Overoxidation of phenol by hexachloroiridate(IV)

It has been previously established that the aqueous oxidation of phenol by a deficiency of IrCl(6)](2-) proceeds through the production of IrCl(6)](3-) and phenoxyl radicals. Coupling of the phenoxyl radicals leads primarily to 4,4'-biphenol, 2,2'-biphenol, 2,4'-biphenol, and 4-phenoxyphenol. Overoxidation occurs through the further oxidation of these coupling products, leading to a rather complex mixture of final products. The rate laws for oxidation of the four coupling products by IrCl(6)](2-) have the same form as those for the oxidation of phenol itself: -dIr(IV)]/dt = {(k(ArOH) + k(ArO(-))K(a)/H(+)])/(1 + K(a)/H(+)])}ArOH](tot)Ir(IV)]. Values for k(ArOH) and k(ArO(-)) have been determined for the four substrates at 25 °C and are assigned to H(2)O-PCET and electron-transfer mechanisms, respectively. Kinetic simulations of a combined mechanism that includes the rate of oxidation of phenol as well as the rates of these overoxidation steps show that the degree of overoxidation is rather limited at high pH but quite extensive at low pH. This pH-dependent overoxidation leads to a pH-dependent stoichiometric factor in the rate law for oxidation of phenol and causes some minor deviations in the rate law for oxidation of phenol. Empirically, these minor deviations can be accommodated by the introduction of a third term in the rate law that includes a "pH-dependent rate constant", but this approach masks the mechanistic origins of the effect.