Kinetics and Mechanism of the Oxidation of Deoxybenzoins by Cobalt (III) Acetate in Acetic Acid

Abstract

The mechanism of the oxidation of deoxybenzoins by cobalt(III) acetate has been studied in acetic acid under nitrogen through product analysis and kinetic measurement. The main products were benzoin acetate and benzil. The ratio of the two products increased with an increase in the ratio of the initial concentration of cobalt(III) acetate to that of the substrate. The rate of oxidation of deoxybenzoins was of second order in cobalt(III) acetate, of inverse first-order in cobaltan acetate and of first order in the substrate. It increased on addition of sodium acetate. The Hammett plots for substituents in benzoyl moiety gave straight lines in the presence (p= +1.03) and absence (p=-0.60) of sodium acetate, while both electron-releasing and -withdrawing substituents in benzyl moiety accelerated the reaction in both media. Oxidation of benzoin, which was an intermediate to benzil, was also studied under similar conditions. The rate was of first-order in both benzoin and cobalt(III) acetate and was independent of cobalt(II) acetate. These facts are explained in terms of competitive reactions between metal-assisted enolization of ketone and direct electron transfer from benzene ring in benzyl moiety followed by the departure of a benzylic proton.