A Search for Carboxyl-Group Catalysis in Ketal Hydrolysis

Abstract

The pH-rate profiles for the hydrolysis of 19 1,3-dioxanes and 1,3-dioxolanes, seven of which were carboxyl substituted, have been determined. The rates of hydrolysis were found not to be sensitive to the concentration of formate and acetate buffers at constant pH showing the compounds not to be susceptible to intermolecular general acid catalyzed hydrolysis. The log of the pseudo-first-order rate constants (kobsd) for the hydrolysis of all dioxolanes and dioxanes not substituted by carboxyl groups when plotted vs. pH provided linear plots of slope ca. – 1.0, indicating the mechanism of hydrolysis is specific acid catalysis. For the carboxyl-substituted dioxolanes and dioxanes, the pH-log kobsd profiles are characterized by the superimposition of a plateau rate upon the specific acid catalyzed region, followed in the low acid region by a descending leg of approximate slope of –1.0. The plateau and second descending leg are found to have kinetically equivalent interpretations as participation by the undissociated carboxyl group in the hydrolysis or the specific acid catalyzed hydrolysis of the carboxylate anion form of the ketals. The latter mechanism has been shown to be correct on the basis that the log krate values for the calculated constants for specific acid catalyzed hydrolysis of the undissociated ketals (K-COOH) and dissociated ketals (K-COO−) exhibit no positive deviations from Hammett plots (σ*) constructed from the rate constants for specific acid catalysis of the hydrolysis of ketals not containing carboxyl groups. Literature reports of the participation of neighboring carboxyl groups in the hydrolysis of acetals are discussed. © 1967, American Chemical Society. All rights reserved.