One-electron transfer reactions of some hydroxynaphthoquinones. Solvent and substitution effect as studied by pulse radiolysis

Abstract

The properties of one-electron reduced species of 1,4-naphthoquinone (NQ), 2-hydroxynaphthoquinone (2HNQ), 5-hydroxynaphthoquinone (5HNQ) and 5,8-dihydroxynaphthoquionone (58DHNQ) in water–isopropanol (PrⁱOH)(5 mol dm^–3)–acetone (1 mol dm^–3) mixed solvent (MS), PrⁱOH and water have been investigated by a pulse radiolysis technique. Semiquinones of 2HNQ produced at acidic pH resemble those produced in pure PrⁱOH and have no absorption beyond 500 nm. These were assigned as neutral semiquinones. The semiquinones produced at higher pH (>11) were assigned as dianionic semi-quinones. Both the dianionic semiquinones and the one-electron oxidised species of 2 HNQ have absorption beyond 600 nm, but to a different extent since the difference of two electrons between the species leads to a difference in the extent of conjugation of the aromatic ring with the oxygen atom of the OH group. The pKs of the parent molecules and the semiquinones were determined in MS. The one-electron reduction potentials (E¹) of the quinones were calculated and compared with those of other related quinones. The kinetics of formation and decay of both the one-electron reduced and the one-electron oxidised species have been calculated. The k_f values for the reduced and the oxidised species were of the order of 10⁹ dm³ mol^–1 s^–1. The corresponding k_d values were of the order of 10⁸ dm³ mol^–1 s^–1 for the oxidised species, but the decay of the semiquinones was very slow. The effect of the position of the OH groups in NQ on the semiquinone characteristics has been discussed for 2HNQ, 5HNQ and 58DHNQ in MS and aqueous solutions.