Theoretical calculations of the kinetics and mechanisms of the gas phase elimination of primary, secondary, and tertiary 2-hydroxyalkylbenzenes

Theoretical study of the elimination kinetics of 2-phenylethanol, 1-phenyl-2-propanol, and 2-methyl-1-phenyl-2-propanol in the gas-phase has been carried out at the MP2/6-31G(d,p), B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31RRG(d,p), PBEPBE/6-31G(d,p), and PBEPBE/6-31++G(d,p) levels of theory. The three substrates undergo two parallel elimination reactions. The first elimination appears to proceed through a six-membered cyclic transition state to give toluene and the corresponding aldehyde or ketone. The second parallel elimination takes place through a four-membered cyclic transition state producing water and the corresponding unsaturated aromatic hydrocarbon. Results from MP2/6-31G(d,p) and MPW1PW91/6-31++G(d,p) methods were found to be in good agreement with the experimental kinetic and thermodynamic parameters in the formation of toluene and the corresponding carbonyl compound. However, the results for PBEPBE/6-31G(d,p) were in better agreement with the experimental data for the second parallel reaction yielding water and the corresponding unsaturated aromatic hydrocarbon. The charge distribution differences in the TS related to the substitution by methyl groups in the substrates can account for the observed reaction rate coefficients. The synchronicity parameters imply semi-polar transition states for these elimination reactions.