Kinetic Study on Alkaline Hydrolysis of Y‐substituted Phenyl Picolinates: Effects of Modification of Nonleaving Group from Benzoyl to Picolinyl on Reactivity and Reaction Mechanism

Abstract

Second‐order rate constants (k OH–) for alkaline hydrolysis of Y‐substituted phenyl picolinates (6a–6i) have been measured spectrophotometrically. A linear Brønsted‐type plot is obtained with βlg = −0.34, which is typical for reactions reported previously to proceed through a stepwise mechanism with formation of an addition intermediate being the rate‐determining step (RDS). However, constants result in a much poorer Hammett correlation than constants. Furthermore, the Yukawa‐Tsuno plot exhibits an excellent linear correlation with ρ Y  = 0.82 and r = 0.72, indicating that a negative charge develops partially on the O atom of the leaving group in the RDS. Thus, the reactions have been concluded to proceed through a forced concerted mechanism with a highly unstable intermediate 7. Comparison of the current kinetic data with those reported previously for the corresponding reactions of Y‐substituted phenyl benzoates has revealed that modification of the nonleaving group from benzoyl to picolinyl causes not only an increase in reactivity but also a change in the reaction mechanism (i.e., from a stepwise mechanism to a forced concerted pathway).