Anodic oxidation aniline

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

The existence of materials now included among the conducting polymers has long been known. The first electrochemical syntheses and their characterization as insoluble systems took place well over a century ago. In 1862 Letheby reported the anodic oxidation of aniline in a solution of diluted sulphuric acid, and that the blue-black, shiny powder deposited on a platinum electrode was insoluble in HjO, alcohol, and other organic solvents. Further experiments, including analytical studies, led Goppelsroeder to postulate in 1876 that oligomers were formed by the oxidation of aniline. 

Historically, the first condnctive polymer was prepared along route (2) but at the time not recognized Letheby in 1862 ( ) subjected aniline to anodic oxidation in sulfuric acid. 

Aniline black was first prepared in the last century [164] when it was found that anodic oxidation of aniline at a Pt electrode in aqueous H2S04 gave a dark green powdery product. Aniline blacks have subsequently produced under a variety of conditions via both chemical and electrochemical routes with a low level of interest... 

Electrochemistry is one of the most promising areas in the research of conducting polymers. Thus, the method of choice for preparing conducting polymers, with the exception of PA, is the anodic oxidation of suitable monomeric species such as pyrrole [3], thiophene [4], or aniline [5]. Several aspects of electrosynthesis are of relevance for electrochemists. First, there is the deposition process of the polymers at the electrode surface, which involves nucleation-and-growth steps [6]. Second, to analyze these phenomena correctly, one has to know the mechanism of electropolymerization [7, 8]. And thirdly, there is the problem of the optimization of the mechanical, electrical, and optical material properties produced by the special parameters of electropolymerization. 

PANI is usually produced by the anodic oxidation of aniline in acidic aqueous solution [5, 139], but can also be produced by chemical oxidation [138b, 140]. Hence, it is not surprising that the oxidation of PANI is pH-dependent and that, therefore, in addition to electron-transfer processes, proton-transfer reactions occur during charging. Although it is usually assumed that PANI has a chain structure (emeraldine) with head-tail connections... 

Anodic oxidation of phenylamines is irreversible and involves the loss of two electrons and a proton to give a delocalised carbonium ion, which reacts further. Oxidation of 2,4-dimethyl-aniline to give 51 illustrates the process [161]. Interae-... 

A number of related couplings have been reported during the synthesis of the alkaloid ( )-cryptopleurine149 and also intramolecular coupling of diaryl amides to dibenzazepine and dibenzazodne structures.150 A versatile method for the preparation of tetrahydroquinolines and jololidines has been developed.151 The method involves the anodic oxidation of AT,AT-dimethyl-aniline in methanol to afford a-methoxylated or a, a -dimethoxylated compounds and subsequent treatment of products with Lewis acids in the presence of olefins. 

Further anodic oxidation of the monomethoxylated compound in methanol gives N,N-bis(methoxymethyl)aniline 24, which yields a julolidine derivative 25 upon reaction with two molecules of ethyl vinyl ether. 

The above process was applied initially [142,144] to destroy 100 ppm aniline and 4-chloroaniline in alkaline solutions of pH between 11 and 13 by anodic oxidation in the presence of H202 electrogenerated at an ODC (54 mM H202 at 600 mA). Both substrates presented pseudo first-order decays with half-lives less than 30 min at 600 mA. After 11 hr of electrolysis at 300 mA, a TOC decay >95% was found (see Table 4). Nitrobenzene and p-chloronitrobenzene were detected, respectively, as intermediates, which degraded via maleic acid. Cl was quantitatively released from 4-chloroaniline, and NH3 was a final product of both substrates. General reaction pathways involving oxidation of organics by OH and H02 were proposed. 

Kirk DW, Sharifian H, Foulkes FR. Anodic oxidation of aniline for waste water treatment. J Appl Electrochem 1985 15 285-292. 

Brillas E, Mur E, Sauleda R, Sanchez L, Peral J, Domenech X, Casado J. Aniline mineralization by AOPs anodic oxidation, photocatalysis, electro-Fenton and photoelectro-Fenton processes. Appl Catal B Environ 1998 16 31-42. 

An enamine (4) prepared from aniline and an aromatic ketone yields indole-type compounds (5 and 6) upon anodic oxidation (equation 3)4. 

Non-Reversible Processes. —Reactions of the non-reversible type, i.e., with systems which do not give reversible equilibrium potentials, occur most frequently with un-ionized organic compounds the cathodic reduction of nitrobenzene to aniline and the anodic oxidation of alcohol to acetic acid are instances of this type of process. A number of inorganic reactions, such as the electrolytic reduction of nitric acid and nitrates to hydroxylamine and ammonia, and the anodic oxidation of chromic ions to chromate, are also probably irreversible in character. Although the problems of electrolytic oxidation and reduction have been the subject of much experimental investigation, the exact mechanisms of the reactions involved are still in dispute. For example, the electrolytic reduction of the compound RO to R may be represented by... 

The oxidation of aniline in sulfuric acid was studied by Kirk et al. on a packed-bed anode made up of 1 mm spherical lead pellets in contact with a pure lead anode collector plate [27]. The lead pellets were oxidized to lead dioxide for an hour in sulfuric acid (pH = 2) at 300 A/m. No mention was made of the corrosion resistance of these anodes. The anodic destruction of aniline to CO2 was found to occur through the formation of benzoquinone and maleic acid. At a current of 2 A, the initial rate of aniline (2.7 mM) oxidation was very rapid with more than 90% of the initial aniline being oxidized within one hour. After 5 h of operation, 72.5% of the... 

Table 1. Influence of electrolysis conditions on product distribution in the anodic oxidation of aniline and A -alkylanilines. ...

Table 1 Influence of Electrolysis Conditions on Product Distribution for Anodic Oxidation of Aniline and A-Alkylanilines...

Polyanilines (Scheme 36) are conjugated polymers whose it electrons are delocalized over the whole molecule. They are important conducting polymers that also act as semiconductors, in a similar manner to inorganic semiconductors121 m. They are made by chemical or electrochemical (anodic) oxidation of aniline. The product, a poor textile colorant, dates from the 1860s, and is still known by the name given at that time, emeraldine. In the electrochemical process, it is possible to produce thin films directly on conductive substrates. Polyanilines have been used in photoelectrochemical devices124-126. 

In this review, attention is focused primarily on the oxidation mechanisms under the given conditions, which is the essential topic of interest for organic chemists. Reaction pathways will be outlined if they seem to be well established. However, even small differences in medium properties used by different researchers can lead to serious variations as will be shown in some examples. Anodic oxidation of unsubstituted aniline is discussed in Section II and electrode reactions of /V-substifilled and C-substituted anilines in Sections III and IV, respectively. In the last case, the oxidation of reactants with monosubstituted ring is presented first (para-substituents separately from the effects of ortho- and mefa-substituents), and next the oxidation of di- and trisubstituted anilines. In each part the processes in dipolar aprotic solvents, in particular in acetonitrile (ACN) and /V. /V-dimethylformamide (DMF), are compared with those proceeding in aqueous solutions, chiefly in commonly used acidic media. 

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