Kinetics and mechanism of the reduction of chromium(VI) and chromium(V) by D-glucitol and D-mannitol

The oxidation of D-glucitol and D-mannitol by Cr(VI) yields the aldonic acid (and/or the aldonolactone) and Cr(III) as final products when an excess of alditol over Cr(VI) is used. The redox reaction occurs through a Cr(VI) -> Cr(V) -> Cr(III) path, the Cr(VI) CrV reduction being the slow redox step. The complete rate laws for the redo,: reactions are expressed by: a) - d[Cr(VI)]/dt=(k(M2H) [H(+)](2) + k(MH) [H(+)]}[mannitol][Cr(VI)], where k(M2H)=(6.7 +/- 0.3). 10(-2) M(-3) s(-1) and k(MH) = (9+/-2) . 10(-3) M(-2) s(-1); b) - d[Cr(VI)]/dt= {k(G2H) [H(+)](2)+ k(GH) [H(+)])[glucitol][Cr(VI)], where k(G2H) = (8.5 . 10(-2)). 10(-2) M(-3) S(-1) and k(GH) = (1.8 +/- 0.1). 10(-2) M(-2) S(-1), at 33 degrees. The slow redox steps are preceded by the formation of a Cr(VI) OXY ester with lambda (max) 371 nm, at pH 4.5. In acid medium, intermediate Cr(V) reacts with the substrate faster than Cr(VI) does. The EPR spectra show that five- and six-coordinate oxo-Cr(V) intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-Cr(V) 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-Cr(V) species are not observed, Cr(V) complexes are stable enough to remain in solution for several days to months.