Cleavage reactions of radical anions that range from homolytic to heterolytic within the same family of compounds

Properties of the radical anions of three alpha-nitrocumenes (alpha-nitrocumene, p-cyano-alpha-nitrocumene, and p-nitro-alpha-nitrocumene, 1a-c) have been determined by electrochemical methods. In particular, the standard potentials of the neutral/radical anion couples were found to be -2.20, -2.04, and -1.43 V with respect to the ferrocenium ion/ferrocene potential and the rate constants for expulsion of nitrite from the radical anions were 3 x 10(6), 5 x 10(6), and 240 s(-1) for 1a-c, respectively. Comparison of these potentials with those of related compounds demonstrates that reduction of the nitroalkyl portion of the molecule occurs in 1a and 1b while in 1c the electron is added to the nitrophenyl group. Thus, using previously defined terminology, the cleavage of the radical anions of 1a and 1b to give nitrite and the corresponding cumyl radicals are examples of homolytic cleavage reactions but the cleavage of the radical anion of 1c is heterolytic. The driving force for the three cleavage reactions has been estimated and it is concluded that the large decrease in magnitude of the cleavage rate constants on going from 1a and 1b to 1c is mainly due to the much larger driving force for the first two.