Properties of hydrogen bonds in substituted 5,8-dihydroxy-1,4-napthoquinones and 1,4-dihydroxy-9,10-anthraquinones; kinetics of the reactions of the monoanions with hydroxide ion and buffers in dimethyl sulphoxide–water mixtures

Abstract

Rate coefficients and equilibrium constants for removal of the hydrogen-bonded protons from the monoanions of two 1,4-dihydroxy-9,10-anthraquinones by hydroxide ion have been measured in 50%(v/v) Me2SO–H2O. The values are compared with results from reactions in aqueous solution and with results for the reactions of the monoanions of 5,8-dihydroxy-1,4-napthoquinone and its 2-methyl derivative with buffers in 50 and 70%(v/v) Me2SO–H2O. The similarity between the rate coefficients for proton removal by buffer bases and by hydroxide ion is compatible with a mechanism involving diffusion-controlled proton removal from a low concentration of a non-hydrogen-bonded intermediate in equilibrium with the hydrogen-bonded monoanions. At high buffer concentrations the dependence of rate on buffer concentration is curvilinear, and formation of the non-hydrogen-bonded intermediate may then be rate-limiting.