Radical cations para-substituted anilines

Amine radical-cations have been generated by the treatment of para-substituted anilines with ceric ion (Stone and Waters, 1962 Fox and Waters, 1964). When the para position is free, the initial radical-cation can react further for example, the oxidation of triphenylamine with lead tetra-acetate in the presence of boron trifluoride (Allara ei al., 1965) or with iodine (Stamires and Turkevich, 1963) gives the radical cation Ph3N+ , and, when excess of triphenylamine is used, the former oxidant leads to the radical-cation of A(, .A/, W, A -tetraphenylbenzidine. The only radical observed by the oxidation of dimethylanihne either electrochemically (Mzoguchi and Adams, 1962) or with lead tetraacetate and boron trifluoride (Allara et al., 1965) is the radical-cation of iV, .A, .A7, iV -tetramethylbenzidine. The relatively stable (hindered) anilino radical (40) has been generated from the corresponding aniline by flash photolysis audits e.s.r. spectrum has been measured in n-hexane (Land and Porter, 1961). The electronic spectrum of this radical is very similar to that of the unsubstituted anilino radical, detected during flash photolysis of aniline, but this radical is so short-lived that it has not yet been detected by e.s.r. 

The general pattern of anodic behavior of para-substituted anilines (68) was established in aqueous acidic media by Bacon and Adams17,106. The postulated one-electron oxidation of the substrate to the radical cation 681 is followed by rapid head-to-tail coupling of 681 with the substrate 68 giving protonated 4 -substituted 4-aminodiphenylamine in the oxidized form (69) as the final main product. The product 69 shows reversible redox peaks at more cathodic potentials, supporting its identification beside the spectral and chemical analysis. The product formation is preceded by elimination of one para-substituent and, if it leaves as an anion (e.g. halide, methoxide or ethoxide ion), then the overall electrochemical process (equation 1) corresponds to a one-electron (two electrons per two reactant molecules) process. However, if it leaves as a neutral group (e.g. CO2 in the oxidation of p-aminobenzoic acid), a dimer is formed in the reduced form and the overall reaction is a two-electron process. 

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