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Homogeneous and unimolecular gas-phase thermal decomposition kinetics of methyl benzoylformate: Experimental and theoretical study

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: log10 k (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.