressure and detonation velocity compared to RDX (Table 6). Due to its reported
performance, TKX-50 has been the subject of scale-up and compatibility studies [
41
]. During that time,
efforts by Damavarapu, Paraskos, and Cooke resulted in producing TKX-50 on a 75 g scale per batch.
Over the course of a year, over three pounds of the material was synthesized for testing purposes.
Table 6. Sensitivities and performance of TKX-50 and RDX.
Data Category TKX-50 (47) RDX
IS [J]
a
20 7.5
FS [N]
b
120 120
ESD
c
[J]
c
0.1 0.2
T
dec
[
˝
C]
d
221 210
CO
2
[%]
e
´27.1 ´21.61
ρ [g/cc]
f
1.918 1.858
P
cj
[kbar]
g
424 380
V
det
[m/s]
h
9698 8983
f
H
0
[kJ/mol]
i
446.6 86.3
ex
U
0
[kJ/kg]
j
6025 6190
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
dec
= Temperature
of decomposition;
e
CO
2
= CO
2
oxygen balance;
f
ρ
= crystalline density;
g
P
cj
= detonation pressure;
h
V
det
= detonation velocity;
i
f
H
0
= molar enthalpy of formation;
j
ex
U
0
= total energy of detonation.
TKX-50 was found to pass the vacuum thermal stability (VTS) test, and thus showed compatibility
when combined with a wide range of metals, metal oxides explosive materials, plasticizers, and
polymers. One notable exception where VTS failed was when TKX-50 was mixed with the DEMN
composition (DEMN is composed of 34.9% diethylenetriamine trinitrate, 33.4% ethylenediamine
dinitrate, 25.5% methyl nitroguanidine, and 6.3% nitroguanidine [
42
]). Very recent work by Sinditskii
has demonstrated that TKX-50 has a burn rate slightly higher than HMX, and that the compound
decomposes via dissociation of the salt moiety, producing diammonium 5,5
'
-bis(tetrazolate) diolate
(ABTOX, Figure 5). Sinditskii has also shown that the enthalpy of formation (
f
H
0
) of TKX-50 is just
111
˘
16 kJ/mol, which is significantly lower than the
f
H
0
= 446.6 kJ/mol that was initially reported,
as shown in Table 6 [43].
Crystals 2016, 6, 5
14
Treatment of (44) with sodium azide produces the highly sensitive diazidoglyoxime
intermediate (45), which is treated with HCl under anhydrous conditions to effect cyclization, thus
yielding bis-adjacent tetrazole (46). Exposure of this latter intermediate to dihydroxylammonium
chloride results in the formation of TKX-50.
Compared to RDX, TKX-50 was found to have a lower impact sensitivity, with comparable
friction and ESD sensitivities. TKX-50 was found to have a higher temperature of decomposition,
density, detonation pressure and detonation velocity compared to RDX (Table 6). Due to its reported
performance, TKX-50 has been the subject of scale-up and compatibility studies [41]. During that
time, efforts by Damavarapu, Paraskos, and Cooke resulted in producing TKX-50 on a 75 g scale per
batch. Over the course of a year, over three pounds of the material was synthesized for testing
purposes.
Table 6. Sensitivities and performance of TKX-50 and RDX.
Data Category TKX-50 (47) RDX
IS [J]
a
20 7.5
FS [N]
b
120 120
ESD
c
[J]
c
0.1 0.2
T
dec [°C]
d
221 210
Ω

[%]
e
27.1 21.61
ρ [g/cc]
f
1.918 1.858
P
cj [kbar]
g
424 380
V
det [m/s]
h
9698 8983
Δ
fH
0
[kJ/mol]
i
446.6 86.3
Δ
exU
0
[kJ/kg]
j
6025 6190
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tdec = Temperature
of decomposition;
e
Ω

= CO2 oxygen balance;
f
ρ = crystalline density;
g
Pcj = detonation pressure;
h
Vdet = detonation velocity;
i
ΔfH
0
= molar enthalpy of formation;
j
ΔexU
0
= total energy of detonation.
TKX-50 was found to pass the vacuum thermal stability (VTS) test, and thus showed
compatibility when combined with a wide range of metals, metal oxides explosive materials,
plasticizers, and polymers. One notable exception where VTS failed was when TKX-50 was mixed
with the DEMN composition (DEMN is composed of 34.9% diethylenetriamine trinitrate, 33.4%
ethylenediamine dinitrate, 25.5% methyl nitroguanidine, and 6.3% nitroguanidine [42]). Very recent
work by Sinditskii has demonstrated that TKX-50 has a burn rate slightly higher than HMX, and that
the compound decomposes via dissociation of the salt moiety, producing diammonium 5,5'-
bis(tetrazolate) diolate (ABTOX, Figure 5). Sinditskii has also shown that the enthalpy of formation
(Δ
fH
0
) of TKX-50 is just 111 ± 16 kJ/mol, which is significantly lower than the ΔfH
0
= 446.6 kJ/mol that
was initially reported, as shown in Table 6 [43].
