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Oxidation of 4-Chloroaniline Studied by On-Line Electrochemistry Electrospray Ionization Mass Spectrometry

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journal contribution
posted on 01.07.2009, 00:00 by Camilla Zettersten, Per J. R. Sjöberg, Leif Nyholm
The oxidation of 4-chloroaniline (4-CA) has been studied by electrochemistry (EC) coupled on-line with electrospray ionization mass spectrometry (ESI-MS) using two electrochemical flow cells of different design. The experimental results, which generally verify previously suggested oxidation pathways for 4-CA, also indicate the presence of an up to now unrecognized comproportionation reaction. The oxidation of 4-CA (m/z 128.2) was found to give rise to the formation of both an oxidized dimer, 4-[(4-chlorophenyl)imino]-2,5-cyclohexadien-1-imine (m/z 217.2), and a reduced dimer, 4-amino-4′-chlorodiphenylamine (m/z 219.2), in addition to a dimer intermediate (m/z 253.2). The unexpected formation of the reduced dimer is shown to stem from a comproportionation reaction involving 4-CA and the oxidized dimer. The presence of the latter reaction was clearly seen by comparing results obtained with two thin-layer flow cells, both with conversion efficiencies of 50% under mass transport controlled conditions but of different design with respect to the influence of the counter electrode reaction on the reaction at the working electrode. The experimental results demonstrate that the formation of the reduced dimer is favored by a decrease in the local pH in the flow cell, and the influence of the pH on the oxidation of 4-CA was also investigated in the pH range between 2.0 and 6.0 using off-line voltammetry. It is concluded that EC/ESI-MS is a powerful tool for the study of the present type of reactions and that studies of the reaction pathways of these systems are best carried out under noncoulometric experimental conditions as the latter facilitates the detection of reaction intermediates. Comproportionation reactions, similar to the reaction present in the 4-CA system, can also be expected to be present during the formation of conducting polymers such as polyaniline and polypyrrole.