The relative ability of aldoses and deoxyaldoses to reduce CrVI and CrV. A comparative kinetic and mechanistic study

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 CrVI yields the aldonic acid and Cr3+ as final products when an excess of sugar over CrVI is used. The redox reaction occurs through CrVI→CrIII and CrVI→CrV→CrIII paths. The rate laws for the CrVI and CrV oxidation reactions are expressed by: −d[CrVI]/dt=kH[Ald][CrVI] and −d[CrV]/dt=kbKb[Ald][CrV]/(1+Kb[Ald]), where kH, kb and Kb are the kinetic parameters independent of the [Ald] and [oxidant], and the relative aldoses reactivity with both CrVI and CrV 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. CrV is formed in a rapid step by the reaction of the sugarradical dot radical with CrVI. The substrate is oxidised at a comparable rate by CrVI and CrV, especially at higher [Ald], and the reduction of CrVI occurs concurrently with CrV. 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 CrV species are present at any [H+], whereas hexa-coordinate ones require stronger acid conditions.