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Permanganate oxidation of glycine: Kinetics, catalytic effects, and mechanisms
Canadian Journal of Chemistry
Abstract
The oxidation of glycine by permanganate ion in aqueous phosphate buffers is autocatalyzed by the soluble form of colloidal manganese dioxide formed as a reaction product. Both the noncatalytic and the catalytic reaction pathways are first order in permanganate, the noncatalytic pathway is also first order in glycine, whereas the catalytic pathway has a kinetic order unity for the autocatalytic agent and a non-integral order (1.31) for glycine. Both reaction pathways are accelerated by an increase in the pH of the medium, whereas an increase in the buffer concentration at constant pH results in an increase in the rate of the noncatalytic pathway and a decrease in the rate of the catalytic one. Additions of potassium chloride to the solutions have no kinetic effect on the reaction. The apparent activation energies of the noncatalytic and catalytic reaction pathways are 64.5 and 62.0 kJ mol−1, respectively. On the other hand, manganese(II), thiosulfate, and hexacyanoferrate(II) ions, as well as benzyltriethylammonium chloride and arabic gum, have all been found to increase the initial reaction rate. Mechanisms in concordance with the experimental findings are proposed.
... In numerous evaluations of MnO 4 − oxidation of individual organic compounds (e.g. amino acids, aromatics, etc.), altering the ratio of MnO 4 − to substrate can substantially alter the rate of oxidation (Verma et al., 1976;Perez-Benito et al., 1987;Brillas et al., 1988). This means that altering the ratio of MnO 4 − to SOC could impact how much MnO 4 − is reduced in a given soil sample. ...
... The operational interpretation using a fixed soil mass would greatly decrease the labor required and-as unexpectedly demonstrated here-slightly reduce the variability of the POXC values (Table 3). The short reaction time (12 min) may limit the effects of complex interactions of reactant stoichiometry, organic matter composition, pH, and other factors that determine the reduction of MnO 4 − (Perez-Benito et al., 1987;Brillas et al., 1988;Perez-Benito, 2011). It is important to note that there is substantial uncertainty on which compounds are or aren't preferentially oxidized by MnO 4 − in complex solutions (Margenot et al., 2017;Romero et al., 2018;Laszakovits et al., 2020;Huang et al., 2021). ...
The use of permanganate oxidizable carbon (POXC) as a soil health indicator has gained traction due to its low analysis cost and potential for high-throughput analysis. Permanganate (MnO4⁻) has long been proposed to fractionate soil organic matter. A methodological alteration was proposed to allow for its use as a rapid soil health indicator (POXC) is to allow the MnO4⁻ to reach with a fixed soil mass, rather than a fixed mass of soil organic carbon (SOC). However, this modification may compromise the robustness of the results by altering the consistency of the stoichiometry in the reduction–oxidation reaction (MnO4⁻ : SOC). Here we use a diverse set of 69 U.S. soils to evaluate whether using a fixed soil mass (2.5 g) substantively undermines the theoretical requirement of a fixed amount of SOC (15 or 25 mg SOC per sample) per unit of oxidant (MnO4⁻). We found that the use of a fixed SOC mass entailed a more consistent reduction of MnO4⁻ than a fixed soil mass and also resulted in a greater range of absolute (mg kg⁻¹) and relative POXC values (as a % of SOC) across soils. This broader range of values was not driven by large differences in the amount of MnO4⁻ reduced per unit of SOC analyzed, but rather resulted from normalizing the amount of MnO4⁻ reduced on a soil mass basis. The underlying distribution of MnO4⁻ reduction did not substantively change, suggesting that the interpretation when comparing relative differences in POXC would similarly be unchanged. Unexpectedly, the use of a fixed SOC mass decreased the repeatability of the metric relative to the use of a fixed soil mass. Given the current interpretation of POXC, we see few upsides of using fixed SOC mass and several downsides (i.e. lower throughput and decreased reliability), relative to the current use of a fixed soil mass. To minimize unfounded assumptions, we further propose that POXC values be reported strictly as MnO4⁻ reduced, specifically as μmol MnO4⁻ reduced kg⁻¹ soil. Our results further underscore that the results of POXC assays should be cautiously interpreted. Specifically, interpretations should be qualified by the operationally-defined nature of the POXC assay as an indirectly measured, chemically defined fraction.