Figure 5. ABTOX (48), a decomposition product of TKX-50.
3.3.2. Dihydroxylammonium Dinitro-Bis(1,2,4-triazole-1,1'-diolate)
Shortly after the initial synthesis appeared for TKX-50, Klapötke reported the synthesis of
Dihydroxylammonium dinitro-bis-1,2,4-triazole-1,1'-diolate (52) [44]. As outlined in Scheme 13, this
compound is produced in a four-step process, first by reacting oxalic acid with aminoguanidinium
Figure 5. ABTOX (48), a decomposition product of TKX-50.
3.3.2. Dihydroxylammonium Dinitro-Bis(1,2,4-triazole-1,1'-diolate)
Shortly after the initial synthesis appeared for TKX-50, Klapötke reported the synthesis of
Dihydroxylammonium dinitro-bis-1,2,4-triazole-1,1
'
-diolate (52) [
44
]. As outlined in Scheme 13, this
Crystals 2016, 6, 5 15 of 22
compound is produced in a four-step process, first by reacting oxalic acid with aminoguanidinium
bicarbonate in acidic solution, followed by cyclization upon addition of base to furnish bis(triazole) (49).
Introduction of the nitro groups via the Sandmeyer reaction, followed by treatment of the resulting
dinitro bis(triazole) (50) oxone, afforded diol (51). Exposure of the diol to hydroxylamine afforded
dihydroxylammonium salt (52).
Crystals 2016, 6, 5
15
bicarbonate in acidic solution, followed by cyclization upon addition of base to furnish bis(triazole)
(49). Introduction of the nitro groups via the Sandmeyer reaction, followed by treatment of the
resulting dinitro bis(triazole) (50) oxone, afforded diol (51). Exposure of the diol to hydroxylamine
afforded dihydroxylammonium salt (52).
Scheme 13. Synthesis of Dihydroxylammonium dinitro-bis(1,2,4-triazole-1,1'-diolate) (52).
Dihydroxylammonium salt (52) shows a remarkably low sensitivity toward impact, friction, and
ESD (Table 7). The explosive performance of this compound appears to be very similar to RDX.
Curiously, the enthalpy of formation of dihydroxylammonium salt (52) is significantly lower than
the initial reported value for TKX-50, despite the presence of two nitro groups in the structure of the
former compound. With the exception of the nitro groups present on (52), these two compounds have
many similarities in their structure. It would be expected that the presence of nitro groups should
lead to a more positive enthalpy of formation. Therefore, it is plausible that the work carried out by
Sinditskii may be valid, and that the original values for TKX-50 may need to be re-evaluated. As one
would expect, dihydroxylammonium salt (52) had a higher enthalpy of formation (Δ
fH
0
= 213 kJ/mol)
than Sinditskii’s adjusted value for TKX-50 (Δ
fH
0
= 111 ± 16 kJ/mol).
Table 7. Sensitivities and performance of Dihydroxylammonium salt 52 and RDX.
Data Category 52 RDX
IS [J]
a
40 7.5
FS [N]
b
>360 120
ESD
c
[J]
c
0.5 0.2
T
dec [°C]
d
217 210
Ω

[%]
e
19.7 21.61
ρ [g/cc]
f
1.9 1.858
P
cj [kbar]
g
390 380
V
det [m/s]
h
9087 8983
Δ
fH
0
[kJ/mol]
i
213 86.3
Δ
exU
0
[kJ/kg]
j
5985 6190
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tdec = Temperature
of decomposition;
e
Ω

= CO2 oxygen balance;
f
ρ = crystalline density;
g
Pcj = detonation pressure;
h
Vdet = detonation velocity;
i
ΔfH
0
= molar enthalpy of formation;
j
ΔexU
0
= total energy of detonation.
3.3.3. Nitryl Cyanide (NCNO2)
Though a simple molecule on paper, the synthesis of nitryl cyanide proved to be elusive for
decades. However, Christe solved this long-standing problem, finally synthesizing this material, and
providing full characterization upon isolation [45]. However, the synthesis of nitryl cyanide requires
specialized equipment at this time, which is a drawback. Nitryl cyanide was prepared by reacting
tert-butyldimethylsilyl cyanide (TBSCN) with nitronium tetrafluoroborate in nitromethane at 30 °C
(Scheme 14). Employing TBSCN in excess resulted in dramatically reduced yields, as well as the
Scheme 13. Synthesis of Dihydroxylammonium dinitro-bis(1,2,4-triazole-1,1'-diolate) (52).
