MECHANISTIC INVESTIGATIONS OF THE OXIDATION OF SUBSTITUTED PHENETHYL ALCOHOLS BY MANGANESE(III) SULFATE CATALYZED BY RUTHENIUM(III) IN ACID SOLUTION

Abstract

The kinetics of the Ru(III) catalysed oxidation of six derivatives of phenethyl alcohol (X-C6H4-CH2CH2OH where X═H, Cl, Br, CH3, OCH3 or NO2) by manganese(III) sulfate, in sulfuric acid solution at 30°C, were followed spectrophotometrically at λmax =500 nm. The rate shows a first-order dependence on [Mn(III)] and Michaelis-Menten kinetics was observed with the substrate. The rate was of fractional-order in [Ru(III)]. Hammett acidity function (─H0) has been invoked to explain the retardation of the rate by acid. Bunnett plots indicated the absence of water molecule participation in the rate-limiting step. Variation of the ionic strength and the dielectric constant and the addition of the reaction product Mn(II) and anions such as C1−, SO2+ 4, and F− have no effect on the rate. The solvent isotope effect k(H2O)/k(D2O) =1.38 was observed. Proton inventory studies were made in H2O-D2O mixtures. A linear free energy relationship was observed when the results were analyzed by Hammett and Okamoto-Brown equations. Activation parameters were computed. A mechanism consistent with the observed kinetics has been proposed.