... For the phosphate buffer solutions, Liu et al. (2012) used an already prepared phosphate buffer solution obtained from Fisher Scientific with no composition details provided, whereas we evaluated over the 0~100 mM KH 2 PO 4 , but regardless of KH 2 PO 4 concentrations, we observed no PFOS loss (Fig. 1). The efficiency of PM for oxidizing contaminants has been shown to be enhanced by water-soluble MnO 2 colloids formed either as long-lived intermediates or as reaction products (referred to as an autocatalytic reaction) (Freeman et al. 1975;Jiang et al. 2016;Mata-Perez and Perez-Benito 1984;Perez-Benito et al. 1987;Arias 1992a, 1992b). The occurrence and stabilization of water-soluble MnO 2 colloids are heavily dependent on the experimental conditions (e.g., pH, temperature, ionic strength, composition, and concentration). ...
... The occurrence and stabilization of water-soluble MnO 2 colloids are heavily dependent on the experimental conditions (e.g., pH, temperature, ionic strength, composition, and concentration). Phosphate ions have been known to play a role on stabilizing water-soluble MnO 2 colloids by adsorption of phosphate ions on their surface (Fieser and Fieser 1967;Mata-Perez and Perez-Benito 1984;Perez-Benito et al. 1987). Given that most phosphate solutions provided from Fisher Scientific are made of Na + or K + -phosphate form and their concentrations are below 1 M, we investigated if the type (Na + or K + ) or concentrations of phosphate salt solutions (0~200 mM) affect the occurrence or stabilization of water-soluble MnO 2 colloids using data for zeta potential and the size of MnO 2 colloids formed. ...
Permanganate (PM) has shown to be able to oxidize a range of organic contaminants including perfluorooctane sulfonate (PFOS). However, mechanisms of PFOS removal by PM have been questioned. To provide clarity to what may be happening to PFOS in PM systems, here we evaluated the ability of PM on PFOS destruction by conducting studies similar to previous studies that reported PFOS destruction which included PM solutions and PM combined with persulfate (PS). We also evaluated if addition of various soluble catalysts could enhance PM’s potential to breakdown PFOS. We observed no PFOS destruction by PM. We also show that the F− and SO42− generation reported in a published study as evidence that PM was breaking bonds in PFOS were found below or not significantly higher than reported limits of quantitation and that SO42− impurities in technical PM approach the reported SO42− levels. For PM-PS systems, heterogeneous PFOS distribution was observed when subsampling reaction vessels at different depths and “salting-out” of PFOS was evident. In addition, subsequent sonication and filtering of the samples led to the apparent disappearance of most of the PFOS, which was an artifact arising from the behavior of PFOS aggregates or potential hemi-micelle formation. Given these findings, addition of salts may have application for collecting or concentrating PFOS and other PFAAs in a remediation or water treatment strategy.
... Kinetics of oxidation of vanillin has been studied previously in alkaline media by few reagents, such as hexacyanoferrate(III) [2], diperiodatoargentate(III) [3], bismuth(V) [4], diperiodatonickelate(IV) [5] and periodate catalyzed by ruthenium(III) [6], and in acid media by cerium(IV) [7]. Permanganate ion is an efficient oxidant in acid, neutral and alkaline media [8][9][10][11][12][13][14]which still remains as one of the most important, eco-friendly and powerful multi-electron oxidants employed in the kinetic studies [15]. The mechanism of oxidation by this multivalent oxidant depends not only on the substrate but also on the medium used for the study. ...
