Substituent effects in the one-electron reduction of anthraquinone derivatives

Polarographic reduction against (C₂H₅)₄NBr as background in dimenthyformamide has been applied to 17 anthraquinones with - and -substituents. There are two pronounced half-waves, which correspond to two one-electron degrees of reduction, a semiquinone anion being formed as intermediate. An ESR signal is produced when this radical is formed by electrolysis with a cathode potential equal to E_1/2. The observed E_1/2 fit Hammett's equation closely. The relation E_1/2 to donor-acceptor aspects of the substituents is discussed.