Oxidation of D-glucose in the presence of 2,2′-bipyridine by CrVI in aqueous micellar media:: a kinetic study

The kinetics of Cr-VI oxidation Of D-glucose to the corresponding lactone in the presence and absence of 2,2'-bipyridine (bipy) has been carried out under the conditions, [D-glucose](T) >> [Cr-VI](T) at different temperatures in aqueous micellar media. The monomeric Cr-VI species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the Cr-VI-bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the Cr-VI-bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (through two-electron transfer) in the rate-determining step leading to the product lactone and Cr-IV-bipy complex. The Cr-IV-bipy complex then takes part in faster steps in the further oxidation Of D-glucose and is ultimately converted into a Cr-III bipy complex. In the uncatalysed path, the Cr-VI-substrate ester experiences acid catalysed redox decomposition (two-electron transfer) in the rate-determining step. The uncatalysed path shows a second order dependence on [H+] and a first order dependence on each of the reactants [D-glucose](T) and [Cr-VI](T). In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H+], [D-glucose](T) and [Cr-VI](T). The bipy-catalysed path is first order in [bipy](T). These observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecyl sulfate (SDS) on both the uncatalysed and bipy-catalysed path has been studied. CPC inhibits both the uncatalysed and bipy-catalysed path, while SDS catalyses these reactions. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the surfactants and reactants. (C) 2005 Elsevier Ltd. All rights reserved.