KINETICS AND MECHANISM OF THE OXIDATION OF (+/-)-2-HYDROXY-3-METHYL BUTANOIC ACID BY CHROMIUM(VI) IN PERCHLORIC-ACID MEDIUM

The oxidation of 2-hydroxy-3-methylbutanoic acid by chromium(VI) in perchloric acid follows the rate law: -d[Cr(VI)]/dt = (k0 + k(H)h0) [S] [HCrO4-]; where [S] represents 2-hydroxy-3-methylbutanoic acid, k0 = 0.0359 M-1 s-1 and k(H) = 0.0183 M-2 s-1. This rate law corresponds to the formation of 2-methylpropanoic acid and carbon dioxide by C-C cleavage when a 10-fold or higher excess of acid over chromium(VI) is employed. The reaction proceeds through a 1:1 acid/chromium(VI) complex, which decomposes by a three-electron step to the products. Chromium(V), formed in subsequent steps, decays at least four times faster than chromium(VI), as observed by EPR and visible spectrophotometry. When an excess of chromium(VI) over the substrate is employed, further C-C cleavage takes place, and the final degradation products are acetone and carbon dioxide.