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 CrVI yielded Cr3+ and methyl α-D-glycofuranurono-6,3-lactone as final products when an excess of the methyl glycoside over CrVI was used. The redox reaction occurs through CrVI → CrIV → CrIII and CrVI → CrV → CrIII paths, the CrVI/CrIV reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: −d[CrVI]/dt = kGH[H+]2[Glc1Me][CrVI], where kGH = (6.75 ± 0.05) × 10−3 M−3 s−1; and −d[CrVI]/dt = (kM0 + kMH [H+]2)[Man1Me][CrVI], where kM0 = (5 ± 1) × 10−4 M−1 s−1 and kMH = (6.5 ± 0.2) × 10−3 M−3 s−1, at 40 °C. In acid medium, intermediate CrV reacts with the substrate faster than CrVI 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.