Ion association effects on rates of alkaline hydrolysis of acetylsalicylate and acetylmandelate ions

Abstract

Salt effects on rates of alkaline hydrolysis of acetylsalicylate and acetylmandelate ions have been studied in water at 35°. The rate for a particular ionic strength is dependent on the nature of the cation and not on the anion. The rate-enhancing effects of cations are found to lie in the order Ba⁺⁺ Li⁺ > Na⁺ K⁺ > Me₄N⁺. In the presence of barium and lithium ions, the individual rate constants for the reactions between the two free ions and between the metal–hydroxide ion-pair and the free ester ion have been calculated from the observed rate constants, by use of the known values of the respective association constants. The catalytic action of cations can be explained on the basis that the more strongly a cation is bound with the ester ion and hydroxyl ion, the more stabilized will be the activated chelate complex, and hence the faster the reaction in the presence of that cation.