Kinetics and mechanisms of the unimolecular elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane in the gas phase: experimental and theoretical study.

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.

Experimental and theoretical studies of the homogeneous, unimolecular gas-phase elimination kinetics of trimethyl orthovalerate and trimethyl orthochloroacetate.

The rates of gas-phase elimination of trimethyl orthovalerate and trimethyl orthochloroacetate have been determined in a static system, and the reaction Pyrex vessels have been deactivated with the product of decomposition of allyl bromide. The reactions are unimolecular and follow a first-order rate law. The working temperature and pressure ranges were 313-410 degrees C and 40-140 Torr, respectively. The rate coefficients for the homogeneous reaction are given by the following Arrhenius expressions: for trimethyl orthovalerate: log k (s(-1)) = [(14.00 +/- 0.28) - (196.3 +/- 1.7) (kJ/mol)] (2.303RT)(-1), (r = 0.9999); and for trimethyl orthochloroacetate: log k (s(-1)) = [(13.54 +/- 0.21) - (209.3 +/- 1.9)(kJ/mol)](2.303RT)(-1), (r = 0.9998). The theoretical calculations of the kinetic and thermodynamic parameters were carried out by using B3LYP, B3PW91, MPW1PW91, and PBEPBE methods. The theoretical results show reasonably good agreement with the experimental energy and enthalpy of activation values when using the B3PW91/6-31++G** method for trimethyl orthovalerate and PBEPBE /6-31++G** for trimethyl orthochloroacetate. These calculations suggest a molecular concerted nonsynchronous mechanism where C-OCH(3) bond polarization, in the sense C(delta+)...(delta-)OCH(3), is the rate-determining step. The increase in electron density of the oxygen atom at OCH(3) eases the abstraction of the hydrogen of the adjacent C-H bond in a four-membered cyclic structure to give methanol and the corresponding unsaturated ketal. The electron-donor substituent enhances decomposition rates by stabilizing the positive charge developing in the transition state at the carbon bearing the three methoxy groups, whereas the electron-withdrawing substituent destabilizes this charge, thus retarding the reaction.