Mechanistic studies of oxidation of maltose and lactose by [H 2OBr] + in presence of chloro-complex of Rh(III) as homogeneous catalyst

Abstract

Kinetic studies in homogeneously Rh(III)-catalyzed oxidation of reducing sugars, i.e. maltose and lactose, by N-bromoacetamide (NBA) in the presence of perchloric acid have been made at 40 °C using mercuric acetate as Br − ion scavenger. The results obtained for the oxidation of both reducing sugars show first-order dependence of the reactions on NBA at its low concentrations, which shifts towards zero-order at its higher concentrations. First-order kinetics in [Rh(III)] and zero-order kinetics in [reducing sugar] were observed. Positive effect of [Cl −] was observed in the oxidation of both maltose and lactose. Order of reaction was found to be one and half (1.5) throughout the variation of [H +] in the oxidation of both maltose and lactose. An increase in the rate of reaction with the decrease in [Hg(OAc) 2] and [NHA] was observed for both the redox systems. The rate of oxidation is unaffected by the change in ionic strength ( μ) of the medium. The main oxidation products of the reactions were identified as formic acid and arabinonic acid in the case of maltose and formic acid, arabinonic acid and lyxonic acid in the case of lactose. A common mechanism for the oxidation of both maltose and lactose, showing the formation of most reactive activated complex, [RhCl 4(H 3O)H 2OBr] +, and an unreactive complex, [RhCl 4(H 2O)(H 2OBrHg)] 2+, has been proposed. Various activation parameters have also been calculated and on the basis of these parameters, a suitable explanation for the reaction mechanism has been given.