Dihydroxylammonium salt (52) shows a remarkably low sensitivity toward impact, friction,
and ESD (Table 7). The explosive performance of this compound appears to be very similar to RDX.
Curiously, the enthalpy of formation of dihydroxylammonium salt (52) is significantly lower than
the initial reported value for TKX-50, despite the presence of two nitro groups in the structure of the
former compound. With the exception of the nitro groups present on (52), these two compounds have
many similarities in their structure. It would be expected that the presence of nitro groups should
lead to a more positive enthalpy of formation. Therefore, it is plausible that the work carried out by
Sinditskii may be valid, and that the original values for TKX-50 may need to be re-evaluated. As one would
expect, dihydroxylammonium salt (52) had a higher enthalpy of formation (
f
H
0
= 213 kJ/mol) than
Sinditskii’s adjusted value for TKX-50 (
f
H
0
= 111 ˘ 16 kJ/mol).
Table 7. Sensitivities and performance of Dihydroxylammonium salt 52 and RDX.
Data Category 52 RDX
IS [J]
a
40 7.5
FS [N]
b
>360 120
ESD
c
[J]
c
0.5 0.2
T
dec
[
˝
C]
d
217 210
CO
2
[%]
e
´19.7 ´21.61
ρ [g/cc]
f
1.9 1.858
P
cj
[kbar]
g
390 380
V
det
[m/s]
h
9087 8983
f
H
0
[kJ/mol]
i
213 86.3
ex
U
0
[kJ/kg]
j
5985 6190
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
dec
= Temperature
of decomposition;
e
CO
2
= CO
2
oxygen balance;
f
ρ
= crystalline density;
g
P
cj
= detonation pressure;
h
V
det
= detonation velocity;
i
f
H
0
= molar enthalpy of formation;
j
ex
U
0
= total energy of detonation.
3.3.3. Nitryl Cyanide (NCNO
2
)
Though a simple molecule on paper, the synthesis of nitryl cyanide proved to be elusive for
decades. However, Christe solved this long-standing problem, finally synthesizing this material,
and providing full characterization upon isolation [
45
]. However, the synthesis of nitryl cyanide
requires specialized equipment at this time, which is a drawback. Nitryl cyanide was prepared by
reacting tert-butyldimethylsilyl cyanide (TBSCN) with nitronium tetrafluoroborate in nitromethane
Crystals 2016, 6, 5 16 of 22
at
´
30
˝
C (Scheme 14). Employing TBSCN in excess resulted in dramatically reduced yields, as well
as the formation of cyanogen. Cyanogen is believed to form from a reaction involving TBSCN and
nitryl cyanide.
Crystals 2016, 6, 5
16
formation of cyanogen. Cyanogen is believed to form from a reaction involving TBSCN and nitryl
cyanide.
Scheme 14. Synthesis of nitryl cyanide (53) and its proposed trimerization to trinitrotriazine (54).
To reduce the amount of cyanogen formed, TBSCN needs to be condensed at 196 °C above a
frozen mixture of nitronium tetrafluoroborate in nitromethane. This mixture must be thawed, and
only small amounts of TBSCN can be dissolved in the mixture at a time. The reaction by-products
from the reaction are TBSF and boron trifluoride (BF
3), which are removed by fractional condensation
through a series of cold traps. Removal of BF
3 is critical, as it will readily react with nitryl cyanide in
the liquid phase, and this transformation dramatically reduces yields.
Pure nitryl cyanide was found to be a stable liquid at room temperature. It undergoes very little
decomposition when heated to 50 °C or 100 °C for an hour, but decomposes readily over the course
of several hours at 140 °C. Nitryl cyanide is not compatible with fluorides, Lewis acids, bases, amines,
K
3PO4, and elemental mercury.
Although not useful on its own as an RDX replacement, trimerization of nitryl cyanide would
yield trinitrotriazine (54); an elusive theoretical compound with a predicted density of 1.99 g/cc, a
detonation pressure of 421 kbar, and a detonation velocity of 9770 m/s. Though the sensitivity of
trinitrotriazine is not known, the high theoretical performance makes synthesizing this target
attractive.
