Kinetics and Mechanism of Ir(III) Catalyzed Oxidation of Acetone by Cerium(IV) in Aqueous Perchloric Acid Media

Abstract

Kinetics and mechanism of iridium(III) (ca. 10-6 mol dm-3) catalyzed oxidation of acetone by cerium(IV) in aqueous perchloric acid medium have been investigated at different temperatures (20-35 ºC) under the conditions [CH3COCH3]_T >> [Ce(IV)]_T >> [Ir]_T. The rate law conforms to -d[Ce(IV) / dt = 2kK₁K₂[S][Ce(IV)][Ir(III)]_T[H⁺] / 1+K₁K₂[S][Ce(IV)] where k, K1 and K2 are constants at a particular temperature at a fixed [H⁺] and [Ir]_T gives the total concentration of iridium added as catalyst. An intermediate involving the oxidant, catalyst and substrate has been proposed as being formed in pre-equilibrium steps before the electron transfer step through the Ir(III)/Ir(IV) catalytic cycle. The electron transfer (inner sphere mechanism) leads to the partially oxidized product which is rapidly oxidized by cerium(IV) at a fast step. The predominant species of Ce(IV) perchlorate in aqueous perchloric acid medium is monomeric, hence unhydrolyzed species Ce⁴⁺ and hydrolyzed species Ce(OH)³⁺ have been found as the kinetically active species.