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

The kinetics of the C₆H₅ reactions with CH ₃OH 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 κ (CH₃OH) = (7.82 ± 0.44) × 10¹¹exp [-(853 ± 30)/T] and κ(C₂H₅OH) = (5.73 ± 0.58) × 10¹¹exp [-(1103 ± 44)/T] cm³ mol ^-1 s^-1 for the temperature range studied. Theoretically, four possible product channels of the C₆H₅ + CH ₃OH reaction producing C₆H₆ + CH₃O, C₆H₆ + CH₂OH, C₆H₅OH + CH₃ and C₆H₅OCH₃ + H and five possible product channels of the C₆H₅ + C ₂H₅OH reaction producing C₆H₆ + C₂H₅O, C₆H₆ + CH₂CH ₂OH, C₆H₆ + CH₃CHOH, C ₆H₅OH + CH₃CH₂ and C ₆H₅OCH₂CH₃ + 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.