1. Kinetics and mechanisms of the unimolecular elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane in the gas phase: experimental and theoretical study
Felix Rosas, Alexis Maldonado, Jesus Lezama, Rosa M Domínguez, José R Mora, Tania Cordova, Gabriel Chuchani J Phys Chem A. 2012 Jan 19;116(2):846-54. doi: 10.1021/jp209596p. Epub 2012 Jan 6.
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.
2. Joint experimental and DFT study of the gas-phase unimolecular elimination kinetic of methyl trifluoropyruvate
María M Tosta, José R Mora, Tania Córdova, Gabriel Chuchani J Phys Chem A. 2010 Aug 5;114(30):7892-7. doi: 10.1021/jp104238a.
The elimination kinetics of methyl trifluoropyruvate in the gas phase was determined in a static system, where the reaction vessel was always deactivated with allyl bromide, and in the presence of at least a 3-fold excess of the free-radical chain inhibitor toluene. The working temperature range was 388.5-430.1 degrees C, and the pressure range was 38.6-65.8 Torr. The reaction was found to be homogeneous and unimolecular and to obey a first-order rate law. The products of the reaction are methyl trifluoroacetate and CO gas. The Arrhenius equation of this elimination was found to be as follows: log k(1) (s(-1)) = (12.48 +/- 0.32) - (204.2 +/- 4.2) kJ mol(-1)(2.303RT)(-1) (r = 0.9994). The theoretical calculation of the kinetic and thermodynamic parameters and the mechanism of this reaction were carried out at the 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/6-31G++(d,p) levels of theory. The theoretical study showed that the preferred reaction channel is a 1,2-migration of OCH(3) involving a three-membered cyclic transition state in the rate-determining step.
3. Kinetics and mechanisms of the thermal decomposition of 2-methyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, and cyclopentanone ethylene ketal in the gas phase. Combined experimental and DFT study
Felix Rosas, Jesus Lezama, José R Mora, Alexis Maldonado, Tania Cordova, Gabriel Chuchani J Phys Chem A. 2012 Sep 20;116(37):9228-37. doi: 10.1021/jp305179n. Epub 2012 Sep 4.
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.
