Kinetics and mechanism of the oxidation of disaccharides by Cr[sup VI]

Rold�n, Viviana; Carlos Gonz�lez, Juan; Santoro, Mabel; Garc�a, Silvia; Casado, Nieves; Olivera, Silvina; Boggio, Juan Carlos; Salas-Peregrin, Juan Manuel; Signorella, Sandra; Sala, Luis F
December 2002
Canadian Journal of Chemistry;Dec2002, Vol. 80 Issue 12, p1676
Academic Journal
The oxidation of D-lactose, D-maltose, D-melibiose, and D-cellobiose by Cr[sup VI] yields the corresponding aldobionic acid and Cr[sup 3+] as final products when an excess of reducing disaccharide over Cr[sup VI] is used. The rate law for the Cr[sup VI] oxidation reaction is expressed by �d[Cr[sup VI] ]/dt = k[sub H] [disaccharide][Cr[sup VI] ], where the second-order kinetic constant, k[sub H] , depends on [H[sup +] ]. The relative reactivity of the disaccharides with Cr[sup VI] is expressed as follows: Mel > Lac > Cel > Mal, at 33�C. In acid medium, intermediate Cr[sup V] forms and reacts with the substrate faster than Cr[sup VI] . The EPR spectra show that five- and six-coordinate oxo-Cr[sup V] intermediates are formed, with the disaccharide acting as bidentate ligand. Five-coordinate oxo-Cr[sup V] species are present at any [H[sup +] ], whereas six-coordinate ones are observed only at pH < 2, where they rapidly decompose to the redox products. In the pH 3�7 range, where hexa-coordinate oxo-Cr[sup V] species are not observed, Cr[sup V] complexes are stable enough to remain in solution from several days to several months.Key words: chromium, saccharides, kinetics, EPR.Lorsqu'on utilise un exc�s de du disaccharide r�ducteur par rapport au Cr(VI), l'oxydation du D-lactose, du D-maltose, du D-melibiose et du D-cellobiose par le Cr(VI) conduit � la formation de l'acide aldobionique correspondant et de Cr[sup 3+] comme produits finaux. La constante de vitesse de la r�action pour la r�action d'oxydation par le Cr(VI) peut �tre exprim�e par l'�quation �d[Cr(VI)]/dt = k[sub H] [disaccharide] [Cr(VI)], dans laquelle la constante cin�tique du deuxi�me ordre, k[sub H] , d�pend de [H[sup +] ]; � 33�C, les r�activit�s relatives des disaccharides vis-�-vis du Cr(VI) sont les suivantes: Mel > Lac > Cel > Mal. En milieu acide, il se forme du Cr(V) comme interm�diaire qui r�agit avec le substrat plus rapidement que le Cr(VI). Les spectres de RPE montrent qu'il se forme des interm�diaires penta- et hexacoordin�s d'oxo-Cr(V) dans lesquels le disaccharide agit comme ligand bidentate. Les esp�ces pentacoordin�es oxo-Cr(V) sont pr�sentes � toutes les [H[sup +] ] alors qu'on n'observe les esp�ces hexacoordin�es qu'� des pH < 2 et que dans ces conditions elles se d�composent rapidement en produits d'oxydor�duction. � des pH allant de 3 � 7 o� on n'observe pas de formation des esp�ces hexacoordin�es d'oxo-Cr(V), les complexes de Cr(V) sont suffisamment stables pour �tre observ�s en solution pour des p�riodes allant de plusieurs jours � plusieurs mois.Mots cl�s : chrome, disaccharides, cin�tique, RPE.[Traduit par la R�daction]


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