Kinetics and mechanism of the chromium(VI) oxidation of methyl α-D-glucopyranoside and methyl α-D-mannopyranoside

Abstract

The oxidation of methyl α-D-glucopyranoside (Glc1Me) and methyl α-D-mannopyranoside (Man1Me) by Cr^VI yielded Cr³⁺ and methyl α-D-glycofuranurono-6,3-lactone as final products when an excess of the methyl glycoside over Cr^VI was used. The redox reaction occurs through Cr^VI → Cr^IV → Cr^III and Cr^VI → Cr^V → Cr^III paths, the Cr^VI/Cr^IV reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: −d[Cr^VI]/dt = k_GH[H⁺]²[Glc1Me][Cr^VI], where k_GH = (6.75 ± 0.05) × 10⁻³ M⁻³ s⁻¹; and −d[Cr^VI]/dt = (k_M0 + k_MH [H⁺]²)[Man1Me][Cr^VI], where k_M0 = (5 ± 1) × 10⁻⁴ M⁻¹ s⁻¹ and k_MH = (6.5 ± 0.2) × 10⁻³ M⁻³ s⁻¹, at 40 °C. In acid medium, intermediate Cr^V reacts with the substrate faster than Cr^VI does. Chromium(V) mono- and bis-chelates were detected by EPR spectroscopy, and the EPR spectra show that the distribution of these species essentially depends on the solution acidity.