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 λ_max =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.

ACKNOWLEDGMENT

We thank the Council of Scientific and Industrial Research, New Delhi, India, for financial support (01/1458/97/EMR-II).