The mechanism of oxidation of α-glycols by periodic acid. Part VI. Oxidation of pinacol at pH 0–6

The oxidation of pinacol by periodic acid has a second-order kinetic form in the region pH 0–6, with a rate maximum at pH 1. The variation of rate with pH is consistent with a mechanism involving the formation of a pinacol–periodate monoester which cyclises to a diester which decomposes to the reaction products. At pH > 2 the reactants and monoester are in equilibrium, but at pH < 2 the formation of the monoester becomes slow. A large kinetic salt effect observed in the range pH 2–4 is probably caused by the salt effect on the dissociation of periodic acid. The reaction rate in the region of the maximum is about 30% slower in D2O than in H2O. There is no appreciable deuterium isotope effect on the apparent first dissociation of periodic acid.