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 °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 B 3LYP, 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 - OCH₃ bond polarization, in the sense C^δ+⋯ ^δ-OCH₃, is the rate-determining step. The increase in electron density of the oxygen atom at OCH₃ 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 electrondonor 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.