... The appearance of these intermediate oxidation states depends upon various reaction conditions and the type of substrate. In neutral or slightly alkaline solutions, permanganate used as a powerful oxidizing agent (Eo = +1.23 V) according to the following equation: Many investigators [8][9][10][11][12][32][33][34][35]suggested that most of the oxidation reactions by permanganate ion especially in neutral and alkaline media proceed through intermediate complex formation between oxidant and substrate. The formation of manganate(VI) and/or hypomanganate(V) short-lived intermediates may be confirmed by the change in the color of the solution mixture as the reaction proceeded from purple-pink, Mn(VII), to blue, Mn(V), to green, Mn(VI). ...
... All solutions were prepared with ACS or trace metal grade chemicals in milli-Q Ò water (Barnstedt). Polymeric or transparent MnO 2 (diameter range of 89-193 nm), thereafter called colloidal MnO 2 , was prepared by reducing KMnO 4 with a stoichiometric amount of Na 2 S 2 O 3 (Perez-Benito et al., 1987Zhu et al., 2012). Poorly crystallized MnO 2 (particle diameter range of 0.2-1.0 ...
... The results of the present study reveal that the reactivity of Mn(IV) oxides may also play an important role in triggering Mn(IV)-catalyzed anaerobic nitrification. Indeed, colloidal MnO 2 , with its large specific surface area, enhances the reactivity and oxidizing capacity of Mn(IV) oxides compared to amorphous MnO 2 (Perez-Benito et al., 1987;Perez-Benito and Arias, 1992;Perez-Benito, 2002). Similarly, Mn(III) oxyhydroxides were more efficient than Mn(IV) oxides in catalyzing anaerobic nitrification (Anschutz et al., 2005), even though thermodynamic considerations indicate the reduction of either Mn(IV) or Mn(III) oxides with hydroxylamine or hydrazine as possible intermediates during NH þ 4 oxidation is favorable over a range of pH (Luther, 2010). ...
The reduction of Mn(IV) oxides coupled to the anaerobic oxidation of NH4+ has been proposed for more than a decade to contribute to the fixed nitrogen pool in marine sediments, yet the existence of this process is still under debate. In this study, surface sediments from an intertidal salt marsh were incubated with MnO2 in the presence of elevated concentrations of NH4+ to test the hypothesis that the reduction of Mn(IV) oxides catalyzes anaerobic NH4+ oxidation to NO2- or NO3-. Geochemical factors such as the ratio of Mn(IV) to NH4+, the type of Mn(IV) oxides (amorphous or colloidal MnO2), and the redox potential of the sediment significantly affect the activity of anaerobic nitrification. Incubations show that the net production of NO3- is stimulated under anaerobic conditions with external addition of colloidal but not amorphous MnO2 and is facilitated by the presence of high concentrations of NH4+. Mass balance calculations demonstrate that anaerobic NH4+ oxidation contributes to the net consumption of NH4+, providing another piece of evidence for the occurrence of Mn(IV)-catalyzed anaerobic nitrification in coastal marine sediments. Finally, anaerobic nitrification is stimulated by the amendment of small concentrations of NO3- or the absence of sulfate reduction, suggesting that moderately reducing conditions favor anaerobic NH4+ oxidation. Overall, these findings suggest that Mn(IV)-catalyzed anaerobic nitrification in suboxic sediments with high N/Mn concentration ratios and highly reactive manganese oxides may be an important source of NO2- and NO3- for subsequent marine nitrogen loss via denitrification or anammox.
... Report of the oxidation of glycine by permanganate ion in aqueous phosphate buffers auto catalyzed by the soluble form of colloidal manganese dioxide as a reaction product has been established [9,10]. Both the catalytic and non-catalytic reaction pathways of permanganate were first order and glycine concentration respectively, however, the catalytic pathway had a kinetic order of unity for the autocatalytic agent and a non-integral order with glycine. ...
... Report of the oxidation of glycine by permanganate ion in aqueous phosphate buffers auto catalyzed by the soluble form of colloidal manganese dioxide as a reaction product has been established [9,10]. Both the catalytic and non-catalytic reaction pathways of permanganate were first order and glycine concentration respectively, however, the catalytic pathway had a kinetic order of unity for the autocatalytic agent and a non-integral order with glycine. ...
