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

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 β 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, σ Y o constants result in a much poorer Hammett correlation than σ Y − 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).