Kinetic Study on Alkaline Hydrolysis of 2‐Pyridyl and 4‐Pyridyl X‐substituted‐Benzoates: Effects of Benzoyl Substituent X and Leaving‐Group Basicity on Reactivity and Reaction Mechanism

Abstract

Second‐order rate constants for alkaline hydrolysis of 2‐pyridyl X‐substituted‐benzoates (4a–4l) and 4‐pyridyl X‐substituted‐benzoates (5a–5l) are reported. The Hammett plots are nonlinear, e.g., substrates possessing a weak electron‐withdrawing group (EWG) or an electron‐donating group (EDG) in the benzoyl moiety deviate negatively from the linear Hammett plots composed of substrates bearing a strong EWG. In contrast, the Yukawa–Tsuno plots exhibit excellent linearity with ρX = 1.57, r = 0.43, and R2 = 0.998 for the reactions of 4a–4l and ρX = 1.47, r = 0.41, and R2 = 0.999 for those of 5a–5l, indicating that the nonlinear Hammett plots are not due to a change in the rate‐determining step (RDS) but are caused by ground‐state (GS) stabilization through resonance interactions. The ρX values found for the reactions of 4a–4l and 5a–5l are similar to those reported previously for reactions that proceed through a stepwise mechanism with formation of an intermediate being the RDS. Small βlg values (e.g., −0.2 ~ −0.3) calculated from the Brønsted‐type correlation of rate constants with leaving‐group basicity also indicate that the reactions proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the RDS.