Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) by D‐Glucitol and D‐Mannitol |
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Authors: | Viviana P. Roldá n,Veró nica A. Daier,Bernard Goodman,Mabel I. Santoro,Juan Carlos Gonzá lez,Nancy Calisto,Sandra R. Signorella,Luis F. Sala |
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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. |
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