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→Cr^V reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: a) ?dCr^VI]/dt {k_{M2 H} H⁺]²+k_MH H⁺]}mannitol]Cr^VI], where k_{M2 H} (6.7±0.3)?10 M s^?1 and k_MH (9±2)?10 M s^?1; b) ?dCr^VI]/dt {k_{G2 H} H⁺]²+k_GH H⁺]}glucitol]Cr^VI], where k_{G2 H} (8.5±0.2)?10 M s^?1 and k_GH (1.8±0.1)?10 M s^?1, at 33°. The slow redox steps are preceded by the formation of a Cr^VI oxy ester with λ_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.