Kinetics and mechanism of the reduction of dodecatungstocobaltate (III) by D-fructose, D-glucose, and D-mannose: comparison between keto- and aldohexoses

Abstract

The kinetics of reduction of dodecatungstocobaltate (III) by D-fructose, D-glucose, and D-mannose in aqueous media have been investigated. A pseudo-zero-order rate was obtained with the ketose and the reaction was acid-dependent. On the contrary, pseudo-second-order rates were obtained for the aldose reactions. The reactions were catalysed by metal ions in the supporting electrolyte; the cation catalytic order K⁺ > Na⁺ Li⁺ observed for aldose oxidation is explained on the basis of electrostatic considerations. A mechanism for the ketose oxidation is offered based on keto–enol tautomerism; the aldose oxidations are explained in terms of a transition state formed via ion-induced dipole bridging.