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Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) by D ‐Glucitol and D ‐Mannitol
Author(s) -
Roldán Viviana P.,
Daier Verónica A.,
Goodman Bernard,
Santoro Mabel I.,
González Juan Carlos,
Calisto Nancy,
Signorella Sandra R.,
Sala Luis F.
Publication year - 2000
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20001220)83:12<3211::aid-hlca3211>3.0.co;2-4
Subject(s) - chemistry , redox , chromium , hexacoordinate , mannitol , kinetics , d mannitol , denticity , medicinal chemistry , inorganic chemistry , metal , organic chemistry , physics , quantum mechanics , silicon
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 ) −d[Cr VI ]/d t { 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 ) −d[Cr VI ]/d t { 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.