The relative ability of aldoses and deoxyaldoses to reduce Cr VI and Cr V. A comparative kinetic and mechanistic study

Abstract

Oxidation of the aldoses (Ald) d-glucose, d-allose, d-mannose, d-galactose, 6-deoxy- d-galactose ( d-fucose) and 2,6-dideoxy- d- ribo-hexose (digitoxose) by Cr VI yields the aldonic acid and Cr 3+ 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 rate laws for the Cr VI and Cr V oxidation reactions are expressed by: −d[Cr VI]/d t= k H[Ald][Cr VI] and −d[Cr V]/d t= k b K b[Ald][Cr V]/(1+ K b[Ald]), where k H, k b and K b are the kinetic parameters independent of the [Ald] and [oxidant], and the relative aldoses reactivity with both Cr VI and Cr V is 2,6-dideoxy- d- ribo-hexose> d-galactose> d-allose> d-glucose≈6-deoxy- d-galactose, at 33 °C. Intermediate sugar alkoxide radicals could be trapped with 5,5-demethyl-1-pyrroline N-oxide (DMPO) and observed by electron paramagnetic resonance (EPR) as a multiline signal at g=2.003. Cr V is formed in a rapid step by the reaction of the sugar radical with Cr VI. The substrate is oxidised at a comparable rate by Cr VI and Cr V, especially at higher [Ald], and the reduction of Cr VI occurs concurrently with Cr V. The EPR spectra show that five- and six-coordinate oxochromate(V) intermediates are formed, with the aldose and/or the aldonic acid acting as bidentate ligands. Penta-coordinate Cr V species are present at any [H +], whereas hexa-coordinate ones require stronger acid conditions.