Kinetics of Ru(III) catalysed oxidation of phenethyl alcohol and <i>para-</i>substituted phenethyl alcohols by Ce(IV) in aqueous nitric acid medium

Abstract

A simple, rapid and sensitive spectroscopic method for the

kinetics of Ru(III) catalysed oxidation of phenthyl alcohol(PA) by

Ce(IV) in aqueous nitric acid medium has been reported under

varying conditions. The reaction is followed spectrophotometrically

by measuring the decrease in absorbance of Ce(IV) at 350 nm. The rates show first order dependence on [Ce(IV)] and the rate constants evaluated at different [Ce(IV)] are found to be almost the same. Increase in [Ru(III)] has linear relation with the rate of oxidation and order in [Ru(III)] has been found to be

fractional. Variation in ionic strength of the medium has significant effect on the rate of reaction. The rates of the reaction have been measured at different temperatures and the activation parameters for all the substrates computed. The rates decrease in the order – OCH₃> -CH₃> -H>-Cl>-NO₂ of <i>para </i>substituted

phenethyl alcohols. Hammett's plot of log <i>k</i>_obs versus σ is found to be valid. The correlation between enthalapies and free energies of activation is reasonably linear with an isokinetic temperature of 425K.The rate constant <i>k </i>obeys corresponding equation, <i>k=Q.</i>e<i> ^–</i>^{Δ<i>E#/RT</i>}<i>. </i>e^{Δs#<i>/R</i>}. <i>Ea </i>increases with introduction of electron-withdrawing groups into the benzene ring. The introduction of electron-releasing groups lowers the <i>Ea </i>for the reaction. Similarly, log<i>A</i> decreases with substitution of electron withdrawing groups and increases with substitution of electron releasing groups. A plausible mechanism consistent with the experimental results has been proposed.