3-ELECTRON OXIDATIONS .16. CHROMIC-ACID OXIDATION OF MANDELIC-ACID

The chromic acid oxidation of mandelic acid (MA) obeys the rate law-d[Cr(VI)]/dt=[HCrO4-] [MA](k1+ k2[H+] + k3[MA]). The reactions corresponding to the two first-order kinetic terms (k, and k2) yield benzaldehyde, benzoic acid, and carbon dioxide in a 1:1:2 ratio, while the second-order reaction (k3) produces a benzaldehyde, phenylglyoxylic acid, and carbon dioxide in a 2:1:2 ratio. Neither benzaldehyde nor phenylglyoxylic acid is intermediate in the formation of benzoic acid. Both the first-order and the second-order reactions are three-electron oxidations. It is proposed that in the first-order reaction mandelic acid is oxidized to carbon dioxide and a benzoyl radical. This reaction represents the first example of an intramolecular three-electron oxidation in which a carbon hydrogen and a carbon-carbon bond to the same carbon atom are broken simultaneously in the rate-limiting step. © 1979, American Chemical Society. All rights reserved.