The kinetics and mechanistic steps to the electron transfer reaction of the peroxo-bridged binuclear cobalt(III) complex of succinimide [(suc)(en)2Co(O2)Co(en)2(suc)2+] hereafter called peroxo-bridged dicobalt(III) complex ‘[Co(O2)Co2+]’ by glycine have been carried out spectrophotometrically at λ = 420 nm and T = 26 ± 1 oC, [H+] = 1 x 10-3 M and ᶙ = 0.5 M (NaCl) in aqueous acidic medium. The reaction was found to be first order with respect to [Co(O2)Co2+] and [(Gly] and experimental data indicates a second-order overall. The reactions obeyed the general rate law: (d[Co(O2)Co2+]/dt) = (a +b)[H+])[Co(O2)Co2+][Gly]. Varying hydrogen ion concentration accelerated the reaction rate and shows first-order dependence while the reactions also affected by changes in the ionic strength of the reaction medium by giving a non-negative salt effect in the course of the reaction. Free radicals were not detected in the reactions. Spectroscopic investigation and Michaelis-Menten plots suggest the absence of intermediate complex formation. The experimental result obtained in this system is concluded in favor of the outer-sphere mechanism. KEY WORDS: Cobalt(III) complex, Kinetic, Electron transfer, Spectroscopic, Mechanistic steps, Michaelis-Menten, Glycine Bull. Chem. Soc. Ethiop. 2021, 35(2), 425-434. DOI: https://dx.doi.org/10.4314/bcse.v35i2.15
... 1,2 Due to lysine and other amino acids' functional role, the oxidation kinetics, with different metal ion oxidants, are of special interest and have undergone extensive investigation. [3][4][5][6] One of the metal ion oxidants of interest is the permanganate heptavalent ion. It is a powerful oxidizing agent and has been used extensively in the oxidation of organic substrates. ...
Kinetics of oxidation of L-lysine by permanganate ion in a perchloric acid medium was investigated to explore the order of the reaction with respect to oxidant and substrate and to study the catalytic behaviour of sodium lauryl sulphate (SLS) and polyethylene glycol (PEG). The reaction was found to be first-order with respect to the oxidant and the substrate and zero-order with respect to hydrogen ion. Changes in the sodium sulphate concentration produce a non-significant variation in the rate of the reaction. SLS and PEG were found to catalyze the reaction. Surfactant catalysis was modelled by Piszkiewicz's cooperativity model, while polymer catalysis was explained with the help of the Benesi-Hildebrand equation. The temperature dependence of the rate of the reaction was elucidated, and activation parameters were obtained. Interestingly, the reaction was found to possess positive activation entropy indicating the dissociative nature of the transition state and outer-sphere electron transfer mechanism. A mechanism of the reaction that is supported by the experimental findings was suggested. Keywords: L-lysine, permanganate ion, micellar catalysis, polymer catalysis, outer sphere electron transfer mechanism.
... and Fe(III); two-electron oxidants chloramine-T and K2S2O8; and multi-electron oxidants like permanganate and bromate [2][3][4][5][6][7]. The oxidation of Gly by permanganate has been studied by Joaquin et al. by both catalytic and noncatalytic pathways [8]. Glycine has been oxidized by KMnO4 in H2SO4 and HClO4 media. ...
... Consequently, at high pH values, it is sometimes difficult to ascertain whether an oxidation is proceeding via a one-or a two-electron process. Many investigators [18][19][20][21][22][23] suggested that most of the oxidation reactions by permanganate ion especially in neutral and basic media proceed through intermediate complex formation between oxidant and substrate. The kinetic evidence that supports formation of an intermediate complex between the oxidant and substrate may be represented by the linearity of the plots of 1/k obs versus 1/[Cit] (Fig. 6), similar to the well-known Michaelis-Menten [33] mechanism for enzyme-substrate reactions. ...
