The Electrooxidation of Halogeno‐Anilines at Microelectrodes in Acetonitrile Solution

Abstract

The electrochemical oxidation of 4-chloro-, 2,4-dichloro-, 2,4,6-trichloro-anilines and 4-bromo-, 2,4-dibromo-, 2,4,6-tribromo-anilines and 4-iodo-aniline were investigated in unbuffered acetonitrile solution. The main reaction at the electrooxidation of these compounds is the dimerization which is accompanied by release of the p-substituent halide and formation of protons. To complete the so far achieved results by the apparent electron exchange number (n app ) determination in the voltammetric time scale have been attempted. These measurements were carried out with the aim, to have a clearer picture on the n app influencing role of the following complicating factors: The rejected halogenides in MeCN solvent at lower potential values as the substrates can be oxidized to the corresponding halogens, and these halogens except the iodine substitute the substrate which takes place even in the voltammetric time scale by increasing the n app value characteristic for the dimerization only. The measurable n app values are influenced by the protons produced at the oxidation as well, in the opposite direction. The explanation for the latter is, that the solvent is aprotic, therefore the differently basic primary amino groups of the diffusing substrate molecules binding partly the protons and in this way voltammetrically inactive substrate species are formed. This reaction is a typical equilibrium type and this inactivation effect decrease the measurable n app value. This effect depends on the quality and number of halogene substituent at a given substrate because these affect the protonation constant of the substrate. Therefore the protonation constants of the substrates had to be determined. In the knowledge of these quantitative data the explanation of the trends in the n app values measured in the voltammetric time scale is attempted. The mechanistic study of the dimerization reaction of these substrates is in progress in our laboratory.

In Situ Electrochemical ESR and Voltammetric Studies on the Anodic Oxidation of para-Haloanilines in Acetonitrile

“…The voltammetry observed for the corresponding bromo-and iodoaniline derivatives was found to be qualitatively very similar to that of para-chloroaniline with peak potentials of 1.03 and 1.02 V, respectively. This behavior is in good agreement with that reported by Farsang and co-workers [14][15][16][17][18] for the oxidation of para-haloanilines in aprotic solvents, where the anodic wave was attributed to the formation of the corresponding radical cation which was shown to be unstable with respect to dimerization on conventional voltammetric time scales. The cathodic feature on the reverse sweep was then ascribed to the two-electron, two-proton reduction of the electrogenerated dimer, the product of which is reoxidized upon subsequent scanning (see Scheme 1).…”

“…The mechanism proposed differs from that postulated by Farsang et. al., − in which the electrogenerated radical cation reacts with a neutral radical, formed via deprotonation of the radical cation, to yield the dimer in a higher, non-paramagnetic oxidation state.…”

“…The electrogenerated radical cation is thought to undergo a deprotonation step, yielding the neutral radical, as was suggested by Farsang et. al., [14][15][16][17][18] and this reacts with a parent haloaniline molecule, present in a large excess under our experimental conditions, in a "head-to-tail" sense. The adduct formed can then collapse with the loss of a halide anion to yield the para halogenated paraaminodiphenylamine radical cation, whose structure is not inconsistent with the ESR spectra observed.…”

“…In a recent collection of papers, Farsang and co-workers − studied the electrochemical oxidation of para -haloanilines in DMF 14 and acetonitrile, − developing earlier work by Adams and Bacon in aqueous media . In all cases, the anodic oxidation was reported to proceed via a single electron transfer to generate the corresponding radical cation, which was found to undergo further chemical reaction to ultimately form an electrogenerated dimer product.…”

“…[1][2][3][4][5][6][7][8] The complexity of the anodic oxidation of aniline has provoked a number of investigations in which a range of substituents occupy various positions of the aromatic ring in order to probe mechanistic aspects of the electrodimerization and ultimately the polymerization which takes place. [9][10][11][12][13] In a recent collection of papers, Farsang and co-workers [14][15][16][17][18][19] studied the electrochemical oxidation of para-haloanilines in DMF 14 and acetonitrile, [15][16][17][18][19] developing earlier work by Adams and Bacon in aqueous media. 13 In all cases, the anodic oxidation was reported to proceed via a single electron transfer to generate the corresponding radical cation, which was found to undergo further chemical reaction to ultimately form an electrogenerated dimer product.…”