Reaction of carbohydrates with hydroperoxides : Part II. Oxidation of ketoses with the hydroperoxide anion

Abstract

When treated with an excess of hydrogen peroxide under alkaline conditions, D-fructose and L-sorbose yield approximately four moles of formic acid and one mole of glycolic acid per mole of hexulose. This observation is rationalized by a reaction mechanism consisting of nucleophilic addition of a hydroperoxide anion to the carbonyl form of the ketose, followed by oxidative cleavage of the hydroperoxide adduct to glycolic acid and the next lower aldose. The aldose is subsequently degraded entirely to formic acid by the mechanism described in the preceding paper of this series. Oxidative cleavage of the hydroperoxide adduct of a hexulose could take place by rupture of either the C-1C-2 or the C-2C-3 bond. The reaction products show that, under the conditions used, the oxidation takes place almost entirely with cleavage of the C-2C-3 bond of the parent sugar. The optical rotations of the reaction mixtures approach zero as the reactions proceed, and there is no indication of accumulation of optically active intermediates. In buffered solutions, the rate of reaction increases linearly with the increase in the concentration of alkali peroxide. With a constant amount of hydrogen peroxide and increasing amounts of alkali, the rate increases to a maximum corresponding to approximately one equivalent of alkali per mole of peroxide. The rate then decreases slightly as the pH is increased from 12.3 to 13.9. Lack of a rate increase in this range suggests a free-radical, rather than a base-catalyzed, mechanism for the oxidative cleavage of the adduct.