Alexis Maldonado

Venezuelan Institute for Scientific Research, Caracas, Capital, Venezuela

Are you Alexis Maldonado?

Claim your profile

Publications (11)20.73 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The kinetics of the gas‐phase thermal decomposition of the α‐ketoester methyl benzoylformate was carried out in a static system with reaction vessel deactivated with allyl bromide, and in the presence of the free radical inhibitor propene. The rate coefficients were determined over the temperature range of 440–481 °C and pressures from 32 to 80 Torr. The reaction was found to be homogenous, unimolecular and obey a first‐order rate law. The products are methyl benzoate and CO. The temperature dependence of the rate coefficient gives the following Arrhenius parameters: log10k (s−1) = 13.56 ± 0.31 and Ea (kJ mol−1) = 232.6 ± 4.4. Theoretical calculations of the kinetic and thermodynamic parameters are in good agreement with the experimental values using PBE1PBE/6‐311++g(d,p). A theoretical Arrhenius plot was constructed at this level of theory, and the good agreement with the experimental Arrhenius plot suggests that this model of transition state may describe reasonably the elimination process. These results suggest a concerted non‐synchronous semi‐polar three‐membered cyclic transition state type of mechanism. The most advanced coordinate is the bond breaking Cδ+‐‐‐δ‐OCH3 with an evolution of 66.7%, implying this as the limiting factor of the elimination process. Copyright © 2014 John Wiley & Sons, Ltd. Kinetic of the gas‐phase thermal decomposition of methyl benzoylformate was carried at 440–481 °C and 32–80 Torr. The product formation suggests both experimentally and theoretically a three‐membered cyclic transition state mechanism.
    Journal of Physical Organic Chemistry 01/2015; 28(1). DOI:10.1002/poc.3396 · 1.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The gas-phase thermal dehydration mechanism of tert-butanol, 2-methyl-2-butanol, 2-methyl-2-pentanol and 2,3-dimethyl-2-butanol by homogeneous catalysis of hydrogen bromide was examined by density functional theory calculations with the hybrid functionals: M062X, CAMB3LYP and WB97XD. Reasonable agreements were found between theoretical and experimental enthalpy values at the WB97XD/6-311++G(d,p) level. The dehydration mechanism of tert-butanol with and without catalysis was evaluated in order to examine the catalyst effect on the mechanism. The elimination reaction without catalysis involves a four-membered transition state (TS), while the reaction with catalysis involves a six-membered TS. The mechanism without catalysis has enthalpy activation over 150 kJ mol–1 greater than the catalysed reaction. In all these reactions, the elongation of the C–O bond is significant in the TS. The un-catalysed reaction is controlled by breaking of C–O bond, and it was found to be more synchronous (Sy ≈ 0.91) than the hydrogen bromide catalysed reactions (Sy ≈ 0.75–0.78); the latter reactions are dominated by the three reaction coordinates associated with water formation. No significant effect on the enthalpies of activation was observed when the size of the alkyl chain was increased.
    Molecular Physics 12/2014; 113:1-12. DOI:10.1080/00268976.2014.938707 · 1.64 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The kinetics of the gas-phase elimination of α-methyl-trans-cinamaldehyde catalyzed by HCl in the temperature range of 399.0–438.7 °C, and the pressure range of 38–165 Torr is a homogeneous, molecular, pseudo first-order process and undergoing a parallel reaction to produce via (A) α-methylstyrene and CO gas and via (B) β-methylstyrene and CO gas. The decomposition of substrate E-2-methyl-2-pentenal was performed in the temperature range of 370.0–410.0 °C and the pressure range of 44–150 Torr also undergoing a molecular, pseudo first-order reaction gives E-2-pentene and CO gas. These reactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor. The rate coefficients are given by the following Arrhenius expressions: Products formation from α-methyl-trans-cinamaldehydeα-methylstyrene: β-methylstyrene: Products formation from E-2-methyl-2-pentenalE-2-pentene: The kinetic and thermodynamic parameters for the thermal decomposition of α-methyl-trans-cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield α-methylstyrene and carbon monoxide, whereas via (B) through a five-membered cyclic transition state to give β-methylstyrene and carbon monoxide. However, the elimination of E-2-methyl-2-pentenal occurs by way of a concerted cyclic five-membered transition state mechanism producing E-2-pentene and carbon monoxide. The present results support that uncatalyzed α-β-unsaturated aldehydes decarbonylate through a three-membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd.
    Journal of Physical Organic Chemistry 05/2014; 27(5):450-455. DOI:10.1002/poc.3282 · 1.23 Impact Factor
  • Computational and Theoretical Chemistry 09/2013; 1019:48-54. DOI:10.1016/j.comptc.2013.06.013 · 1.37 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The kinetics of the gas-phase thermal decomposition of 2-methyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, and cyclopentanone ethylene ketal were determined in a static system and the reaction vessel deactivated with allyl bromide. The decomposition reactions, in the presence of the free radical suppressor propene, are homogeneous, are unimolecular, and follow first-order law kinetics. The products of these reactions are acetaldehyde and the corresponding ketone. The working temperature range was 459-490 °C, and the pressure range was 46-113 Torr. The rate coefficients are given by the following Arrhenius equations: for 2-methyl-1,3-dioxolane, log k = (13.61 ± 0.12) - (242.1 ± 1.0)(2.303RT)(-1), r = 0.9997; for 2,2-dimethyl-1,3-dioxolane, log k = (14.16 ± 0.14) - (253.7 ± 2.0)(2.303RT)(-1), r = 0.9998; for cyclopentanone ethylene ketal, log k = (14.16 ± 0.14) - (253.7 ± 2.0)(2.303RT)(-1), r = 0.9998. Electronic structure calculations using DFT methods B3LYP and MPW1PW91 with 6-31G(d,p), and 6-31++G(d,p) basis sets suggest that the decomposition of these substrates takes place through a stepwise mechanism. The rate-determining step proceeds through a concerted nonsynchronous four-centered cyclic transition state, and the elongation of the C-OCH(3) bond in the direction C(α)(δ+)...OCH(3)(δ-) is predominant. The intermediate products of these decompositions are unstable, at the working temperatures, decomposing rapidly through a concerted cyclic six-centered cyclic transition state type of mechanism.
