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

摘要

Second-order rate constants (k(OH-))for alkaline hydrolysis of Y-substituted phenyl picolinates (6a-6i) have been measured spectrophotometrically. A linear Bronsted-type plot is obtained with beta(lg) = -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, sigma(Y)degrees constants result in a much poorer Hammett correlation than sigma(-)(Y) constants. Furthermore, the Yukawa-Tsuno plot exhibits an excellent linear correlation with rho(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).