Pulse radiolytic one-electron oxidation of some dihydroxy-substituted anthraquinones

Abstract

The spectroscopic characteristics and the kinetic parameters associated with the transients formed on one-electron oxidation of quinizarin (1,4-dihydroxy-9, 10-anthraquinone), quinizarin 2- and 6-sulfonates, 1,5-dihydroxy-9, 10-anthraquinone and 1,8-dihydroxy-9, 10-anthraquinone have been studied by pulse radiolysis and kinetic spectrophotometric techniques, using OH^•, O^•-, N₃^•, Br and ^•CH₂CHO as the oxidising radicals. The pK_a and the disproportionation equilibria of the semi-oxidised quinones have been studied for the water-soluble sulfonates. In contrast to the complex decay of the semi-oxidised naphthazarin (5,8-dihydroxy-1,4-naphtho-quinone), the semi-oxidised anthraquinone derivatives decay by simple second-order kinetics. The pK_a values of the latter are also much higher (ca. 8) compared to the former (ca. <4). The differences observed are attributed to the loss in symmetry in the free radical structures of the semi-oxidised anthraquinone derivatives.