3.3.4. 2,6-Diamino-3,5-Dinitropyrazine-N-Oxide (LLM-105)
LLM-105, initially synthesized at Lawrence Livermore National Laboratories by Pagoria and co-
workers in 1994, is an insensitive secondary explosive material with 80% of the power of HMX. LLM-
105 possesses a density of 1.918 g/cc, a detonation velocity of 8730 m/s, a detonation pressure of 35.9
GPa, a high thermal decomposition temperature in excess of 350 °C, and very low sensitivities to
impact, friction, and electrostatic discharge. The most efficient synthesis to-date developed for LLM-
105 is summarized in Scheme 15 [46]. Diaminopyrazine N-oxide (DAPO, 57) can be prepared by
treating N-chloro bis(cyanomethyl) amine (55) or N-nitroso bis(cyanomethyl) amine (56) with
triethylamine. Reaction of the former compound proceeds in 14% yield, whereas reaction of the latter
compound is 66%. Mixed acid nitration of DAPO affords LLM-105 in a reasonable yield of 64%. Very
recent efforts by Zuckerman have detailed some success at synthesizing LLM-105 using a flow
microreactor synthesis [47]. Although no increases in yield were realized compared to traditional
synthetic processes, reduced reaction times, higher purities, and an increase in safety to the operator
have been demonstrated.
Scheme 15. Synthesis of LLM-105 (58).
Scheme 14. Synthesis of nitryl cyanide (53) and its proposed trimerization to trinitrotriazine (54).
To reduce the amount of cyanogen formed, TBSCN needs to be condensed at
´
196
˝
C above
a frozen mixture of nitronium tetrafluoroborate in nitromethane. This mixture must be thawed, and
only small amounts of TBSCN can be dissolved in the mixture at a time. The reaction by-products
from the reaction are TBSF and boron trifluoride (BF
3
), which are removed by fractional condensation
through a series of cold traps. Removal of BF
3
is critical, as it will readily react with nitryl cyanide in
the liquid phase, and this transformation dramatically reduces yields.
Pure nitryl cyanide was found to be a stable liquid at room temperature. It undergoes very little
decomposition when heated to 50
˝
C or 100
˝
C for an hour, but decomposes readily over the course of
several hours at 140
˝
C. Nitryl cyanide is not compatible with fluorides, Lewis acids, bases, amines,
K
3
PO
4
, and elemental mercury.
Although not useful on its own as an RDX replacement, trimerization of nitryl cyanide would
yield trinitrotriazine (54); an elusive theoretical compound with a predicted density of 1.99 g/cc,
a detonation pressure of 421 kbar, and a detonation velocity of 9770 m/s. Though the sensitivity of
trinitrotriazine is not known, the high theoretical performance makes synthesizing this target attractive.
3.3.4. 2,6-Diamino-3,5-Dinitropyrazine-N-Oxide (LLM-105)
LLM-105, initially synthesized at Lawrence Livermore National Laboratories by Pagoria and
co-workers in 1994, is an insensitive secondary explosive material with 80% of the power of HMX.
LLM-105 possesses a density of 1.918 g/cc, a detonation velocity of 8730 m/s, a detonation pressure of
35.9 GPa, a high thermal decomposition temperature in excess of 350
˝
C, and very low sensitivities
to impact, friction, and electrostatic discharge. The most efficient synthesis to-date developed for
LLM-105 is summarized in Scheme 15 [
46
]. Diaminopyrazine N-oxide (DAPO, 57) can be prepared
by treating N-chloro bis(cyanomethyl) amine (55) or N-nitroso bis(cyanomethyl) amine (56) with
triethylamine. Reaction of the former compound proceeds in 14% yield, whereas reaction of the latter
compound is 66%. Mixed acid nitration of DAPO affords LLM-105 in a reasonable yield of 64%.
Very recent efforts by Zuckerman have detailed some success at synthesizing LLM-105 using a flow
microreactor synthesis [
47
]. Although no increases in yield were realized compared to traditional
synthetic processes, reduced reaction times, higher purities, and an increase in safety to the operator
have been demonstrated.
Crystals 2016, 6, 5
16
formation of cyanogen. Cyanogen is believed to form from a reaction involving TBSCN and nitryl
cyanide.
Scheme 14. Synthesis of nitryl cyanide (53) and its proposed trimerization to trinitrotriazine (54).
To reduce the amount of cyanogen formed, TBSCN needs to be condensed at 196 °C above a
frozen mixture of nitronium tetrafluoroborate in nitromethane. This mixture must be thawed, and
only small amounts of TBSCN can be dissolved in the mixture at a time. The reaction by-products
from the reaction are TBSF and boron trifluoride (BF
3), which are removed by fractional condensation
through a series of cold traps. Removal of BF
3 is critical, as it will readily react with nitryl cyanide in
the liquid phase, and this transformation dramatically reduces yields.
Pure nitryl cyanide was found to be a stable liquid at room temperature. It undergoes very little
decomposition when heated to 50 °C or 100 °C for an hour, but decomposes readily over the course
of several hours at 140 °C. Nitryl cyanide is not compatible with fluorides, Lewis acids, bases, amines,
K
3PO4, and elemental mercury.
Although not useful on its own as an RDX replacement, trimerization of nitryl cyanide would
yield trinitrotriazine (54); an elusive theoretical compound with a predicted density of 1.99 g/cc, a
detonation pressure of 421 kbar, and a detonation velocity of 9770 m/s. Though the sensitivity of
trinitrotriazine is not known, the high theoretical performance makes synthesizing this target
attractive.
