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

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 Bronsted-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.