Superoxide Radical Reactions in Aqueous Solutions of Pyrogallol and N-propyl Gallate: The Involvement of Phenoxyl Radicals. A Pulse Radiolysis Study

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Abstract
The reactions of O2·− in aqueous solutions of pyrogallol 1 and the antioxidant n-propyl gallate 2 have been studied. In both cases the initial reaction gives hydrogen peroxide and the corresponding phenoxyl radical (k(1 + O2·−) = 3·4 × 105, k(2 + O2·−) = 2·6 × 105 dm3 mol−1 s−1). These phenoxyl radicals have been produced independently by reacting 1 and 2 with Br2·− and their spectra and first pKa values measured (pKa(phenoxyl radical from 1) = 5·1, pKa(phenoxyl radical from 2) = 4·1). It is necessary to correct the observed spectra for the contribution of the H-adducts, formed by the reaction of radiolytically produced H atoms with the substrates (k(1 + H·) = 2·5 × 109, k(2 + H·) = 3·8 × 109 dm3 mol−1 s−1). The H-adduct spectra are given. In the reactions of O2·− with the substrates the initial transient absorbances are characteristic of the phenoxyl radicals; however at longer times a new transient absorbing around 500 nm (ε ≃ 104 dm3 mol−1 cm−1) appears. This is believed to be the deprotonated hydroxy-orthoquinone, formed by the reaction of phenoxyl radicals with O2·− (k ≃ 1·5 × 108 dm3 mol−1 s−1, from kinetic curve-fitting). The absorbance due to the hydroxy-orthoquinones decays by first-order kinetics (1·6 × 102 in the case of 1 and 1·1 × 102 s−1 in the case of 2). This is thought to be mainly the result of the conversion of the hydroxy-orthoquinone into its hydrate. Similar experiments were carried out with catechol and ethyl protocatechuate. The chemistry appears to be similar to that of the pyrogallol derivatives. The rate constant for reaction of these compounds with O2·− is, however, only ≤ × 104 dm3 mol−1 s−1.