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 (k_f) for the one-electron transfer step, which are defined as k₀=k_f for X = H and CH₃ and as k₀= 2k_f 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 k_f, standard redox potentials for the radical cations p-C₆H₄(X)OH˙⁺ were estimated and found to be well correlated with the Hammett constants (σ_p) for the para substituents X.