Mechanistic studies of aquachlororhodium(III) catalysis in potassium iodate oxidation of 1,2-propanediol in acidic medium: A kinetic approach

Abstract

The kinetics of homogeneously Rh(III) catalysed oxidation of 1, 2-propanediol by acidic solution of potassium iodate at constant ionic strength of the medium has been investigated at 308 K. The reaction follows complex kinetics, being first order in [KIO₃], zero order in 1, 2-propanediol and inverse fractional order in both [H⁺] and [Cl^¯]. The first order rate constant increases linearly with increase in [Rh(III)] in its lower concentration range but as the concentration of Rh(III) is increased towards the higher range, the rate constant becomes almost constant, indicating positive fractional order in [Rh(III)]. Variation of ionic strength of the medium and successive addition of mercuric acetate (used as I^¯ ions scavenger) has a positive effect on the rate of the reaction. IO₃^¯ and [RhCl₅(H₂O)]^-2 have been postulated as the reactive species of KIO₃ and Rh(III), respectively. Activation parameters for the slow and rate determining step of the proposed mechanism, which involves the formation of the most activated complex, [Cl₅Rh(HO→Hg)]^¯, prior to the slowest step, have been obtained from rate measurements at 30, 35, 40 and 45 °C. The rate law conforming to the observed kinetic results has been derived as <img src='http://www.niscair.res.in/jinfo/IJCA/IJCA 50A(5) 650-657-image.gif'>. The main oxidation products have been identified as acetic acid and formic acid