Kinetics and correlation analysis of reactivity in the oxidation of some α-hydroxy acids by benzimidazolium dichromate

Kinetic and mechanistic studies of the oxidation of mandelic acid and nine monosubstituted mandelic acids by benzimidazolium dichromate (BIDC) in dimethyl sulfoxide are discussed with an emphasis on correlation of structure and reactivity. The reactions were of first order with respect to BIDC. However, Michaelis-Menten type kinetics were observed with respect to hydroxy acids. The reactions are catalysed by protons. The deuterium isotope effect for the oxidation of mandelic acid (k(H)/k(D) = 5.91 at 298 K) indicated an alpha-C-H bond cleavage in the rate-determining step. An analysis of the solvent effect showed that the role of cation-solvation is major. The reaction showed an excellent correlation with the Hammett sigma values, the reaction constant being negative. Based on the kinetic data, analysis of the solvent effect and results of structure-reactivity correlation along with some non-kinetic parameters, a mechanism involving rate-determining oxidative decomposition of the complex through hydride-ion transfer via a cyclic transition state to give the corresponding oxoacid is suggested.