Kinetics and mechanism of the reduction of CrVI to CrIII by D-Ribose and 2-deoxi-D-Ribose.

DAIER, VERÓNICA; SANDRA SIGNORELLA,; MARCELA RIZZOTTO,; MARIA INES, FRASCAROLI; CLAUDIA PALOPOLI,; CARLOS BRONDINO,; JUAN MANUEL SALAS PELEGRIN; LUIS SALA,

CANADIAN JOURNAL OF CHEMISTRY

NATL RESEARCH COUNCIL CANADA-N R C RESEARCH PRESS

Lugar: Canada; Año: 1999 vol. 77 p. 57 - 64

0008-4042

The oxidation of D-Ribose and 2-deoxy-D-Ribose by CrVI yields the aldonic acid and Cr3+ as final products when an excess of sugar over CrVI is used. The redox reaction occurs through CrVI®CrIII and CrVI®CrV®CrIII paths. The complete rate laws for the CrVI oxidation reactions are expressed by: -d[CrVI]/dt = kH [H+]2 [Rib][CrVI], where kH = (5.9 ± 0.1) x 10-2 mol-3 dm9 s-1; and -d[CrVI]/dt = (k0 + kH’ [H+]2) [2dRib][CrVI], where k0 = (1.3 ± 0.5) x 10-3 mol-1 dm3 s-1 and kH’ = (4.2 ± 0.1) x 10-2 mol-3 dm9 s-1, at 33°C. An intermediate sugar alkoxide radical could be trapped with DMPO and observed by EPR as a multiline signal at g = 2.003. CrV is formed in a rapid step by reaction of the sugar• radical with CrVI. CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-co-ordinate oxochromate(V) intermediates are formed, and the distribution of these CrV species in the reaction mixture essentially depends on the solution acidity.