Oxovanadium(IV)-catalysed and uncatalysed acid bromate oxidation of mandelic acid

The oxidation of mandelic acid (MA) by bromate in acidic aqueous solutions exhibits first order each in [Br(V)], [substrate] and second order in [H2SO 4]. The reaction exhibits an inverse solvent effect (k(D2O)/k(H2O) = 0.66 a t 313K) and no primary isotope effect. The mechanism consists of rate deter mining esterfication of mandelic acid with H2Br+O3, followed by a fast disp roportionation of the ester through C-C bond cleavage to form benzaldehyde. Oxovanadium(IV)-catalysed Br(V) oxidation of the substrate in perchloric a cid medium exhibits first order each in [MA], [VOSO4] and is independent of [oxidant]. The hydride ion-abstration mechanism is proposed to account for the oxovanadium(IV)-catalysed reaction. The rate law is given by -d[Br(V)] /dt=k[MA][V(IV)]=k(o). The activation parameters of catalysed and uncatalys ed reactions are evaluated and discussed.