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The widespread occurrence of quinones in nature and their importance in biological electron transport has led to extensive studies of their electrochemical behavior (1,2). Much of what is known about the redox behavior of biologically significant quinones is related to the behavior of simple quinones such as 1,4-benzoquinone and 9,10-anthraquinone (AQ). Although the parent quinones have little visible absorption, their reduced forms have distinct colors arising from intense π → π* transitions and this has led to the frequent use of UV-vis spectroscopy in distinguishing quinone reduction products.Successive one-electron reduction of quinones was clearly demonstrated by Wawzoneck et al. (3) in a classic polarographic study of AQ and other quinones in dimethylformamide (DMF) and acetonitrile solvents. They also found that quinone anions are significantly stabilized when associated with alkali metal cations. Furthermore this association in solution and the electrosorption of alkali metal cations reduces electrostatic repulsion at the electrode giving faster electron transfer kinetics (4).Wawonzek et al.(3) also studied the reduction behavior when the solvents contained small quantities of water and of benzoic acid as proton donors. They attributed the reduction behavior of AQ largely to the ready protonation of the dianions. Given and Peover (5), in a further study of AQ in DMF, suggested that the radical anion is protonated by benzoic acid and immediately undergoes further reduction. More recently Wightman et al. (6) carried out a thorough study of AQ reduction reactions in DMF with added benzoic acid. They showed that protonation of the radical anion is followed by disproportionation.The following experiment shows how cyclic voltammetry (7) may be used to study the reduction behavior of AQ in dimethylformamide, a typical laboratory aprotic solvent, and to examine the influence of a proton donor, benzoic acid, on this behavior. Digital simulations of the cyclic voltammograms (8,9) have been used to test protonation reaction mechanisms. The UV-vis spectroelectrochemistry of this system has also been employed to characterize the reduction products of AQ under conditions of different proton availability. This experiment is part of an Otago thirdyear undergraduate course in Chemical and Electrode Kinetics. The experimental data are collected in a four-hour laboratory period. TheoryQuinones (Q) undergo two successive one-electron reductions in aprotic solvents to form stable radical anions (Q៛) and quasistable dianions (Q 2᎑ ).The equilibrium potentials for these two processes (E 1 and E 2 ) are related to the concentrations of the quinone species by(3)The tendency of the radical anion to disproportionateis given by the disproportionation constantBy considering eq 6 and eqs 3 and 4 one obtainsSuccessive electron transfers of quinones in aprotic solvents usually occur at quite separate potentials. Thus E 2° -E 1° is generally large and negative. This results in very low K disp values (eq 7) and there is little tendency f...

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