    The Journal of Physical Chemistry A 08/2012; 116(37):9228-37. DOI:10.1021/jp305179n · 2.78 Impact Factor
  • The Journal of Physical Chemistry A 07/2012; · 2.78 Impact Factor
  • Source
    The Journal of Physical Chemistry A 07/2012; · 2.78 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The gas-phase thermal elimination of 2,2-diethoxypropane was found to give ethanol, acetone, and ethylene, while 1,1-diethoxycyclohexane yielded 1-ethoxycyclohexene and ethanol. The kinetics determinations were carried out, with the reaction vessels deactivated with allyl bromide, and the presence of the free radical suppressor cyclohexene and toluene. Temperature and pressure ranges were 240.1-358.3 °C and 38-102 Torr. The elimination reactions are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 2,2-diethoxypropane, log k(1) (s(-1)) = (13.04 ± 0.07) - (186.6 ± 0.8) kJ mol(-1) (2.303RT)(-1); for the intermediate 2-ethoxypropene, log k(1) (s(-1)) = (13.36 ± 0.33) - (188.8 ± 3.4) kJ mol(-1) (2.303RT)(-1); and for 1,1-diethoxycyclohexane, log k = (14.02 ± 0.11) - (176.6 ± 1.1) kJ mol(-1) (2.303RT)(-1). Theoretical calculations of these reactions using DFT methods B3LYP, MPW1PW91, and PBEPBE, with 6-31G(d,p) and 6-31++G(d,p) basis set, demonstrated that the elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane proceeds through a concerted nonsynchronous four-membered cyclic transition state type of mechanism. The rate-determining factor in these reactions is the elongation of the C-O bond. The intermediate product of 2,2-diethoxypropane elimination, that is, 2-ethoxypropene, further decomposes through a concerted cyclic six-membered cyclic transition state mechanism.
    The Journal of Physical Chemistry A 12/2011; 116(2):846-54. DOI:10.1021/jp209596p · 2.78 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The kinetics and mechanisms of the dehydrochlorination of 2-chloro-1- phenylethane, 3-chloro-1-phenylpropane, 4-chloro-1-phenylbutane, 5-chloro-1-phenylpentane, and their corresponding chloroalkanes were examined by means of electronic structure calculation using density functional theory methods B3LYP/6–31G(d,p), B3LYP/6–31++G(d,p), MPW1PW91/6–31G(d,p), MPW1PW91/6–31++G(d,p), PBEPBE/6–31G(d,p), and PBEPBE/6–31++G(d,p). The potential energy surface was investigated for the minimum energy path. Calculated enthalpies and energies of activation are in good agreement with experimental values using the MPW1PW91 and B3LYP methods. The transition state of these reactions is a four-centered cyclic structure. The reported experimental results proposing neighboring group participation by the phenyl group was not supported by theoretical calculations. The rate-determining process in these reactions is the breaking of ClC bond. The reactions are described as concerted moderately polar and nonsynchronous. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 292–302, 2011
    International Journal of Chemical Kinetics 06/2011; 43(6). DOI:10.1002/kin.20548 · 1.57 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The kinetic and mechanism of the unimolecular gas-phase elimination of 2-(dimethylamino)ethyl chloride were examined by using density functional theory methods to explain the enhanced reactivity in gas-phase elimination compared to the parent compound ethyl chloride. The plausible anchimeric assistance of the dimethylamino proposed in the literature was investigated. The theoretical calculations were carried out at B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p), PBEPBE/6-31G(d,p), and PBEPBE/6-31++G(d,p) levels of theory. The previous proposed reaction path of anchimeric assistance has an energy of activation 60 kJ/mol higher than the experimental value. The located transition state in the minimum energy path is a four-centered cyclic configuration comprising chlorine, hydrogen and two carbon atoms. Calculation results give a lower energy of activation of 2-(dimethylamino)ethyl chloride when compared to the parent compound ethyl chloride. This result is due to the stabilization of the transition state because of electron delocalization involving the dimethylamino substituent.
    Journal of Molecular Structure THEOCHEM 12/2010; DOI:10.1016/j.theochem.2010.08.037 · 1.37 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The rates of gas-phase elimination of several β-substituted diethyl acetals have been determined in a static system and seasoned with allyl bromide. The reactions, inhibited with toluene, are homogeneous, unimolecular, and follow first-order law kinetics. These elimination processes involve two parallel reactions. The first parallel reaction yields ethanol and the corresponding ethyl vinyl ether. The latter product is an unstable intermediate and further decomposes to ethylene and the corresponding substituted aldehyde. The second parallel reaction gives ethane and the corresponding ethyl ester. The kinetics has been measured over the temperature range of 370–441 °C and pressure range of 23–160 torr. The rate coefficients are given by the following Arrhenius equations: The differences in the rates of ethanol formation may be attributed to electronic transmission of the β-substituent. The comparative kinetic and thermodynamic parameters of the parallel reactions suggest two different concerted polar four-membered cyclic transition state types of mechanisms. Copyright © 2010 John Wiley & Sons, Ltd.
    Journal of Physical Organic Chemistry 09/2010; 23(9). DOI:10.1002/poc.1672 · 1.23 Impact Factor