Effect of o-methyl group on rate, mechanism, and resonance contribution: aminolysis of Y-substituted phenyl X-substituted 2-methylbenzoates.

Second-order rate constants have been determined spectrophotometrically for the reactions of 4-nitrophenyl X-substituted 2-methylbenzoates (2a-e) and Y-substituted phenyl 2-methylbenzoates (3a-e) with alicyclic secondary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The o-methyl group in the benzoyl moiety of 2a-e retards the reaction rate but does not influence the reaction mechanism. The Hammett plots for the reactions of 2a-e are nonlinear, while the corresponding Yukawa-Tsuno plots are linear with large r values (1.06-1.70). The linear Yukawa-Tsuno plots suggest that stabilization of the ground-state through resonance interaction between the electron donating substituent X and the carbonyl group is responsible for the nonlinear Hammett plots, while the large r values imply that the ground-state resonance interaction is significant. The reactions of 2a-e resulted in smaller rho(X) values but larger r values than the corresponding reactions of 4-nitrophenyl X-substituted benzoates (1a-e). The small rho(X) value for the reactions of 2a-e (e.g., rho(X) = 0.22) is suggested to be responsible for the large r value (e.g., r = 1.70). The reactions of 3a-e with piperidine are proposed to proceed in a stepwise manner with a change in the rate-determining step on the basis of the curved Brønsted-type plot obtained. Microscopic rate constants associated with the reactions of 3a-e are also consistent with the proposed mechanism.