Three-electron oxidations. X. Cooxidation of isopropyl alcohol and glycolic acid

The chromic acid oxidation of a mixture of glycolic acid and isopropyl alcohol is a fast cooxidation process yielding acetone and gloxylic acid in an approximately 2:1 ratio. The mechanism consists in the formation of a termolecular complex of chromium(VI) with glycolic acid and isopropyl alcohol, which decomposes in a rate-limiting step in a three-electron oxidation--reduction process to a molecule of acetone, chromium(III), and a free radical intermediate HOCHCO2H which is subsequently oxidized to glyoxylic acid. The substantial isotope effect observed for both substrates (HOCH2CO2H--(CH3)2CDOH, 6.0; HOCD2CO2H--(CH3)2CHOH, 5.8; HOCD2CO2H--(CH3)2CDOH, 34.4) provides clear evidence for the synchronous breaking of two different C--H bonds in the rate-limiting step. It also represents convincing proof for the proposed three-electron oxidation mechanism. (U.S.)