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| - Effect of o-Methyl Group on Rate, Mechanism, and ResonanceContribution: Aminolysis of Y-Substituted Phenyl X-Substituted2-Methylbenzoates
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| - Second-order rate constants have been determined spectrophotometrically for the reactions of4-nitrophenyl X-substituted 2-methylbenzoates (2a−e) and Y-substituted phenyl 2-methylbenzoates(3a−e) with alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. Theo-methyl group in the benzoyl moiety of 2a−e retards the reaction rate but does not influence thereaction mechanism. The Hammett plots for the reactions of 2a−e are nonlinear, while thecorresponding 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 betweenthe electron donating substituent X and the carbonyl group is responsible for the nonlinear Hammettplots, while the large r values imply that the ground-state resonance interaction is significant.The reactions of 2a−e resulted in smaller ρX values but larger r values than the correspondingreactions of 4-nitrophenyl X-substituted benzoates (1a−e). The small ρX value for the reactions of2a−e (e.g., ρX = 0.22) is suggested to be responsible for the large r value (e.g., r = 1.70). The reactionsof 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 constantsassociated with the reactions of 3a−e are also consistent with the proposed mechanism.
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