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

The oxidation reactions kinetics of a series of related saccharides by aqua-oxo chromium(IV) ion, (H2O)(5)(CrO2+)-O-IV, 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 0.1-1.0 M. Relative reactivities of these saccharides toward CrO2+ reduction are 1-methyl-alpha-D-glucopyranose << 1-methyl-alpha-D-galactopyranose similar to 3-O-methyl-D-glucopyranose similar to 6-desoxi-L-galactopyranose similar to 2-desoxi-D-glucopyranose similar to 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. (c) 2012 Elsevier Ltd. All rights reserved.