Kinetics of oxidation of L-citrulline (Cit) by permanganate ion in both acidic and basic media has been investigated spectrophtometrically at constant ionic strengths and at 25°C. In both media the reactions exhibited first order dependence in [permanganate] and less than unit order dependences in L-citrulline concentration. A fractional-second order dependence with respect to [H + ] and a fractional-first order dependence with respect to [OH - ] were revealed in acidic and basic media, respectively. Increasing ionic strength in basic medium increased the oxidation rate of L-citrulline, whereas it had a negligible effect on the oxidation rate in acidic medium. The rate-determining step in both media is suggested to involve a one-electron change, but the stoichiometry (L-citrulline: permanganate) was different, being 5:2 in acidic medium and 1:2 in basic medium. The proposed oxidation mechanisms involve formation of 1:1 intermediate complexes between kinetically active species of both L-citrulline and permanganate ion in pre-equilibrium steps. The final oxidation products of L-citrulline were identified in both acidic and basic media as the corresponding aldehyde (4-(carbamoylamino) butyraldehyde), ammonia and carbon dioxide. The appropriate rate laws are deduced.
... The MnO 2 is unstable due to its reaction with acid producing Mn(II). The appearance of a new band at a wavelength of 418 nm suggests intervention of Mn(IV) [37,38] as a reduced product of the oxidant. Also, Mn(IV) formation may be due to the reaction between Mn(V) and ...
The kinetics of oxidations of caffeine by permanganate ion in both perchloric and sulfuric acids solutions have been investigated spectrophotometrically at a constant ionic strength of 1.0 mol dm-3 and at 25°C. In both acids, the reaction-time curves were obtained with a sigmoid profile suggesting an autocatalytic effect caused by Mn(II) ions formed as a reaction product. Both catalytic and non-catalytic processes were determined to be first order with respect to the permanganate ion and caffeine concentrations, whereas the orders with respect to [H + ] and [Mn(II)] were found to be less than unity. Variation of either ionic strength or dielectric constant of the medium had no significant effect on the oxidation rates. Spectroscopic studies and Michaelis-Menten plots showed no evidence for the formation of intermediate complexes in both acids suggesting that the reactions point towards the outer-sphere pathway. The reactions mechanism adequately describing the kinetic results was proposed. In both acids, the main oxidation products of caffeine were identified as 1,3,7-trimethyluric acid. Under comparable experimental conditions, the oxidation rate of caffeine in perchloric acid was slightly higher than that in sulfuric acid. The constants involved in the different steps of the reactions mechanism have been evaluated. With admiration to the rate-limiting step of these reactions, the activation parameters have been evaluated and discussed.
... The above phenomenon is shown in Figure 11 (see theExperimental Section), revealing a sigmoid type of curve, which is characteristic of and corresponds to self/autocatalytic reactions. 44 Reaction Rate and Modeling Results. The overall equation for the SGC reaction with Ca(OH) 2 is usually expressed by eq 1. ...
The effect of temperature, pressure, time, solid ionic liquid (SIL) mass (here, the SIL used is tetra-n-heptyl-ammonium bromide, abbreviated as THepAmBr), reuse of THepAmBr, type of SIL, and additives on the solid–gas (Ca(OH)2–CO2) carbonation system was investigated. Results showed that conversion increased as the temperature increased from 25.0 °C up to 50.0 °C and then decreased thereafter; but increased consistently with increasing pressure. Similarly, conversion increased as the mass of THepAmBr increased, leading to complete conversion at a THepAmBr/Ca(OH)2 mass ratio of 0.1:1 with the production of rhombohedral calcites. Stability tests revealed that THepAmBr was active and stable. The effect of BmimBr and BmimCl indicated that they gave conversions of >96%. Furthermore, studies on polymorphic control to aragonite gave interesting results: 30.2% aragonite was synthesized with MgCl2 and 54.7% with PEG6000. Moreover, time-dependent conversion showed that the reaction mechanism for the system was self-catalytic. Consequently, the reaction rate equation that was derived described the experimental conversion satisfactorily.