3.3.4. 2,6-Diamino-3,5-Dinitropyrazine-N-Oxide (LLM-105)
LLM-105, initially synthesized at Lawrence Livermore National Laboratories by Pagoria and co-
workers in 1994, is an insensitive secondary explosive material with 80% of the power of HMX. LLM-
105 possesses a density of 1.918 g/cc, a detonation velocity of 8730 m/s, a detonation pressure of 35.9
GPa, a high thermal decomposition temperature in excess of 350 °C, and very low sensitivities to
impact, friction, and electrostatic discharge. The most efficient synthesis to-date developed for LLM-
105 is summarized in Scheme 15 [46]. Diaminopyrazine N-oxide (DAPO, 57) can be prepared by
treating N-chloro bis(cyanomethyl) amine (55) or N-nitroso bis(cyanomethyl) amine (56) with
triethylamine. Reaction of the former compound proceeds in 14% yield, whereas reaction of the latter
compound is 66%. Mixed acid nitration of DAPO affords LLM-105 in a reasonable yield of 64%. Very
recent efforts by Zuckerman have detailed some success at synthesizing LLM-105 using a flow
microreactor synthesis [47]. Although no increases in yield were realized compared to traditional
synthetic processes, reduced reaction times, higher purities, and an increase in safety to the operator
have been demonstrated.
Scheme 15. Synthesis of LLM-105 (58).
Scheme 15. Synthesis of LLM-105 (58).
Crystals 2016, 6, 5 17 of 22
3.3.5. Energetic co-Crystallization of HMX and CL-20
In a recent new development in applying pharmaceutical applications to the field of energetic
materials, Matzger pioneered the concept of developing the first known energetic co-crystals. The most
successful energetic co-crystal reported to date was developed by co-crystallizing a 2:1 molar mixture
of CL-20 (39) and HMX (40), whose individual structures are provided in Figure 4 [
48
]. Surprisingly,
the co-crystallized material was shown to have a low impact sensitivity comparable to HMX rather
than CL-20, which is believed to be attributed to the high degree of C–H bonding interactions present
in the co-crystal. These interactions are absent from CL-20 and HMX in their individual, pure form.
The co-crystal has a crystalline density of 1.945 g/cc, was also theoretically predicted to have a detonation
velocity of 9484 m/s, which is higher than HMX (V
det
= 9100 m/s), and undergoes decomposition
at 235
˝
C, followed by detonation at 240
˝
C. Friction sensitivity, ESD sensitivity, and detonation
pressure values were not reported. In future years, as the technology becomes more mature, it will be
interesting to determine whether an energetic co-crystal, such as the aforementioned example, will
maintain its integrity, structure and power when used in explosive formulations, when subjected to
high loading pressures, and when stored in high temperature/high humidity environments for long
periods of time.
3.4. High-Performing Melt-Castable Materials
3.4.1. 2,3-Dinitro-2,3-Bis((nitrooxy)methyl)Butane-1,4-Diyl Dinitrate
Melt-castable materials are preferred over press-cured materials due to their less labor-intensive
processes, as well as their enhanced safety issues. There is also an increasing interest
in the development of melt-castable materials that demosntrate performance exceeding TNT.
Work at Los Alamos National Laboratory spearheaded by Chavez led to the synthesis of a new
melt-castable material that appears to satisfy these requirements, as summarized in Scheme 16 [
49
].
Commercially available (though expensive) dioxane (59) was homocoupled to yield dinitro derivative
(60). Removal of the diacetal, followed by exhaustive nitration of the resulting tetrol afforded
2,3-Dinitro-2,3-bis((nitrooxy)methyl)butane-1,4-diyl dinitrate (62).
Crystals 2016, 6, 5
17
3.3.5. Energetic co-Crystallization of HMX and CL-20
In a recent new development in applying pharmaceutical applications to the field of energetic
materials, Matzger pioneered the concept of developing the first known energetic co-crystals. The
most successful energetic co-crystal reported to date was developed by co-crystallizing a 2:1 molar
mixture of CL-20 (39) and HMX (40), whose individual structures are provided in Figure 4 [48].
Surprisingly, the co-crystallized material was shown to have a low impact sensitivity comparable to
HMX rather than CL-20, which is believed to be attributed to the high degree of C–H bonding
interactions present in the co-crystal. These interactions are absent from CL-20 and HMX in their
individual, pure form. The co-crystal has a crystalline density of 1.945 g/cc, was also theoretically
predicted to have a detonation velocity of 9484 m/s, which is higher than HMX (V
det = 9100 m/s), and
undergoes decomposition at 235 °C, followed by detonation at 240 °C. Friction sensitivity, ESD
sensitivity, and detonation pressure values were not reported. In future years, as the technology
becomes more mature, it will be interesting to determine whether an energetic co-crystal, such as the
aforementioned example, will maintain its integrity, structure and power when used in explosive
formulations, when subjected to high loading pressures, and when stored in high temperature/high
humidity environments for long periods of time.
