One-electron reactions of 1,5- and 1,8-dihydroxy-9, 10-anthraquinones. A pulse radiolysis study

Abstract

Absorption characteristics of the semiquinone free radicals formed by the one-electron reduction (using e^–_aq, CO^˙–₂ and CH₃ĊOHCH₃ as the reductant) as well as the oxidation (using OH˙, O˙^– and N^˙₃ as the oxidants) of 1,5-dihydroxy-9, 10-anthraquinone (1,5-DHAQ) and 1,8-dihydroxy-9, 10-anthraquinone (1,8-DHAQ) have been studied by pulse radiolysis in pure isopropyl alcohol and in aqueous solutions containing various appropriate additives. The first acid dissociation constants for the reduced semiquinones were measured as pK_a= 3.65 and 3.95 for 1,5-DHAQ and 1,8-DHAQ, respectively. Second-order rate constants for various formation and decay reactions have been determined. The one-electron reduction potentials (vs. NHE) were determined at pH 11, as E¹₁₁=–350 mV (for 1,5-DHAQ) and E¹₁₁=–377 mV (for 1,8-DHAQ), respectively. Differences with 1,4-dihydroxy-9, 10-anthraquinone (quinizarin) are discussed.