Pulse radiolysis of 9,10-anthraquinones. Part 1.—Radicals

Abstract

Radicals derived from 1- and 2-substituted 9,10-anthraquinones have been studied using pulse radiolysis. In aqueous solution, both sodium anthraquinone-1- and -2-sulphonates react rapidly with hydrated electrons, and more slowly with formate radicals, to form anthrasemiquinone radical ions (A·^–). The absorption spectra and extinction coefficients of both A·^– radical ions were very similar; the corresponding protonated semiquinones (AH·) had similar spectral properties also. No significant difference was found in the reactivities of both A·^– radicals towards oxygen, but the pK values for the equilibria A·^–+ H⁺⇌ AH· differed significantly. OH radicals react rapidly with the 1- and 2-sulphonate to form transient products which have similar spectral properties. Kinetic salt effect studies indicate that such transient products are OH adducts rather than semi-oxidized anthraquinones. The rates of reduction of 1- and 2-piperidinoanthraquinones in alcoholic solution by solvated electrons, or by ĊH₂O^– radicals, to form the corresponding semiquinones, are similar. Reduction by ĊH₂OH in methanol acidified with 10^–2 M H₂SO₄ is much slower for the 1-piperidino derivative than for the 2- derivative, due to differences in the states of protonation of the two quinones. The absorption spectra were found for both unprotonated and protonated forms of these semiquinones, and the corresponding species derived from 1- and 2-aminoanthraquinone.