Linear Free Energy Models for Ester Solvolysis with a Critical Examination of the Alcohol andPhenol Dissociation Model

Abstract

Rate constants for the alkaline hydrolysis of para-substituted benzoate esters, with a variety of aliphatic and aromatic leaving groups, were determined under constant reaction conditions. It was found that acyl variation encompassing both aliphatic and aromatic leaving groups could be correlated using the Hammett equation, since the interaction mechanisms involved in both rate processes are apparently proportional to those involved in the equilibrium process. Application of either the Hammett equation or phenol dissociation model to aromatic substituents in the alkyl portion of the ester resulted in nonlinearity. The incomplete success observed in the comparison of the rate processes to these models was not due to a change in mechanism or in rate-determining step, as was shown by solvent isotope, temperature, and catalytic studies, but was apparently due to differences in resonance and solvation interactions in rate and equilibrium processes. Aromatic substituent effects in the alkyl portion of the ester could be quantitated by using a linear combination of the Hammett and phenol dissociation model analogous to the Yukawa-Tsuno equation. In addition, the Swain and Lupton approach quantitated the effect of the alkyl substituent on the rate of reaction. Application of the alcohol dissociation model for aliphatic substituents in the alkyl portion of the ester was not totally effective in correlating rates of reaction with pKa of the leaving group because curvature was noted, when substituents with low pKa were employed, in the plot of rate constant versus pKa. In this case the nonlinearity was ascribed to mesomeric stabilization of the anion in the equilibrium process. Besides the nonlinearity in the free energy plots of rate against phenol or alcohol dissociation, there was a dispersion of approximately 5 pKa units between the lines for aliphatic and aromatic leaving groups; this was attributed to the resonance differences observed in the pKa's of alcohols and phenols.