The Solvent Effect on Semiquinone Redox Equilibria

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Abstract
From the facts presented, it is evident that the stability of semi‐ quinones is largely dependent upon the medium in which they are formed. The stability, as measured by K c, has been shown to be influenced not only by the acidity of the medium, but also to a large extent by the dielectric constant of the solvent and by the ionic strength of the solution. The stability of the semiquinone of sodium 2‐anthra‐quinone‐sulfonate is increased considerably with decrease in dielectric constant and is decreased by an increase in ionic strength. Semiquinone formation in other quinone redox systems should behave in similar fashion. That the semiquinone ion is a stable entity, exhibiting the electrical properties of ordinary ions, is emphasized by the fact that semiquinone equilibria are affected by the dielectric constant and by the ionic strength, in essentially the same manner as are other more familiar ionic equilibria. Few of the numerous studies of solvent effects have dealt with oxidation‐reduction equilibria. The magnitude of the effects observed in the present investigation on semiquinone formation suggests that further study of the solvent effect on redox equilibria would probably be fruitful from both a practical and a theoretical viewpoint. Thus it is conceivable that, since the solvent effect depends on the charge type of the reactants, the order of the normal oxidation‐reduction potentials or “redox strength” (which may be defined as the tendency to give off an electron when the ratio is unity) may be found to change with change in solvent, just as the sequence of acid strengths has been found to vary from one solvent to another.

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