Reactions between iron(III) and catechol (o-dihydroxybenzene). Part II. Equilibria and kinetics of the redox reaction in aqueous acid solution
- 1 January 1973
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 23,p. 2609-2614
- https://doi.org/10.1039/dt9730002609
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, Keq=(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.Keywords
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