Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) byD-Glucitol andD-Mannitol

Abstract

The oxidation of D-glucitol and D-mannitol by CrVI yields the aldonic acid (and/or the aldonolactone) and CrIII as final products when an excess of alditol over CrVI is used. The redox reaction occurs through a CrVI→CrV→CrIII path, the CrVI→CrV reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: a) −d[CrVI]/dt {kM2 H [H+]2+kMH [H+]}[mannitol][CrVI], where kM2 H (6.7±0.3)⋅10 M s−1 and kMH (9±2)⋅10 M s−1; b) −d[CrVI]/dt {kG2 H [H+]2+kGH [H+]}[glucitol][CrVI], where kG2 H (8.5±0.2)⋅10 M s−1 and kGH (1.8±0.1)⋅10 M s−1, at 33°. The slow redox steps are preceded by the formation of a CrVI oxy ester with λmax 371 nm, at pH 4.5. In acid medium, intermediate CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxo-CrV intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-CrV species are present at any [H+], whereas hexacoordinate ones are observed only at pH<2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-CrV species are not observed, CrV complexes are stable enough to remain in solution for several days to months.