Oxidation of carbohydrates of biological importance by the aquachromium(IV) ion

Abstract

The oxidation reactions kinetics of a series of related saccharides by aqua-oxo chromium(IV) ion, (H2O)5CrIVO2+, were carried out in perchloric acid aqueous solutions. These reactions yield superoxochromium(III) ion, CrO22+, providing evidence that the two-electron reduction of CrO2+ to Cr2+ occurred in a single step. In all of these reactions, Cr2+ is the immediate product and could be trapped as CrO22+ when an excess of oxygen was present. The bimolecular rate constants for different aldoses and d-glucitol are independent of [H+] in the range 0.1–1.0M. Relative reactivities of these saccharides toward CrO2+ reduction are 1-methyl-α-d-glucopyranose<<1-methyl-α-d-galactopyranose∼3-O-methyl-d-glucopyranose∼6-desoxi-l-galactopyranose∼2-desoxi-d-glucopyranose∼d-glucopyranose<<d-galactopyranose<<d-glucitol. The oxidation of aldonic acid such as d-gluconic by CrO2+ showed the same mechanism but the redox process is strongly inhibited when [H+] increases. Activation parameters were also determined for selected reactions. On the basis of the kinetic result, activation parameters data and oxidized organic products, the mechanism of saccharides oxidation by CrO2+ is proposed to be a direct hydride-ion transfer.