Kinetics and mechanism of the oxidation of disaccharides by Cr^VI

Abstract

; à 33°C, les réactivités relatives des disaccharides vis-à-vis du Cr(VI) sont les suivantes: Mel > Lac > Cel > Mal. En milieu acide, il se forme du Cr(V) comme intermédiaire qui réagit avec le substrat plus rapidement que le Cr(VI). Les spectres de RPE montrent qu'il se forme des intermédiaires penta-et hexacoordinés d'oxo-Cr(V) dans lesquels le disaccharide agit comme ligand bidentate. Les espèces pentacoordinées oxo-Cr(V) sont présentes à toutes les [H + ] alors qu'on n'observe les espèces hexacoordinées qu'à des pH < 2 et que dans ces conditions elles se décomposent rapidement en produits d'oxydoréduction. À des pH allant de 3 à 7 où on n'observe pas de formation des espèces hexacoordinées d'oxo-Cr(V), les complexes de Cr(V) sont suffisamment stables pour être observés en solution pour des périodes allant de plusieurs jours à plusieurs mois.

Evidence for the involvement of CrII and free radicals as intermediates in the reduction of by saccharides, alcohols and hydroxyacids

“…However, this mechanism does not explain the polymerization induced by free radicals observed in the oxidation of a number of substrates by Cr VI [9][10][11][12][13][14][15][16][17][18][19][20], which implies that a pathway involving a one-electron step must also occur. Recently, based on the observation of CrO 2þ 2 and the successful trap of organic radicals using acrylonitrile, a combination of the Cr VI !…”

Kinetics and mechanism of the reduction of CrVI and CrV by d-lactobionic acid

“…Chelates V or VI, with lba bound to Cr VI as a tridentate ligand, with one alcoholate group of the gal moiety as the third O-donor site of the ligand, could also form and behave as precursor of the slow electron transfer steps. In the case of lactose, this additional coordination site was found to have an stabilizing effect on the Cr VI intermediate resulting in a redox rate slower than for glucose [26] step 9, k 2 , is 5-times slower than found for glca, could be attributed to the gal stabilization effect. After the slow redox step, Cr IV is predicted to react with excess lba to yield Cr III and an lba radical or Cr II and 2klba through two alternative fast steps (Eqs.…”

“…4-O-b-D D -galactopyranosyl-D D -gluconic acid, D D -lactobionic acid (lba), is obtained from the stoichiometric oxidation of D D -lactose by Pseudomonas graveolens [25], and is employed as complexing agent of metal ions in the industry of foods and pharmacist. This acid saccharide was found in cow milk polluted with Cr VI , and Cr V -lba species have been detected as redox intermediates in lactose/Cr VI reaction mixtures [26,27]. In this work, we study the Cr VI oxidation of lba and provide information on the mechanism of this reaction as well as on the structure of Cr V species formed as redox intermediates. )…”

Quinic acid and hypervalent chromium: a spectroscopic and kinetic study

“…A similar situation has arisen with Cr IV . In the course of the examination of the interactions of aldohexoses [29][30][31][32][33][34] and carboxylic acids 26,[35][36][37] with Cr VI , we were able to demonstrate the interactions of Cr VI , Cr V and Cr IV with different sugars present in biological systems. perform intramolecular competition experiments between different functional groups (vic-diol versus tert-2-hydroxy acid) or different orientations of the same functional group (trans versus cis diol).…”