1200 Journal of Chemical Education • Vol. 74 No. 10 October 1997 In the Laboratory 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 biologi- cally significant quinones is related to the behavior of simple quinones such as 1,4-benzoquinone and 9,10-an- thraquinone (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 po- larographic study of AQ and other quinones in dimethyl- formamide (DMF) and acetonitrile solvents. They also found that quinone anions are significantly stabilized when asso- ciated with alkali metal cations. Furthermore this associa- tion in solution and the electrosorption of alkali metal cat- ions reduces electrostatic repulsion at the electrode giving faster electron transfer kinetics (4). Wawonzek et al. (3) also studied the reduction behav- ior 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 reduc- tion. More recently Wightman et al. (6) carried out a thor- ough 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 behav- ior of AQ in dimethylformamide, a typical laboratory aprotic solvent, and to examine the influence of a proton donor, ben- zoic 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 pro- ton availability. This experiment is part of an Otago third- year undergraduate course in Chemical and Electrode Kinet- ics. The experimental data are collected in a four-hour labora- tory period. Theory Quinones (Q) undergo two successive one-electron re- ductions in aprotic solvents to form stable radical anions (Q) and quasistable dianions (Q 2 ). Q + e  ↔ Q (1) Q + e  ↔ Q 2 (2) The equilibrium potentials for these two processes (E 1 and E 2 ) are related to the concentrations of the quinone spe- cies by E 1 = E 1 ° – RT/F ln [Q]/[Q] (3) E 2 = E 2 ° – RT/F ln [Q 2 ]/[Q] (4) The tendency of the radical anion to disproportionate 2Q ↔ Q + Q 2 (5) is given by the disproportionation constant K disp = [Q] [Q 2 ]/[Q] 2 (6) By considering eq 6 and eqs 3 and 4 one obtains ln K disp = F/RT (E 2 ° – E 1 °) (7) Successive 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 for radical anions to disproportionate. Conversely, if Q 2 is formed in the pres- ence of Q the tendency for conproportionation (the reverse of disproportionation) is high and Q 2 is expected to revert to Q . The mechanism and products of two-electron reduction of quinones can be rather different when proton donors are present in aprotic solvents. If the proton donor HA is suffi- ciently acidic to cause Q protonation, the resultant QH  species may undergo immediate disproportionation to form QH 2 . This can occur because QH  species are usually more readily reduced than their Q parents (10). Q + HA ↔ QH  + A  (8) 2QH  ↔ QH 2 + Q (9) The net reaction, via eqs 1, 8, and 9, is a two-electron, two- proton reduction of Q to QH 2 . If direct reduction of QH  oc- curs instead of disproportionation the resultant QH  spe- cies is readily protonated and the net reaction is to QH 2 as before. UV-Visible Spectroelectrochemistry of Reduction Products of Anthraquinone in Dimethylformamide Solutions An Advanced Undergraduate Experiment Ali Babaei, Paul A. Connor, and A. James McQuillan* Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Siva Umapathy Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India *Corresponding author.