Studies to Determine the Transition Structure in Aspirin Hydrolysis.

Rate constants for hydrolyses of 2-(dichloroacetoxy)benzoic acid and its four ring-substituted methyl analogs in acidic media at 25(DEGREES)C are given. Rate constants are also given for solvolyses in deuterium oxide an 0.5 deuterium mole fraction solutions of water and deuterium oxide. Solvent isotope effects calculated from these rate constants are given and proton inventory analyses for some of the substrates are presented. The data support a previously proposed hydrolysis mechanism in which carboxylate is participating in the reaction. Rate constants for hydrolyses of 2-(acetoxy)benzoic acid (aspirin), 2-{1-(\u2713)C}(acetoxy)benzoic acid, and 2-{2-(\u272)H(,3)}(acetoxy)benzoic acid, measured in buffered solutions at pH 5.3 between 25(DEGREES) and 39(DEGREES)C, are given. Calculated isotope effects and Arrhenius and Eyring plots are shown. The data indicate that there is no change in mechanism or rate-determining step within this temperature range and that heavy atom rearrangement of the ester is a significant feature of the rate-determining step. The results of BEBOVIBIV and ab initio calculations for a series of transition-structure models are given and indicate that the transition state is somewhat early and that no proton is in mid transfer