Protolytic equilibria of dihydroxyanthraquinones in non-aqueous solutions Original Research Article

Acid dissociation and anionic homoconjugation equilibria of 1,4-dihydroxy-, 1,5-dihydroxy-, and 1,8-dihydroxyanthraquinone were studied by means of potentiometry and UV-spectroscopy in the following polar non-aqueous solvents of varying dielectric and acid–base properties: acetonitrile, dimethyl sulfoxide, and methanol. The measurements were carried out under anaerobic conditions, in order to prevent the oxidation of phenolic parts. The determined pKa’s range from about 12–14 in dimethyl sulfoxide and methanol to about 22–24 in acetonitrile; the most acidic compound being the 1,8-derivative and the least acidic the 1,5-derivative. The logarithms of anionic homoconjugation constants are of the order of 2–3 in all solvent studied.