... The reaction was shown to be autocatalytic. 3 The autocatalysis was attributed to the soluble form of colloidal manganese dioxide, which formed as the end product in reaction 1. A detailed chemical mechanism was suggested for reaction 1 by Perez-Benito. ...
Oscillatory behavior is reported in the permanganate oxidation of glycine in the presence of Na2HPO4 in a stirred flow reactor. In near-neutral solutions long period sustained oscillations were recorded in the potential of a Pt electrode and in the light absorbance measured at λ=418 and 545 nm, characteristic wavelengths to follow the evolution of the intermediate [Mn(IV)] and the reagent [MnO4- ] during the course of the reaction. No evidence of bistability was found. The chemical and physical backgrounds of the oscillatory phenomenon are discussed. In the oscillatory cycle the positive feedback is attributed to the autocatalytic formation of a soluble Mn(IV) species, while the negative feedback arises from its removal from the solution in the form of solid MnO2. A simple model is suggested which simulates qualitatively the experimental observations in the batch runs and the dynamics that appears in the flow system.
Waste printed circuit boards represent one of the most challenging parts of e-waste recycling. The extraction of precious metals (gold, silver, palladium and platinum) from waste printed circuit boards has commonly been done using highly toxic, corrosive, or expensive lixiviants, such as sodium cyanide, aqua regia and iodine/iodide. The present study provides a greener approach to extract precious metals from waste printed circuit boards using alkaline glycine solution in the presence of an oxidant. The common strong oxidant of potassium permanganate and the non-hazardous potassium ferricyanide were investigated and compared in alkaline media. Decrease of the particle size of sample, and the rise of oxidant concentration (0.04–0.16 M) enhanced gold extraction significantly. However, the increase of temperature (∼55 °C) and glycine concentration (0.5–1 M), and the staged addition of oxidant did not significantly improve gold extraction. Control of solution Eh could not reduce permanganate consumption whereas the ferricyanide consumption could be reduced by>50% in 72 h. Under the recommended conditions, 86.8% gold, 70.2% silver, 89.3% palladium and 87.9% copper could be extracted using glycine-permanganate leaching system. In comparison, 79.3% gold, 69.0% silver, 68.5% palladium and 83.1% copper could be extracted using glycine-ferricyanide leaching system. These extractions are comparable with what is achievable from cyanide (NaCN)-based leaching systems. Both of the leaching systems showed fairly good selectivity to copper and precious metals. The present study paves a way for the further development of non-NaCN glycine process for e-waste recycling.
The importance of autocatalysis spans from practical applications such as in chemically amplified photoresists, to autocatalysis playing a fundamental role in evolution as well as a plausible key role in the origin of life. The phenomenon of autocatalysis is characterized by its kinetic signature rather than by its mechanistic aspects. The molecules that form autocatalytic systems and the mechanisms underlying autocatalytic reactions are very diverse. This chemical diversity, combined with the strong involvement of chemical kinetics, creates a formidable barrier for entrance to the field. Understanding these challenges, we wrote this Review with three main goals in mind: (i) To provide a basic introduction to the kinetics of autocatalytic systems and its relation to the role of autocatalysis in evolution, (ii) To provide a comprehensive overview, including tables, of synthetic chemical autocatalytic systems, and (iii) To provide an in‐depth analysis of the concept of autocatalytic reaction networks, their design, and perspectives for their development.
The kinetics of the decomposition of aqueous hydrogen peroxide by water-soluble colloidal manganese dioxide (obtained by reduction of potassium permanganate with sodium thiosulphate) has been studied in the absence and presence of aqueous perchloric acid solution. The reaction followed pseudo first order kinetics with respect to [MnO2], fractional order with respect to [HClO4] and first order kinetics with respect to [H 2O2].
