Study of the effects of ion pairing and activity coefficients on the separation in standard potentials for two-step reduction of dinitroaromatics

The electrochemical reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, occurs with potential inversion. That is, the standard potential for formation of the anion radical is shifted in the negative direction from the standard potential for the anion radical/dianion couple. This behavior has been attributed to significant structural changes accompanying the reduction steps. In this work, an assessment was made of the magnitude of the effects of activity coefficients and ion pairing, two effects which contribute to potential inversion. 1,4-Dinitrobenzene, 3, and 2,5-dimethyl-1,4-dinitrobenzene, 4, were studied in acetonitrile and N,N-dimethylformamide with R4N+ salts as electrolytes (R = CH3-, CH3CH2-, CH3(CH2)(3)(-), and CH3(CH2)(7)-) at concentrations from 0.010 to 0.100 M. Significant ion pairing between the dianion, A(2-), and R4N+ was found for (CH3)(4)N+ with both 3 and 4 while the effects of the other electrolytes were smaller. The data were successfully interpreted without recourse to other ion pairs, e.g., ion pairing between the anion radical and the electrolyte cation. Ion pair formation constants are reported along with the infinite-dilution values of the difference in the two standard potentials. The effects of activity coefficients and ion pairing at 0.10 M electrolyte do not exceed 100 mV for (CH3N+) and are only 20 to 60 mV for (CH3)(4)N+, a cation commonly used in studies of potential inversion. It is concluded that structural changes accompanying the reduction, rather than activity and ion pairing effects, are the dominant factors underlying potential inversion.