Kinetics for the reactions of phenyl with methanol and ethanol: Comparison of theory and experiment

The kinetics of the C6H5 reactions with CH3OH and C2H5OH has been measured by pulsed-laser photolysis/mass-spectrometry (PLP/MS) employing acetophenone as the radical source. Kinetic modeling of the benzene formed in the reactions over the temperature range 306–771 K allows us to reliably determine the total rate constants for H-abstraction reactions. In order to improve our low temperature measurements down to 304 K we have also applied the cavity ring-down spectrometric technique using nitrosobenzene as the radical source. Both sets of data agree closely. A weighted least-squares analysis of the two complementary sets of data for the two reactions gave the total rate constants k(CH3OH) = (7.82 ± 0.44) × 1011 exp [−(853 ± 30)/T] and k(C2H5OH) = (5.73 ± 0.58) × 1011 exp [−(1103 ± 44)/T] cm3 mol−1 s−1 for the temperature range studied. Theoretically, four possible product channels of the C6H5 + CH3OH reaction producing C6H6 + CH3O, C6H6 + CH2OH, C6H5OH + CH3 and C6H5OCH3 + H and five possible product channels of the C6H5 + C2H5OH reaction producing C6H6 + C2H5O, C6H6 + CH2CH2OH, C6H6 + CH3CHOH, C6H5OH + CH3CH2 and C6H5OCH2CH3 + H have been computed at the G2M//B3LYP/6−311+G(d, p) level of theory. The hydrogen abstraction channels were predicted to have lower energy barriers than those for the substitution reactions and their rate constants were calculated by the microcanonical variational transition state theory at 200–3000 K. The predicted rate constants are in good agreement with the experimental values. Significantly, the rate constant for the CH3OH reaction with C6H5 was found to be greater than that for the C2H5OH reaction and both reactions were found computationally to be dominated by H-abstraction from the hydroxyl group attributable to the affinity of the phenyl toward the OH group and the predicted lower energy barriers for the OH attack.