Reaction of OH radicals with benzoquinone in aqueous solutions. A pulse radiolysis study

Abstract

Hydroxyl radicals have been generated by pulse radiolysis in N₂O-saturated aqueous solutions. Their addition to 1,4-benzoquinone BQ (k₃ = 6.6 × 10⁹ dm³ mol^–1 s^–1 by competition with thiocyanate) in neutral solution leads to a build-up of optical absorption that shows different rates at wavelengths at around 330 and at >400 nm. At 330 nm the rate of build-up is proportional to the benzoquinone concentration, and its rate constant agrees with the value (k₃) obtained by competition. At the longer wavelengths, it becomes independent of benzoquinone concentration beyond 4 × 10^–4 mol dm^–3 (k₆ = 6.9 × 10⁵ s^–1). Kinetic analysis in the ns time-range show that the primarily-generated benzoquinone–OH-adduct radical 1 undergoes rapid (k₄ = 2.5 × 10⁶ s^–1) keto–enol tautomerization yielding the 2,4-dihydroxyphenoxyl radical 2. To gain support for this proposed reaction, radical 2 [pK_a(2) ≈ 4.9 ± 0.2] has been independently generated by one-electron oxidation of 1,2,4-trihydroxybenzene using ˙OH in acidic or N₃˙ in neutral and basic solution. Its absorption characteristics compare favourably with those observed in the benzoquinone system. On the basis of spectrophotometric and conductometric data it is proposed that in neutral solution the radical anion 2a is rapidly oxidized by benzoquinone itself (k₁₅ ⩾ 2 × 10⁹ dm³ mol^–1) into the end product 2-hydroxy-1,4-benzoquinone anion 4a [pK_a(4) = 4.1 ± 0.1; λ_max(4) = 380 nm; λ_max(4a) = 482 nm] and the semibenzoquinone radical anion 3a. The latter decays bimolecularly into benzoquinone and hydroquinone (2k₁₆ = 3.1 × 10⁸ dm³ mol^–1 s^–1). In acidic solution the rate of oxidation of 2 by benzoquinone is considerably slower (k₁₃ ⩽ 2.4 × 10⁷ dm³ mol^–1 s^–1). The assignment of the final product to 2-hydroxy-1,4-benzoquinone 4 has been confirmed by the absorption characteristics and pK_a value of the authentic material obtained by the two-electron electrochemical oxidation of 1,2,4-trihydroxybenzene.

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