3.4. High-Performing Melt-Castable Materials
3.4.1. 2,3-Dinitro-2,3-Bis((nitrooxy)methyl)Butane-1,4-Diyl Dinitrate
Melt-castable materials are preferred over press-cured materials due to their less labor-intensive
processes, as well as their enhanced safety issues. There is also an increasing interest in the
development of melt-castable materials that demosntrate performance exceeding TNT. Work at Los
Alamos National Laboratory spearheaded by Chavez led to the synthesis of a new melt-castable
material that appears to satisfy these requirements, as summarized in Scheme 16 [49]. Commercially
available (though expensive) dioxane (59) was homocoupled to yield dinitro derivative (60). Removal
of the diacetal, followed by exhaustive nitration of the resulting tetrol afforded 2,3-Dinitro-2,3-
bis((nitrooxy)methyl)butane-1,4-diyl dinitrate (62).
Scheme 16. Synthesis of dinitro tetranitrate ester (62).
This material melts at 85–86 °C, but does not decompose until 141 °C (Table 8). The detonation
velocity of tetranitrate ester 62 was calculated to be identical to HMX (9100 m/s) with a calculated
detonation pressure of 400 kbar (P
cj of HMX = 393 kbar). The impact, friction, and ESD sensitivity of
this material was found to be comparable to pentaerythritol tetranitrate (PETN).
Scheme 16. Synthesis of dinitro tetranitrate ester (62).
This material melts at 85–86
˝
C, but does not decompose until 141
˝
C (Table 8). The detonation
velocity of tetranitrate ester 62 was calculated to be identical to HMX (9100 m/s) with a calculated
detonation pressure of 400 kbar (P
cj
of HMX = 393 kbar). The impact, friction, and ESD sensitivity of
this material was found to be comparable to pentaerythritol tetranitrate (PETN).
Crystals 2016, 6, 5 18 of 22
Table 8. Performance and sensitivities of tetranitrate ester 62.
Data Category 62
IS [J]
a
2.7
FS [N]
b
74.5
ESD
c
[J]
c
0.625
T
melt
[
˝
C]
d
85–86
T
dec
[
˝
C]
e
140
CO
2
[%]
f
0
ρ [g/cc]
g
1.917
P
cj
[kbar]
h
400
V
det
[m/s]
i
9100
f
H
0
[kJ/mol]
j
´371
d
T
melt
= Tempterature of melting;
e
T
dec
= Temperature of decomposition;
f
CO
2
= CO
2
oxygen balance;
g
ρ
= crystalline density;
h
P
cj
= detonation pressure;
i
V
det
= detonation velocity;
j
f
H
0
= molar enthalpy
of formation.
3.4.2. 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole-N-Oxide (BNFF, DNTF)
BNFF (aka DNTF) can be synthesized depicted in Scheme 17 [
50
]. Formation of furazan (63) from
malononitrile, followed by diazotization and substitution furnished chloroxime (64). Exposure of the
chloroxime to silver carbonate afforded difurazano furoxan (65), which was nitrated to yield BNFF (66).
Crystals 2016, 6, 5
18
Table 8. Performance and sensitivities of tetranitrate ester 62.
Data Category 62
IS [J]
a
2.7
FS [N]
b
74.5
ESD
c
[J]
c
0.625
T
melt [°C]
d
85–86
T
dec [°C]
e
140
Ω

[%]
f
0
ρ [g/cc]
g
1.917
P
cj [kbar]
h
400
V
det [m/s]
i
9100
Δ
fH
0
[kJ/mol]
j
371
d
Tmelt = Tempterature of melting;
e
Tdec = Temperature of decomposition;
f
Ω

= CO2 oxygen balance;
g
ρ = crystalline density;
h
Pcj = detonation pressure;
i
Vdet = detonation velocity;
j
ΔfH
0
= molar enthalpy
of formation.
3.4.2. 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole-N-Oxide (BNFF, DNTF)
BNFF (aka DNTF) can be synthesized depicted in Scheme 17 [50]. Formation of furazan (63) from
malononitrile, followed by diazotization and substitution furnished chloroxime (64). Exposure of the
chloroxime to silver carbonate afforded difurazano furoxan (65), which was nitrated to yield BNFF
(66).
Scheme 17. Synthesis of BNFF (66).
