ACIDITIES OF RADICAL ANIONS (PKHA-.) DERIVED FROM NITRO-SUBSTITUTED AROMATIC WEAK ACIDS AND THE FORMATION OF RADICAL DIANIONS

A simple method has been devised for measuring the equilibrium acidities in DMSO of radical anions by using the equation pKHA−• = pKHA + 16.8[Erd(HA) − Erd(A-) ], which is based on a thermodynamic cycle. The acidities of 24 nitro-substituted aromatic radical anions have been measured in this way. The reversible reduction potentials to form the radical anions and those of their conjugate bases to form the radical dianions were obtained by cyclic voltammetry. The points for the irreversible Erd(HA) values for 6 nitroaromatic compounds were found to fit on or close to the line of a plot of Erd(HA) vs σp- for the reversible Erd(HA) values for 11 nitroaromatic compounds. The σp- value for p-NH2 was estimated to be about −1.0 from this plot. Relative radical dianion stabilization energies have been derived from the equilibrium acidities of the radical anions and the reduction potentials of the parent acids to provide an estimate of the stabilities of the radical dianions, relative to that of the p--OC6H4NO2-• r...