Aromatic hydroxylation. 8. A radiation chemical study of the oxidation of hydroxycyclohexadienyl radicals

Substituted hydroxycyclohexadienyl radicals have been generated by pulse radiolysis of N2O-saturated aqueous solutions of monosubstituted benzene derivatives. Their rates of disproportionation/dimerization and of oxidation by Fe(CN)63- and by IrCl62- have been measured by optical and conductimetric methods. For the fluoro- and chlorohydroxycyclohexadienyl radicals disproportionation is shown to involve hydrogen atom abstraction rather than electron transfer. However, the Fe(CN)63- oxidation of the substituted hydroxycyclohexadienyl radicals occurs by a rate-determining electron transfer followed by a fast proton loss rather than by hydrogen atom abstraction or synchronous removal of an electron and a proton. A plot of the log of the rate constants for oxidation against Hammett σ+ constants gives a good linear relationship with a rho value of -3.0 +/- 0.1. With the more powerful oxidant IrCl6-, the electron-transfer oxidation of all but the least reactive hydroxycyclohexadienyl radicals is diffusion controlled