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

2-(beta-Hydroxyethyl)aminobenzo-1,4-quinones are electrochemically reduced in two steps. The more positive wave corresponds to the reduction of the qu inone 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 th e equilibrium constants for the addition of hydroxide ions to the 1-carbony l group. Changes of polarographic limiting current with pH indicated that t he 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 beta-carbon in the side chain. Structural effects on the m easured overall constant K indicate that substituents on the quinone ring a nd in the side chain exert different effects on addition of OH- and on the rate-determining step. Oxidation-reduction potentials of these quinones fol low linear free energy relationships as expected. As 2-(beta-hydroxyethyl)a minobenzoquinones are formed by hydrolysis of ethyleneiminoquinones, used a s cancerostatics, observed equilibria may occur under physiological conditi ons.