Quinone–quinol equilibria in solutions of 2-(β-hydroxyethyl)aminobenzo-1,4-quinones. A spectrophotometric and polarographic study

Abstract

2-(β-Hydroxyethyl)aminobenzo-1,4-quinones are electrochemically reduced in two steps. The more positive wave corresponds to the reduction of the quinone form, the second at a potential more negative by 0.7 V is attributed to the reduction of the less conjugated quinol. Electronic spectra enabled determination of equilibrium constants (K = [quinone]/[quinol]) as well as the equilibrium constants for the addition of hydroxide ions to the 1-carbonyl group. Changes of polarographic limiting current with pH indicated that the conversion of the quinone into quinol is base catalyzed. The addition of OH⁻ ions is followed by a ring formation, resulting from an attack of the -O⁻ group on the β-carbon in the side chain. Structural effects on the measured overall constant K indicate that substituents on the quinone ring and in the side chain exert different effects on addition of OH⁻ and on the rate-determining step. Oxidation–reduction potentials of these quinones follow linear free energy relationships as expected. As 2-(β-hydroxyethyl)aminobenzoquinones are formed by hydrolysis of ethyleneiminoquinones, used as cancerostatics, observed equilibria may occur under physiological conditions.