Chromic oxidation of 2-deoxy-D-glucose .2. Comparative study of oxidation by chromium(V) and chromium(IV)

The kinetics and mechanism of the oxidation of 2-deoxy-D-glucose (dGlc) by Cr-VI which yields 2-deoxy-D-gluconic acid and Cr-III as final products when a ten-fold or higher excess of sugar over Cr-VI is used, have been studied. The redox reactions occur through Cr-VI --> Cr-IV --> Cr-III and Cr-VI --> Cr-V --> Cr-III paths. The experimental data were fitted with a multilinear regression program. The complete rate law for the chromium(VI) oxidation reaction is expressed by -d[Cr-VI]/dt = {c[H+] + (d + e[H+] + f[H+](2)) [dGlc]}[Cr-VI], where c = (5 +/- 1) x 10(-4) dm(3) mol(-1) s(-1), d = (3 +/- 2) x 10(-4) dm(3) mol(-1) s(-1), e = (115 +/- 13) x 10(-4) dm(6) mol(-2) s(-1) and f = (402 +/- 17) x 10(-4) dm(9) mol(-3) s(-1), at 50 degrees C. Chromium(V) is formed in a rapid step by reaction of the radical dGlc and Cr-VI and Cr-V reacts with dGlc faster than does Cr-VI. The chromium(V) oxidation of dGlc follows the rate law -dCr(V)/dt = (k(1) + k(2)[H+])[dGlc][Cr-V], where k(1) = 2.52 x 10(-4) dm(3) mol(-1) s(-1) and k(2) = 54.0 dm(6) mol(-2) s(-1), at 25 degrees C. The EPR spectra show that three 1:1 Cr-V:dGlc intermediate complexes (g(1) = 1.9781, g(2) = 1.9752, g(3) = 1.9758) are formed in rapid pre-equilibria before the redox steps.