Kinetics and mechanism of phosphate-catalyzed hydrolysis of benzoate esters: comparison with nucleophilic catalysis by imidazole and o-iodosobenzoate

Abstract

Phosphate-catalyzed hydrolysis of 2,4-dinitrophenyl 4-X-benzoate, and 3- or 4-Y-phenyl 3,5-dinitrobenzoates, where X and Y are substituents, has been studied spectrophotometrically. The following conclusions are based on catalytic rate constants, solvent kinetic isotope effect, detection of a mixed anhydride by FTIR, and application of the Hammett equation: (i) the catalytically active species is HPO₄²⁻, (ii) the mechanism of catalysis is nucleophilic, the reaction proceeds via the irreversible formation of acyl phosphate, (iii) the formation of a tetrahedral species by attack of the HPO₄²⁻ on the ester CO group is rate limiting. Comparison of these results with our previous data on hydrolysis of benzoate esters with imidazole, or o-iodosobenzoate anion showed that: (i) the catalytic efficiency observed, o-iodosobenzoate > imidazole > phosphate, is due to a combination of steric and solvation effects, and the enhancement of the nucleophilicity of the former catalyst by the α-effect, (ii) the nonlinear Brønsted-type plot between log (catalytic constant) and pK_a of the leaving 4-Y-phenol indicates that the rate-limiting step for o-iodosobenzoate changes as a function of changing Y, (iii) the reaction rate-limiting steps are different for the above-mentioned nucleophiles.