Mechanistic studies of oxidation of d-arabinose and d-mannose by acidic solution of N-bromoacetamide in presence of chloro-complex of Ru(III) as homogeneous catalyst

The present paper deals with the kinetics and mechanism of Ru(III)-catalyzed and Hg(II)-co-catalyzed oxidation of d-arabinose(ara) and d-mannose(man) by acidic solution of N-bromoacetamide(NBA). First-order dependence of the reaction on both NBA and sugar at low concentrations shifts to zero-order at their higher concentrations. First-order kinetics with respect to [Ru(III)] throughout its variation was observed in the oxidation of both arabinose and mannose. A positive effect on the oxidation rate is observed for [Hg(II)] and [Cl−] whereas a negative effect is observed for [H+] and [acetamide]. Ionic strength does not influence the oxidation rate. The decrease in the rate of reaction with the increase in dielectric constant (D) of the medium was observed in the oxidation of both reducing sugars and the kinetic data thus obtained were utilized to calculate dAB, the size of activated complex. Various activation parameters have also been calculated. Formic acid and erythronic acid for arabinose and formic acid and arabinonic acid for mannose were identified as main oxidation products of the reactions. Kinetic and equivalence studies together with spectral information, observed effect of dielectric constant on the rate of reaction, activation parameters and multiple regression analysis were made a basis for the formation of a common reaction mechanism for the catalyzed oxidation of both, arabinose and mannose.

Kinetics and mechanism of Ru(III)-catalyzed oxidation of d-arabinose and d-mannose by acidic solution of N-bromoacetamide have been studied at 40 °C. HOBr as the reactive species of NBA and RuCl3(H2O)2OH]− as the reactive species of Ru(III) chloride in acidic medium have been proposed for the oxidation of aforesaid reducing sugars. The size of the activated complex (dAB) and various activation parameters have been calculated. On the basis of spectrophotometric and kinetic analysis of the data, a most probable reaction scheme, common for both arabinose and mannose, has been proposed. The formation of most unstable activated complex, , by the interaction of two oppositely charged species, i.e. [Hg ← OBr]+and [RuCl3·S·(H2O) OH]− is supported by the observed positive entropy of activation.Figure optionsDownload as PowerPoint slide