Electron spin resonance studies. Part 56. An investigation of the role of aromatic radical-zwitterions and acyloxyl radicals in the one-electron oxidation of aromatic carboxylates and reduction of aromatic peroxyacids

Abstract

The results are described of an e.s.r. investigation into the reactions of some aromatic radical-zwitterions ⁺˙ArCO₂^–(derived by one-electron oxidation of benzoate and related ions with SO₄^–˙) and some aroyloxyl radicals ArCO₂·(derived by reduction of some peroxybenzoic acids with Ti^III). The radicals have been generated in an aqueous flow system at high pH; the nitromethane anion CH₂:NO₂^– has been employed as a spin trap in order to elucidate the reaction mechanisms. Evidence is presented which accords with the suggestion that the species ⁺˙ArCO₂^– and ArCO₂˙ are distinct chemical entities, rather than canonical forms of a single intermediate. A variety of differently substituted aroyloxyl radicals (ArCO₂˙) rapidly lose CO₂ to give aryl radicals (Ar˙); an exceptional case is the rapid intramolecular hydrogen atom abstraction reactions of 2-methylbenzoyloxyl. There is evidence that the radical-zwitterions⁺˙ArCO₂^– undergo a variety of reactions depending on their structure: most undergo conversion into aroyloxyl radicals; in addition, 2- and 4-methyl-substituted derivatives also rapidly deprotonate (to yield the appropriate benzyl species), whereas methoxy-substituted species undergo ready one-electron reduction by Ti^III. Rate constants are reported for the one-electron reduction of persulphate ion by titanium(III) at pH 2.5 and 9.