Chromic acid oxidation of benzhydrols-kinetics and mechanism

Abstract

The kinetics of the oxidation of benzhydrol (diphenylmethanol) and several of its substituents by chromium(VI) oxide in aqueous acetic acid have been studied in detail. The rate of oxidation is expressed as

                      v = k₃[ArAr'CHOH][Cr^VI][H⁺]²

The effect of ring substituents on the rate gives a p⁺ value of -0.546 (r = 0.959) pointing to the development of an electron-deficient centre in the transition state. The oxidation of benzhydrol[α-D] proceeds 6.5 times slower than that of its protio analogue at 55°C. From the Arrhenius plots, activation parameters have beep evaluated and the isokinetic relationship has been analysed. The results are discussed in terms of a mechanism involving a rate-determining loss of hydride ion from the α-carbon of benzhydroxy chromate ester.