Kinetics of Pb(IV) Oxidation of Substituted Methyl Mandelates — A L.F. E. R. Study

Abstract

Lead tetra acetate (Pb(IV) or LTA) oxidation of the esters X -C6H4-CH(OH)-COOCH3(X = H, m-NO2, p-NO2, m-Cl, p-Cl, p-Br. p-CH3 and p-C2H5) gives corresponding keto esters. The reaction is first order in [Pb(IV)] and second order in [Ester], and is catalysed by pyridine and 2,6-lutidine. The pyridine catalysed reaction is first order each in [Pb(IV)]. [Ester] and [Pyridine]. A kinetic isotope effect is observed in oxidation of methyl m andelate for both uncatalysed reaction (kH/kD = 4.2) and pyridine catalysed reaction (kH/kD = 1.8). Activation param eters for both uncatalysed and pyridine catalysed reactions are evaluated. The results are in accord with Linear Free Energy Relationship (L.F.E.R.) and the reaction constant (ϱ = +0.75) obtained supports proton transfer in the ɑ-C-H bond rupture in the rate limiting step. The role pf pyridine in catalysis, as a ligand or as a base is discussed