Reactions between iron(III) and catechol (o-dihydroxybenzene). Part II. Equilibria and kinetics of the redox reaction in aqueous acid solution

Abstract

The redox reaction between iron(III) and catechol (H²cat) has been investigated at 25·0 °C and 1·0 > [H⁺] 0·1M with the stopped-flow technique. The overall reaction (i)(qno =o-quinone) is limited by the reverse reaction 2Fe³⁺+ H₂cat ⇌ 2Fe²⁺+ qno + 2H⁺(i) and the equilibrium constant, obtained from spectrophotometric data, K_eq=(2·2 ± 0·1)× 10^–2 mol² l^–2; ε(qno)=(1·46 ± 0·07)× 10³ l mol^–1 cm^–1 at 390 nm. The mechanism proposed involves intermediate semi-quinone radical (Hcat˙) formation as the rate-determining step; Hcat˙ is then oxidized to o-quinone, according to steps (ii) and (iii). The quantities k₁′ and k₂′/k₃′ vary with hydrogen-ion concentration according to k₁′=k₁/[H⁺] Fe³⁺+ H₂cat [graphic omitted] Fe₂₊+ Hcat˙+ H⁺(ii), Fe³⁺+ Hcat˙ [graphic omitted] Fe²⁺+ qno + H⁺(iii), andk₂′/k₃′=k₂[H⁺]/k³, where k₁= 0·19 ± 0·01 s^–1, k₂/k₃= 0·88 ± 0·08 l mol^–1, and k₄= 10·8 ± 1·01 mol^–1 s^–1(independent of [H⁺]). A comparison between kinetic and equilibrium data showed satisfactory agreement. The hydrogen-ion dependence of the reaction rates indicates that one of the reactants is deprotonated and alternative paths for reaction are discussed.