Oxidation of 4-Chloroaniline Studied by On-Line Electrochemistry Electrospray Ionization Mass Spectrometry
作者:Camilla Zettersten、Per J. R. Sjöberg、Leif Nyholm
DOI:10.1021/ac802563f
日期:2009.7.1
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.
使用两个不同设计的电化学流动池,通过电化学(EC)在线耦合电喷雾离子化质谱法(ESI-MS)对 4-氯苯胺(4-CA)的氧化过程进行了研究。实验结果从总体上验证了之前提出的 4-CA 氧化途径,同时也表明存在一种至今尚未认识到的比例反应。研究发现,4-CA 的氧化反应(m/z 128.2)除了生成二聚体中间体(m/z 253.2)外,还生成了氧化二聚体 4-[(4-氯苯基)亚氨基]-2,5-环己二烯-1-亚胺(m/z 217.2)和还原二聚体 4-氨基-4′-氯二苯胺(m/z 219.2)。还原二聚体的意外形成是由于 4-CA 和氧化二聚体发生了比例反应。通过比较两个薄层流动池获得的结果,可以清楚地看到后一种反应的存在。这两个薄层流动池在质量传输控制条件下的转化效率均为 50%,但在对电极反应对工作电极反应的影响方面设计不同。实验结果表明,流动池中局部 pH 值的降低有利于还原二聚体的形成,同时还利用离线伏安法研究了 pH 值在 2.0 至 6.0 pH 范围内对 4-CA 氧化的影响。结论是,EC/ESI-MS 是研究这类反应的有力工具,研究这些体系的反应途径最好在非电容实验条件下进行,因为后者有利于检测反应中间产物。在聚苯胺和聚吡咯等导电聚合物的形成过程中,预计也会出现与 4-CA 系统中的反应类似的配比反应。