Rh(III)-CATALYSED OXIDATION OF D(+) TREHALOSE BY N-BROMOACETAMIDE IN ACIDIC MEDIUM. A KINETIC AND MECHANISTIC STUDY

摘要

The kinetics of oxidation of D(+) trehalose by N-bromoacetamide in the presence of chloro-complex of Rh(III) as homogeneous catalyst in acidic medium at 45°C has been investigated. Order of reaction with respect to [NBA], [Rh(III)] and [substrate] was found to be unity throughout their variations. The linear dependence of the reaction rate with [H~+] was observed. Variation in [Hg(II)], [NHA] and [Cl~-] did not affect the oxidation rate of D(+) trehalose. Negligible effects of ionic strength (μ) and dielectric constant (D) of the medium on the rate of oxidation of D(+) trehalose have been observed. NBAH~+ and [RhCl5(H2O)]~(2-) were postulated as the reactive species of N-bromoacetamide and Rh(III) chokide in acidic medium, respectively. The main products of the oxidation are the corresponding arabinonic and formic acids. A mechanism, consistent with the kinetic data and supported by the observed effect of ionic strength, dielectric constant of the medium and spectrophotometric evidence, has been proposed. The rate-determining step involves the formation of most reactive activated complex formed by the interaction of a charged complex species and a neutral sugar molecule, which in the subsequent steps disproportionates into the reaction products with the regeneration of catalyst. The reaction has also been studied at 4 different temperatures and with the help of observed first order rate constant (A:,) values, various activation parameters have been computed for the oxidation of D(+) trehalose.