The performance parameters of BNFF, as reported by Sinditskii, are provided in Table 6 [51]. It
is worth noting that there are two exothermic events during the decomposition of BNFF, with the
first taking place at 292 °C, and the second taking place at 310 °C. Although reported densities of
BNFF are north of 1.90 g/cc, these are likely at cold temperatures, as the density reported in Table 6
is the experimentally determined density at room temperature. The detonation velocity, enthalpy of
formation, and detonation energy values of BNFF detailed in Table 9 represent experimentally
determined values.
Table 9. Performance and sensitivities of BNFF.
Data Category BNFF (66)
Tmelt [°C]
a
109–111
Tdec[°C]
b
292
ρ [g/cc]
c
1.875
V
det [m/s]
d
8930
Δ
fH
0
[kJ/mol]
e
657.23
Δ
exU
0
[kJ/kg]
f
5797
Scheme 17. Synthesis of BNFF (66).
The performance parameters of BNFF, as reported by Sinditskii, are provided in Table 6 [
51
].
It is worth noting that there are two exothermic events during the decomposition of BNFF, with the
first taking place at 292
˝
C, and the second taking place at 310
˝
C. Although reported densities of
BNFF are north of 1.90 g/cc, these are likely at cold temperatures, as the density reported in Table 6
is the experimentally determined density at room temperature. The detonation velocity, enthalpy
of formation, and detonation energy values of BNFF detailed in Table 9 represent experimentally
determined values.
Table 9. Performance and sensitivities of BNFF.
Data Category BNFF (66)
T
melt
[
˝
C]
a
109–111
T
dec
[
˝
C]
b
292
ρ [g/cc]
c
1.875
V
det
[m/s]
d
8930
f
H
0
[kJ/mol]
e
657.23
ex
U
0
[kJ/kg]
f
5797
a
T
melt
= melting temperature;
b
T
dec
= temperature of decomposition;
c
ρ
= crystalline density;
d
V
det
= detonation velocity;
e
f
H
0
= molar enthalpy of formation;
f
ex
U
0
= total energy of detonation.
Crystals 2016, 6, 5 19 of 22
3.4.3. 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole (LLM-172, BNFF-1)
A close molecular derivative of BNFF is LLM-172 (aka BNFF-1), as shown in Figure 6. The only
difference existing in the structure is that BNFF has the presence of an N-oxide moiety, whereas
LLM-172 does not. As summarized in Table 10, LLM-172 has a melting point of 85
˝
C, which is
significantly lower than the melting point of DNTF, thus shifting the former material further into the
melt-castable range [
52
]. Predictably, due to the absence of the N-oxide functionality, the performance of
LLM-172 is lower than that of DNTF, yet the material also has reduced sensitivity to ignition stimuli as well.
Crystals 2016, 6, 5
19
a
Tmelt = melting temperature;
b
Tdec = temperature of decomposition;
c
ρ = crystalline density;
d
Vdet =
detonation velocity;
e
ΔfH
0
= molar enthalpy of formation;
f
ΔexU
0
= total energy of detonation.
3.4.3. 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole (LLM-172, BNFF-1)
A close molecular derivative of BNFF is LLM-172 (aka BNFF-1), as shown in Figure 6. The only
difference existing in the structure is that BNFF has the presence of an N-oxide moiety, whereas LLM-
172 does not. As summarized in Table 10, LLM-172 has a melting point of 85 °C, which is significantly
lower than the melting point of DNTF, thus shifting the former material further into the melt-castable
range [52]. Predictably, due to the absence of the N-oxide functionality, the performance of LLM-172
is lower than that of DNTF, yet the material also has reduced sensitivity to ignition stimuli as well.
N
O
N
67
N
O
N
N
O
N
O
2
N NO
2
Figure 6. Molecular structure of LLM-172 (67).
Table 10. Performance and sensitivities of LLM-172 (67).
Data Category LLM-172 (67)
IS [J]
a
11.5
FS [N]
b
188
ESD
c
[J]
c
>1.0
T
melt [°C]
d
84
T
dec [°C]
e
293
Ω

[%]
f
27
ρ [g/cc]
g
1.836
P
cj [kbar]
h
340
V
det [m/s]
i
8870
Δ
fH
0
[kJ/mol]
j
644
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tmelt = Tempterature
of melting;
e
Tdec = Temperature of decomposition;
f
Ω

= CO2 oxygen balance;
g
ρ = crystalline
density;
h
Pcj = detonation pressure;
i
Vdet = detonation velocity;
j
ΔfH
0
= molar enthalpy of formation.
3.4.4. 3-(4-Amino-1,2,5-Oxadiazol-3-yl)-4-(4-Nitro-1,2,5-Oxadiazol-3-yl)-1,2,5-Oxadiazole (ANFF-1)
A final furazan-based compound that is of interest for melt-castable applications is ANFF-1,
whose preferred method of synthesis is given in Scheme 18 [53]. BAFF-1 (68) is treated with sulfuric
acid and hydrogen peroxide to produce the asymmetrical ANFF-1 via the conversion of one of the
two amino groups to a nitro group.
