Redox equilibria of aromatic cation radicals and aromatic compounds

Abstract

Equilibrium constants involving aromatic cation radicals with aromatic compounds were determined spectrometrically in a solvent mixture consisting of trifluoroacetic acid–dichloromethane (1 : 4). The observed equilibrium constants were compared to values calculated from voltammetric data using the Nernst equation. Deviations between calculated and observed equilibrium constants were small for compounds of similar structural type and considerably greater when more drastic structural changes were involved. It was concluded that the deviations were not due to errors associated with relating electrode potentials to E° values but rather to changes in activity coefficients from one structural type as compared to another. The maximum deviation observed in log K was 0·8 and it was concluded that equilibrium constants can safely be estimated from voltammetric data with an order of magnitude reliability. Furthermore, it was concluded that disproportionation equilibrium constants for aromatic cation radicals can be estimated with a high degree of reliability from voltammetric data.