Oxidation of Substituted Benzyl Alcohols by Quinoxalinium Dichromate. A Kinetic Study

Quinoxalinium dichromate (QxDC) oxidizes benzyl alcohol and substituted benzyl alcohols smoothly in dimethyl sulfoxide (DMSO) and in the presence of acid to the corresponding aldehydes. The reaction has unit dependence on each of the alcohol, QxDC, and acid concentrations. Electron-releasing substituents accelerate the reaction, whereas electron-withdrawing groups retard the reaction, and the rate data obey Hammetts relationship. The reaction constant was –1.09±0.01 at 303K. Oxidation of ,-dideuteriobenzyl alcohol indicated the presence of a substantial primary kinetic isotope effect (k_H/k_D=6.78 at 303K). The reaction failed to induce the polymerization of acrylonitrile. The rates of oxidation were determined at different temperatures and the activation parameters were evaluated. The analysis of the dependence of the kinetic isotope effect on temperature indicated that the reaction involves a symmetrical cyclic transition state. A suitable mechanism is proposed.