Kinetic Studies of the Oxidation Reactions of o-, m-, and p-Benzenediols with Tris(1,10-phenanthroline)iron(III). An Estimation of the Redox Potentials of the Organic Radicals by Application of the Marcus Theory

Abstract

Abstract Kinetic studies of the oxidation of o-, m-, and p-benzenediols (H2A) by tris(1,10-phenanthroline)iron(III) were made in dilute perchloric acid solution at an ionic strength of 1.0 mol dm−3 at four temperatures between 10 and 30 °C. The second-order rate constants (k0) which follow the observed rate law of –d [Fe(phen)33+]/dt=k0[Fe(phen)33+][H2A] were determined under varied conditions. The order of the rate constants was m-<o-<p-benzenediols, with the ratio being approximately 1 : 103 : 104. By the application of the Marcus theory to the kinetic parameters obtained, the standard redox potentials of the dihydroxyphenyl radicals (H2A\underset.+) were estimated in 0.11 mol dm−3 perchloric acid solution to be 1.41, 1.10, and 0.97 V for m-, o-, and p-benzenediols, respectively. The energies of the highest occupied molecular orbitals for the H2A molecules (εHOMO) were calculated to be −0.4468, −0.4332, and −0.4224 a.u. for m-, o-, and p-benzenediols, respectively. The differences of the total electronic energies between H2A\underset.+ and H2A molecules (ΔET) were calculated to be 0.4183, 0.4033, and 0.3957 a.u. for m-, o-, and p-benzenediols, respectively. Thus, the standard redox potentials of the free-radicals estimated are found to be well correlated with the HOMO energies and also with the differences of the total electronic energies between the cation radicals and the parent molecules.