N-Bromosuccinimide oxidation of maltose and d-galactose using chloro-complex of Rh(III) in its nano-concentration range as homogeneous catalyst: A kinetic and mechanistic study

Abstract

Kinetics of oxidation of maltose (mal) and d-galactose (gal) by protonated N-bromosuccinimide (N +BSH) using chloro-complex of Rh(III) in its nano-concentration range as homogeneous catalyst have been investigated at 40 °C for the first time. Almost constant values of pseudo-first-order rate constant ( k 1) throughout the variation of N-bromosuccinimide (NBS) in the oxidation of both the reducing sugars clearly demonstrate that order of reaction with respect to [NBS] is unity. First-order kinetics with respect to each [Rh(III)], [Sugar] and [H +] is evident from the observed values of k 1 which increase in the same proportion in which the concentration of each reactant is increased. Negligible effects of variations of [Hg(II)], [Cl −] and [succinimide] on the rate of oxidation of each reducing sugar have been observed. Variations in ionic strength ( μ) and dielectric constant ( D) of the medium have not influenced the oxidation rates. Protonated N-bromosuccinimide, N +BSH, and chloro-complex of Rh(III), [RhCl 5(H 2O)] 2−, have been postulated as the reactive species of NBS and Rh(III) chloride in acidic medium, respectively. Various activation parameters have been calculated using pseudo-first-order rate constant ( k 1) values observed at four different temperatures. The proposed mechanism, involving most reactive activated complex formed as a result of interaction between the complex species [RhCl 5·NHBr] − and a sugar molecule is supported by kinetic orders, spectrophotometric evidence, positive entropy of activation and observed zero effect of dielectric constant and ionic strength of the medium. Almost constant values of composite rate constant ( k′) observed for the variations of [sugar], [NBS], [Rh(III)] and [H +] in the oxidation of each reducing sugar provide further support to the proposed reaction path. The main oxidation products of the reactions were identified as arabinonic acid and formic acid in case of maltose and lyxonic acid and formic acid in case of d-galactose.