The effect of proton donors on the electrochemical behavior of aromatic compounds has been studied in recent years 1-3. In general, the aromatic radical anions or dianions are formed initially by the electron-transfer process at the electrode, and the stability of these anions varies depending on their basicity and the nature of the chemical environment.
The contribution of protonation to the electrode reaction of aromatic compounds often leads to profound changes in the polarographic characteristics of the reduction processes. The controlled addition of proton donors to the solution has led to valuable information on the role of protonation in organic electrochemical processes. Most of the work has dealt almost exclusively with the changes in the polarogram relevant to the mole ratio of proton donor to organic compound. Little attention has been paid to the explanation of the results in terms of correlation with the proton donating strength of acid in the given solvent.
Our previous work was initiated in order to show the influence of proton donors on the polarographic reduction of quinones in non-aqueous solvents 4-6. In aprotic solvents, quinones are reduced first to the semiquinone anion and then to the dianion x, whereas in amphiprotic solvents, two reduction waves merge together and give a single wave (approximately equal in height to the sum of the two waves) because of the sufficiently high rates of proton addition from the solvents. In previous papers 5'6 it was found that the prewave caused by the addition of acid to the solution preceded the original reduction waves of quinones, and it was pointed out briefly that the features of the prewave were affected by the strength of the added acid and the nature of the solvent.
It appeared of interest to investigate quantitatively the prewave of quinone in relation to the acid strength and the rate of supply of proton donating species from the bulk solution to the electrode surface. The present paper deals with the comparison of the features of the prewaves of methyl-p-benzoquinone obtained in the presence of acid in aqueous and non-aqueous solvents. The results are discussed on the basis that the prewave is attributed to the protonation of quinone, being involved in the electrode processes, and that the current is controlled by the diffusion of the proton donors to the electrode surface. EXPERIMENTAL Guaranteed-reagent-grade methyl cellosolve (2-methoxyethanol) was refluxed J. EIectroanal. Chem., 31 (1971) 225-232 * The alternative reaction paths.