Oligoaniline intermediates in the aniline-peroxydisulfate system

Chemical oxidative polymerization of aniline using peroxydisulfate oxidant in aqueous pH 2.5–10.0 buffers yields electrically insulating brown powders that are believed to be mixtures of Michael-type adducts of benzoquinone monoimine and aniline at various stages of hydrolysis. A spectroscopically similar product is formed when solid 1,4-benzoquinone is added to an aqueous solution of aniline at room temperature in the absence of peroxydisulfate. This suggests that the peroxydisulfate oxidant in the aniline/S2O82− system provides a pathway for the formation of benzoquinone monoimine as an intermediate. Benzoquinone monoimine intermediate could be formed as a result of a Boyland–Sims rearrangement of aniline proceeding via the intermediacy of p-aminophenyl sulfate. Benzoquinone monoimine undergoes a series of conjugate 1,4-Michael-type addition/reoxidation/coupling steps with aniline or p-aminophenyl sulfate yielding the oligoaniline product. The precipitate that is isolated is also in the midst of two simultaneous pH dependent hydrolysis reactions: (i) hydrolysis of the imine groups to quinone, and (ii) hydrolysis of arylsulfates to phenols. The ratio of hydrolysis in each case was determined by the C/N ratio and sulfur elemental analysis values yielding analytical data that is consistent with experimentally determined values and also with our proposed reaction scheme. These findings offer a rationale for the high C/N ratios (>6.0) frequently observed in these systems while tracing the genesis of the residual sulfur in the product to unhydrolyzed arylsulfate. The oligoaniline product has previously been reported to have a novel poly-aza structure consisting of continuously linked –N–N–N– bonds, and alternately also reported to consist of phenazine-type linkages. This study is consistent with the latter and describes a pathway to phenazine coupling through a second and third stage hydrolysis of the arylsulfate and reoxidation with peroxydisulfate. There is no pathway for the formation of linear –N–N–N– linkages in the aniline/benzoquinone adduct and the striking similarity between its spectroscopic properties and the aniline/S2O82− adduct suggests that it is not a preferred pathway under these experimental conditions.