Kinetics of oxidation of d(+)melibiose and cellobiose by N-bromoacetamide using a rhodium(III) chloride catalyst

Abstract

The kinetics of oxidation of the sugars d(+)Melibiose (mel) and Cellobiose (cel) by N-bromoacetamide (NBA) in the presence of Rh(III) chloride as homogeneous catalyst in acidic medium at 45 °C have been investigated. The reactions are first-order with respect to [NBA], [Rh(III)] and [substrate]. The rate is proportional to [H+]. No effects of [Hg(II)], [NHA] or [Cl−] on the rates were observed. Ionic strength and dielectric constant also have little effect. The observed kinetic data, available literature and spectroscopic evidence lead us to conclude that NBAH+ and [RhCl5(H2O)]2− are the reactive species of NBA and Rh(III) chloride, respectively. The rate-determining step of the proposed reaction path common for both sugars gives an activated complex by the interaction of a charged complex species and neutral sugar molecule, which in the subsequent steps disproportionates into the reaction products with the regeneration of catalyst. The reactions have been studied at four different temperatures and with the help of first-order rate constant values, various activation parameters have been calculated. The main oxidation products of the reactions were identified as arabinonic acid, formic acid and lyxonic acid in the case of mel and arabinonic acid and formic acid in the case of cel.