Kinetics of Pb(IV) Oxidation of Substituted Methyl Mandelates

Lead tetra acetate (Pb(IV) or LTA) oxidation of the esters X —C,,H4—C H(OH ) — COO CH, (X = H, m-NO: , p-N O : , /?;-Cl. /?-Cl. p-Br. p-CH, and p-C:H s) 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 mandelate 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 (o = +0.75) obtained supports proton transfer in the a -C —H bond rupture in in catalysis, as a ligand or as a base is discussed.