Reinterpretation of curved Hammett plots in reaction of nucleophiles with aryl benzoates: Change in rate-determining step or mechanism versus ground-state stabilization

A kinetic study is reported for the reaction of the anionic nucleophiles OH-, CN-, and N-3(-) with aryl benzoates containing substituents on the benzoyl as well as the aryloxy moiety, in 80 mol % H2O-20 mol % dimethyl sulfoxide at 25.0 degreesC. Hammett log k vs a plots for these systems are consistently nonlinear. However, a possible traditional explanation in terms of a mechanism involving a tetrahedral intermediate with curvature resulting from a change in rate-determining step is considered but rejected. The proposed explanation involves ground-state stabilization through resonance interaction between the benzoyl substituent and the electrophilic carbonyl center in the two-stage mechanism. Accordingly, the data are nicely accommodated on the basis of the Yukawa-Tsuno equation, which gives linear plots for all three nuceophiles. Literature reports of the mechanism of acyl transfer processes are reconsidered in this light.