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

Abstract

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.

Perspective on Electrospray Ionization and Its Relation to Electrochemistry

“…We refer the interested reader to a selection of published reviews pertaining to electrospray ionization and/or EC/ESI coupling available elsewhere . The focus in the current paper is kept on features of electrochemistry/ electrospray that have been largely neglected but might be important for analytical practice, especially if one considers how the development of electrochemical derivatization of analytes has become an established and fruitful method [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38].…”

Electrochemical Aspects of Mass Spectrometry: Atmospheric Pressure Ionization and Ambient Ionization for Bioanalysis

“…EC‐MS cells have been used to monitor biologically‐relevant redox reactions, such as oxidation of dopamine,, metabolites, such as uric acid, pharmaceutical drugs,, such as zotepine, and environmental pollutants, such as triclocarban . Additionally, electropolymerization can be induced and monitored via EC‐MS, as found in the cases of methylene blue, aniline, 4‐chloroaniline, and caffeic acid . Reaction mechanism elucidation is another application of EC‐MS cells, as demonstrated by reduction of tetracyanoquinodimethane and oxidation of triphenylamine with a thin‐layer flow cell …”

Diamond Nanowires: A Novel Platform for Electrochemistry and Matrix-Free Mass Spectrometry

“…Desorption electrospray ionization (DESI) or nanoDESI and direct analysis in real time (DART) are other recent methods to perform ambient soft ionization with little or no sample preparation [ 69 ]. Applications of EC/MS are various and numerous and among them we can cite mechanistic elucidation of electrochemical reactions [ 70 , 71 , 72 , 73 ], mimicking of metabolic pathways [ 65 ], tagging of protein/peptide thiol groups using various electrochemical generated species to electrochemically enhance MS signals [ 74 , 75 ], pre-concentrate target via electrochemical deposition [ 66 ], and following protein/peptide cleavage and online MS analysis that is very notably useful in the proteomics field [ 76 , 77 ].…”