Kinetics of electron-transfer reactions of para-substituted phenols p-C6H4(X)OH with [Fe(phen)3]3+(phen = 1,10-phenanthroline) and with [IrCl6]2– in aqueous acidic solutions: correlation between the Hammett constant of X and the one-electron redox potential of p-C6H4(X)OH
Abstract
Kinetic studies of the electron-transfer reactions of p-C6H4(X)OH (X = H, CH3, OCH3, OH, NH2, or NH3+) with tris(1,10-phenanthroline)iron(III), [Fe(phen)3]3+, and with hexachloroiridate(IV), [IrCl6]2–, have been made in aqueous acidic solutions. The second-order rate constants (k0) follow the rate law –d[A]/dt=k0[A][p-C6H4(X)OH], where A is the one-electron acceptor [Fe(phen)3]3+ or [IrCl6]2–, and were determined at an ionic strength of 1.0 mol dm–3 and 25 °C. The order of the forward rate constants (kf) for the one-electron transfer step, which are defined as k0=kf for X = H and CH3 and as k0= 2kf for the other X, was H < NH3+ < CH3 < OCH3 < OH < NH2 in approximate ratio 1:35:76:7 × 104:1.5 × 105:1.1 × 107 in the case of [Fe(phen)3]3+ and 1:150:75:4 × 104:7.2 × 104:1.0 × 108 in the case of the [IrCl6]2– reactions. By applying Marcus theory to kf, standard redox potentials for the radical cations p-C6H4(X)OH˙+ were estimated and found to be well correlated with the Hammett constants (σp) for the para substituents X.