Scheme 18. Synthesis of ANFF-1 (69).
The high density, low melting point, and low level of sensitivity, as outlined in Table 11, makes
ANFF-1 a candidate molecule that ought to be explored for further explosive development.
Figure 6. Molecular structure of LLM-172 (67).
Table 10. Performance and sensitivities of LLM-172 (67).
Data Category LLM-172 (67)
IS [J]
a
11.5
FS [N]
b
188
ESD
c
[J]
c
>1.0
T
melt
[
˝
C]
d
84
T
dec
[
˝
C]
e
293
CO
2
[%]
f
´27
ρ [g/cc]
g
1.836
P
cj
[kbar]
h
340
V
det
[m/s]
i
8870
f
H
0
[kJ/mol]
j
644
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
T
melt
= Tempterature
of melting;
e
T
dec
= Temperature of decomposition;
f
CO
2
= CO
2
oxygen balance;
g
ρ
= crystalline density;
h
P
cj
= detonation pressure;
i
V
det
= detonation velocity;
j
f
H
0
= molar enthalpy of formation.
3.4.4. 3-(4-Amino-1,2,5-Oxadiazol-3-yl)-4-(4-Nitro-1,2,5-Oxadiazol-3-yl)-1,2,5-Oxadiazole (ANFF-1)
A final furazan-based compound that is of interest for melt-castable applications is ANFF-1,
whose preferred method of synthesis is given in Scheme 18 [
53
]. BAFF-1 (68) is treated with sulfuric
acid and hydrogen peroxide to produce the asymmetrical ANFF-1 via the conversion of one of the
two amino groups to a nitro group.
Crystals 2016, 6, 5
19
a
Tmelt = melting temperature;
b
Tdec = temperature of decomposition;
c
ρ = crystalline density;
d
Vdet =
detonation velocity;
e
ΔfH
0
= molar enthalpy of formation;
f
ΔexU
0
= total energy of detonation.
3.4.3. 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-Oxadiazole (LLM-172, BNFF-1)
A close molecular derivative of BNFF is LLM-172 (aka BNFF-1), as shown in Figure 6. The only
difference existing in the structure is that BNFF has the presence of an N-oxide moiety, whereas LLM-
172 does not. As summarized in Table 10, LLM-172 has a melting point of 85 °C, which is significantly
lower than the melting point of DNTF, thus shifting the former material further into the melt-castable
range [52]. Predictably, due to the absence of the N-oxide functionality, the performance of LLM-172
is lower than that of DNTF, yet the material also has reduced sensitivity to ignition stimuli as well.
N
O
N
67
N
O
N
N
O
N
O
2
N NO
2
Figure 6. Molecular structure of LLM-172 (67).
Table 10. Performance and sensitivities of LLM-172 (67).
Data Category LLM-172 (67)
IS [J]
a
11.5
FS [N]
b
188
ESD
c
[J]
c
>1.0
T
melt [°C]
d
84
T
dec [°C]
e
293
Ω

[%]
f
27
ρ [g/cc]
g
1.836
P
cj [kbar]
h
340
V
det [m/s]
i
8870
Δ
fH
0
[kJ/mol]
j
644
a
IS = impact sensitivity;
b
FS = friction sensitivity;
c
ESD = electrostatic discharge;
d
Tmelt = Tempterature
of melting;
e
Tdec = Temperature of decomposition;
f
Ω

= CO2 oxygen balance;
g
ρ = crystalline
density;
h
Pcj = detonation pressure;
i
Vdet = detonation velocity;
j
ΔfH
0
= molar enthalpy of formation.
3.4.4. 3-(4-Amino-1,2,5-Oxadiazol-3-yl)-4-(4-Nitro-1,2,5-Oxadiazol-3-yl)-1,2,5-Oxadiazole (ANFF-1)
A final furazan-based compound that is of interest for melt-castable applications is ANFF-1,
whose preferred method of synthesis is given in Scheme 18 [53]. BAFF-1 (68) is treated with sulfuric
acid and hydrogen peroxide to produce the asymmetrical ANFF-1 via the conversion of one of the
two amino groups to a nitro group.
Scheme 18. Synthesis of ANFF-1 (69).
The high density, low melting point, and low level of sensitivity, as outlined in Table 11, makes
ANFF-1 a candidate molecule that ought to be explored for further explosive development.
Scheme 18. Synthesis of ANFF-1 (69).
The high density, low melting point, and low level of sensitivity, as outlined in Table 11, makes
ANFF-1 a candidate molecule that ought to be explored for further explosive development.