The kinetics of permanganate oxidation of two substituted azinylformamidines (Azn-Fs), namely N,N-dimethyl-N’-(pyridin-2-yl)formamidine (Py) and N,N-dimethyl-N’-(pyrimidin-2-yl)formamidine (Pym), in sulfuric acid were investigated using conventional spectrophotometry. Kinetic evidence for the formation of 1:1 intermediate complexes between the oxidant and substrates was obtained. The reactions of both substrates with permanganate showed similar kinetics, i.e. first order in [MnO4−]0 and fractional-first-order with respect to both [Azn-F]0 and [H+]. The initial product, Mn2+, was found to autocatalyze the oxidation process. Changes in the ionic strength and dielectric constant of the medium had no significant effect on the rate. The final oxidation products of Py and Pym were identified as 2-aminopyridine and 2-aminopyrimidine, respectively, in addition to dimethylamine and carbon dioxide. A plausible reaction mechanism is suggested and the reaction constants involved in the mechanism were evaluated.
The reduction of permanganate ion to MnO2-Mn2O3 soluble colloidal mixed oxide by L-phenylalanine in aqueous phosphate-buffered neutral solutions has been followed by a spectrophotometric method, monitoring the decay of permanganate ion at 525 nm and the formation of the colloidal oxide at 420 nm. The reaction is autocatalyzed by the manganese product and three rate constants have been required to fit the experimental absorbance-time kinetic data. The reaction shows base catalysis and the values of the activation parameters at different pHs have been determined. A mechanism including both the non-autocatalytic and the autocatalytic reaction pathways, and in agreement with the available experimental data, has been proposed. Some key features of this mechanism are the following: (i) of the two predominant forms of the amino acid, the anionic form exhibits a stronger reducing power than the zwitterionic form; (ii) the non-autocatalytic reaction pathway starts with the transfer of the hydrogen atom in the alpha position of the amino acid to permanganate ion; and (iii) the autocatalytic reaction pathway involves the reduction of Mn(IV) to Mn(II) by the amino acid and the posterior re-oxidation of Mn(II) to Mn(IV) by permanganate ion.
The kinetics and mechanism of the KMnO4-oxidative catalysed condensation reaction of L-proline, as a cyclic secondary amino acid, has been investigated spectrophotometrically. Product analysis has revealed, that in contrast to the usual reaction of α-amino acids, which results in an aldehyde species, a valuable diketopiperazine, namely cyclo(L-pro-L-pro), is obtained as the main product, providing a cheap, simple, efficient, and novel method. Conclusive evidence has demonstrated delayed autocatalytic activity involving Mn2+ in this reaction, analogous to that shown by chain primary α-amino acids. It has been revealed that such activity can emerge when a certain concentration of Mn2+ is built up in the medium, to which we refer as the "critical concentration". Moreover, the "critical concentration" for the appearance of "autocatalytic behaviour" in this oxidative process is affected by the nature of the amino acid used, as reported in previous studies, while the magnitude of the critical concentration depends on the sulfuric acid concentration. In considering "delayed autocatalytic behaviour" of Mn2+ ions, rate equations satisfying experimental data for both catalytic and non-catalytic routes have been presented. The reaction shows a first-order dependence on permanganate ion concentration, both in the catalytic and non-catalytic pathways, and an apparent first-order dependence on Mn2+ ions in the catalytic pathway. The correspondence of the pseudo-order rate constants of the catalytic and non-catalytic pathways to the Arrhenius and Eyring laws verified both the "critical concentration" and "delayed autocatalytic behaviour" concepts. The activation parameters associated with the reaction pseudo-rate constants k 1′ and k 2′ are computed and discussed. Proposed mechanisms for both catalytic and non-catalytic routes include a diacyclperoxide intermediate, which leads to a C2-symmetric chiral product having a diketopiperazine skeleton.
The reaction kinetics for the oxidation of l-histidine by permanganate ions have been investigated spectrophotometrically in sulfuric acid medium at constant ionic strength and temperature. The order with respect to permanganate ions was found to be unity and second in acid concentration, whereas a fractional order is observed with respect to histidine. The reaction was observed to proceed through formation of a 1:1 intermediate complex between oxidant and substrate. The effect of the acid concentration suggests that the reaction is acid catalyzed. Increasing the ionic strength has no significant effect on the rate. The influence of temperature on the rate of reaction was studied. The presence of metal ion catalysts was found to accelerate the oxidation rate, and the order of effectiveness of the ions was Cu2+ > Ni2+ > Zn2+. The final oxidation products were identified as aldehyde (2-imidazole acetaldehyde), ammonium ion, manganese(II), and carbon dioxide. Based on the kinetic results, a plausible reaction mechanism is proposed. The activation parameters were determined and discussed with respect to a slow reaction step.
