An in situ UV-vis spectroelectrochemical investigation of the initial stages in the electrooxidation of selected ring- and nitrogen-alkylsubstituted anilines

The electrooxidation of a selection of N-alkylsubstituted (N-ethyl- and N,N-dimethyl-) and ring-substituted (o-, m-, p-methyl-, o-ethyl-) aniline derivatives at ITO glass electrodes was investigated in acidic solutions with in situ UV-vis spectroelectrochemistry. With N-ethylaniline well defined absorbance peaks of an intermediate and of an end product were observed after an anodic potential step. The conversion of the electrogenerated intermediate into an end product was demonstrated. With N,N-dimethylaniline the formation of an electrogenerated intermediate as well as its consumption after interruption of electrolysis were observed. However. no end product was detected. This is interpreted in terms of a C-C (tail-to-tail) coupling, leading to a benzidine type end product which is not detectable in the investigated spectral range. Absorbance transients were detected at lambda = 450 nm for ring-alkylsubstituted anilines, showing the typical time dependent behaviour of an intermediate species. Absorbance transients at lambda = 750 nm were identified as being caused by end products of the electrooxidation of ring-substituted anilines. A reaction sequence is proposed based on the results obtained. At sufficiently high anodic potentials all investigated aniline derivatives undergo electrooxidation, yielding intermediates. In the following chemical step, intermediates react with solution phase molecules of the substrate. The fastest chemical reaction proceeds with unsubstituted aniline, whereas ring-alkylsubstituted derivatives react somewhat slower. The slowest chemical reaction proceeds with N-alkylsubstituted aniline derivatives. (C) 1999 Elsevier Science Ltd. All rights reserved.