Gas phase elimination kinetics of methyl mandelate: experimental and DFT studies.

The gas phase elimination kinetics of racemic methyl mandelate was determined in a static system, and yielded on decomposition benzaldehyde, methanol, and carbon monoxide. The reaction was homogeneous, unimolecular, and follows a first-order law in the temperature range 379.5-440 degrees C and pressure range of 21.5-71.1 Torr. The variation of the rate coefficient with temperature is expressed by the following Arrhenius equation: log k1 = (12.70 +/- 0.14)-(206.5 +/- 1.9) kJ/mol (2.303RT)(-1). The theoretical estimations of the kinetics and thermodynamics parameters were carried out using DFT methods B3LYP, B3PW91, MPW1PW91, and PBEPBE. Calculation results are in reasonably good agreement with the experimental energy and enthalpy values when using the PBEPBE DFT functional. However, regarding the entropy of activation, the MPW1PW91 functional is more adequate to describe the reaction. These calculations imply a molecular concerted nonsynchronous mechanism involving a two-step process, where the formation of the unstable alpha-lactone intermediate is the rate-determining factor. The lactone intermediate rapidly decarbonylates to produce benzaldehyde and carbon monoxide. The transition state is late in the reaction coordinate, resembling the lactone configuration.

Joint experimental and theoretical studies of the mechanism for the gas phase elimination kinetics of methyl 2,2-dimethyl-3-hydroxypropionate.

Methyl 2,2-dimethyl-3-hydroxypropionate was found to decompose, in a static system, mainly to methyl isobutyrate and formaldehyde. The reaction rates were affected in packed and unpacked clean Pyrex vessels, demonstrating little but significant surface effect. However, in vessels seasoned with allyl bromide this reaction was homogeneous and unimolecular and followed a first-order law. The working temperature range was 349-410 degrees C and the pressure range was 64-162 Torr. The variation of the rate coefficient with temperature is expressed by the following Arrhenius expression: log k(1) (s(-1)) = [(11.43 +/- 0.57) - (180.4 +/- 7.2) kJ mol(-1)] x (2.303RT)(-1). Methyl 2,2-dimethyl-3-hydroxypropionate was found to be 1.4 times greater in the rate of elimination than methyl 3-hydroxypropionate. Apparently, steric acceleration may be considered responsible in the process of decomposition. The theoretical calculation of the kinetics and thermodynamics parameters, at the B3LYP/6-211G** level of theory, are in reasonably good agreement with the experimental values obtained. These calculations imply a molecular mechanism involving a concerted nonsynchronous transition state where abstraction of the hydroxyl hydrogen by the oxygen of the carbonyl ester is a determining factor and the transition state is late in the reaction coordinate.