Kinetics and mechanism of the reduction of CrVI to CrIII by D-ribose and 2-deoxy-D-ribose

Abstract

The oxidation of D-ribose and 2-deoxy-D-ribose by CrVI yields the aldonic acid and Cr3+ as final products when an excess of sugar over CrVI is used. The redox reaction occurs through CrVI–>CrIII and CrVI–>CrV–>CrIII paths. The complete rate laws for the CrVI oxidation reactions are expressed by -d[CrVI]/dt = kH[H+]2 [ribose][CrVI], where kH = (5.9 ± 0.1) × 10-2 mol-3 dm9 s-1, and -d[CrVI]/dt = (k0 + kH'[H+]2) [2-deoxyribose][CrVI], where k0 = (1.3 ± 0.5) × 10-3 mol-1 dm3 s-1 and kH' = (4.2 ± 0.1) × 10-2 mol-3 dm9 s-1, at 33°C. An intermediate sugar alkoxide radical could be trapped with DMPO and observed by EPR as a multiline signal at g = 2.003. CrV is formed in a rapid step by reaction of the sugar radical with CrVI. CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxochromate(V) intermediates are formed, and the distribution of these CrV species in the reaction mixture essentially depends on the solution acidity.Key words: ribose, 2-deoxyribose, chromium, redox, mechanism, kinetics.