Properties of hydrogen bonds in 5,8-dihydroxy-1,4-naphthoquinone and 1,4-dihydroxy-9,10-anthraquinone; reaction with bases in aqueous solution

Abstract

Rate coefficients and equilibrium constants for the dissociation of 5,8-dihydroxy-1,4-naphthoquinone (1),pK₁⁰8.51 and pK₂⁰ 11.49, and 1,4-dihydroxy-9,10-anthraquinone (3), pK₁⁰ 9.85 and pK₂⁰ 12.28, have been measured in aqueous solution. The effects of a methyl substituent in (1) and a sulphonate substituent in (3) have been studied. The rate coefficient for the first and second dissociation are 25- and 1 000-fold respectively lower than those for normal proton transfers because of a weak intramolecular hydrogen bond in the undissociated acids and a moderately strong hydrogen bond in the monoanions.