Protolytic equilibria of dihydroxyanthraquinones in non-aqueous solutions

Acid dissociation and anionic homoconjugation equilibria of 1,4-dihydroxy-, 1,5-dihydroxy-, and 1,8-dihydroxyanthraquinone were studied by means of po tentiometry and UV-spectroscopy in the following polar non-aqueous solvents of varying dielectric and acid-base properties: acetonitrile, dimethyl sul foxide, and methanol. The measurements were carried out under anaerobic con ditions, in order to prevent the oxidation of phenolic parts. The determine d pK(a)'s range from about 12-14 in dimethyl sulfoxide and methanol to abou t 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 homoconj ugation constants are of the order of 2-3 in all solvent studied. (C) 1999 Elsevier Science B.V. All rights reserved.