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The mechanism of the homogeneous, unimolecular gas-phase elimination kinetic of 1,1-dimethoxycyclohexane: Experimental and theoretical studies

Abstract:

The gas-phase elimination of 1,1-dimethoxycyclohexane yielded 1-methoxy-1-cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310-360 °C and the pressure was 25-85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s-1)=[(13.82±0.07) - (193.9±1.0)(kJ mol-1)](2.303RT)-1; r=0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31G++(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6-31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four-membered cyclic transition state. The transition state structure of methanol elimination from 1,1-dimethoxycyclohexane is characterized by a significantly elongated C-O bond, while the Cβ-H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS. Copyright © 2010 John Wiley & Sons, Ltd.