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

The oxidation of D-ribose and 2-deoxy-D-ribose by Cr-VI yields the aldonic acid and Cr3+ as final products when an excess of sugar over Cr-VI is used. The redox reaction occurs through Cr-VI-->Cr-III and Cr-VI-->Cr-V-->Cr-III paths. The complete rate laws for the Cr-VI oxidation reactions are expressed by -d[Cr-VI]/dt = k(H)[H+](2) [ribose][Cr-VI], where k(H) = (5.9 +/- 0.1) x 10(-2) mol(-3) dm(9) s(-1), and -d[Cr-VI]/dt = (k(0) + k(H)'[H+](2)) [2-deoxyribose] [Cr-VI], where k(0) = (1.3 +/- 0.5) x 10(-3) mol(-1) dm(3) s(-1) and k(H)' = (4.2 +/- 0.1) x 10(-2) mol(-3) dm(9) s(-1) at 33 degrees C. An intermediate sugar alkoxide radical could be trapped with DMPO and observed by EPR as a multiline signal at g = 2.003. Cr-V is formed in a rapid step by reaction of the sugar radical with Cr-VI. Cr-V reacts with the substrate faster than Cr-VI does. The EPR spectra show that five- and six-coordinate oxochromate(V) intermediates are formed, and the distribution of these Cr-V species in the reaction mixture essentially depends on the solution acidity.