Melamine, a basic organic chemical intermediate, can cause renal failure because of the formation of insoluble melamine cyanurate crystals in the kidneys. This study examined a novel fluorescence spectrometry for the determination of trace melamine in dairy. The method was based on the inhibitory effect of melamine on the decolorizing reaction of the uncatalytic oxidation of acridine red by potassium permanganate in a sulfuric acid medium. The calibration graph was linear for melamine concentrations of 2.1 × 10−4–1.6 mg/L, and a detection limit of 61.5 ng/L was obtained. The melamine was determined in dairy samples by the proposed method after liquid–liquid and solid phase extraction. The results correlated with the high-performance liquid chromatography. The recoveries were within the range of 99.6–103.3% for liquid milk and 98.5–111.1% for milk powder. The possible mechanism of the reaction was also investigated by ultraviolet spectra.
The oxidative treatment of chlorinated ethylenes in ground water using permanganate was investigated in a series of batch kinetic tests. Five chlorinated ethylenes including tetrachloroethylene (PCE), trichloroethylene (TCE), and three isomers of dichloroethylenes (DCEs) were examined. The degradation process was rapid with pseudo-first-order rate constants ranging from 4.5×10−5 to 0.03 s−1 at MnO4−=1 mM. The rate increased with a decreasing number of chlorine substituents on the ethylene. The higher reactivity of trans-DCE (kobs=30×10−3 s−1 at MnO4−=1 mM) as compared to cis-DCE (kobs=0.9×10−3 s−1 at MnO4−=1 mM) is thought to be caused by a significant steric effect due to the formation of a large cyclic activated complex. TCE oxidation as a second-order reaction was confirmed and the rate constant, k=0.67±0.03 M−1 s−1, is independent of pH over the range of 4–8. The activity of both Cl− and hydrogen ions was monitored over time and suggests essentially complete dechlorination, making the degradation products less harmful than the parent compounds. Competition for MnO4− from other organic compounds in ground water or highly contaminated ground water was also evaluated in experiments. A simple and quick approach was demonstrated to estimate permanganate consumption by other organic compounds for field applications and to predict the TCE degradation rate in a system involving multiple contaminants. The modeling results suggest that the effect of autocatalysis by MnO2 on TCE degradation is significant when the system contains high concentration levels of MnO4− and TOC.
The reactions of permanganate ion with seven α-amino acids in aqueous KH(2)PO(4)/K(2)HPO(4) buffers have been followed spectrophotometrically at two different wavelengths: 526 nm (decay of MnO(4)(-)) and 418 nm (formation of colloidal MnO(2)). All of the reactions studied were autocatalyzed by colloidal MnO(2), with the contribution of the autocatalytic reaction pathway decreasing in the order glycine > l-threonine > l-alanine > l-glutamic acid > l-leucine > l-isoleucine > l-valine. The rate constants corresponding to the nonautocatalytic and autocatalytic pathways were obtained by means of either a differential rate law or an integrated one, the latter requiring the use of an iterative method for its implementation. The activation parameters for the two pathways were determined and analyzed to obtain statistically significant correlations for the series of reactions studied. The activation enthalpy of the nonautocatalytic pathway showed a strong, positive dependence on the standard Gibbs energy for the dissociation of the protonated amino group of the α-amino acid. Linear enthalpy-entropy correlations were found for both pathways, leading to isokinetic temperatures of 370 ± 21 K (nonautocatalytic) and 364 ± 28 K (autocatalytic). Mechanisms in agreement with the experimental data are proposed for the two reaction pathways.
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