Comparative study of oxidation by chromium(V) and chromium(VI)

Abstract

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)× 10^–4 dm³ mol^–1 s^–1, d=(3 ± 2)× 10^–4 dm³ mol^–1 s^–1, e=(115 ± 13)× 10^–4 dm⁶ mol^–2 s^–1 and f=(402 ± 17)× 10^–4 dm⁹ mol^–3 s^–1, at 50 °C. Chromium(V) is formed in a rapid step by reaction of the radical dGlc and Cr^VI and C^V reacts with dGlc faster than does Cr^VI. The chromium(V) oxidation of dGlc follows the rate low –dCr^V/dt=(k₁+k₂[H⁺])[dGlc][Cr^V], where k₁= 2.52 × 10^–4 dm³ mol^–1 s^–1 and k₂= 54.0 dm⁶ mol^–2 s^–1, at 25 °C. The EPR spectra show that three 1:1 Cr^V:dGlc intermediate complexes (g₁= 1.9781, g₂= 1.9752, g₃= 1.9758) are formed in rapid pre-equilibria before the redox steps.