Electrochemical oxidation of aromatics

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

Intermediates generated at an electrode surface may react while still near the electrode. If so, one side of the intermediate may be wholly or partly shielded from attack by other reactants by the electrode itself. Such behavior is particularly common in the electrochemical oxidation of aromatic compounds since, as we have already seen with coumarin, aromatic compounds are generally tightly adsorbed parallel to the electrode surface at potentials positive of the p.z.c. For example, electrochemical oxidation of the stilbenes in alkaline methanol affords a mixture of dl and meso-1,2 dimethoxy-1,2-diphenylethane (1) 10>. It is found that c/s-stilbene affords a mixture of isomers of 1 in which the... 

A similar electrochemical oxidation of aromatic aldehydes to their corresponding methyl esters was achieved by using flavo-thiazolio-cyclophane as a mediator [87]. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

The electrochemical oxidations of aromatic compounds in the presence of a fluoride ion sources have been widely studied by a number of workers to produce a range of partially fluorinated compounds [9-12]. 

Scheme 1. Pathways for the indirect electrochemical oxidation of aromatic compounds by metal salts...

Table 3. Indirect electrochemical oxidation of aromatic compounds to quinones using metal salts as redox catalysts... 

A principally different approach for the indirect electrochemical oxidation of aromatic compounds goes via the formation of hydroxyl radicals from cathodically generated hydrogen peroxide and from reductively formed iron(II) ions. The thus in situ formed Fenton reagent can lead to side-chain as well as nuclear oxidations of aromatic compounds. Side-chain oxidations to form benzaldehydes according to Eqs. (18)—(24) can also be initiated by the redox pairs and Cu instead of... 

The electrochemical oxidation of aromatic aldehydes (1) must be studied in strongly alkaline media. Acidity functions for strongly alkaline aqueous solutions of alkali metal and quaternary ammonium hydroxides, corresponding to dissociation of proton (H ), are well established (2, 3). Substituted anilines and diphenylamines (4,5) and indoles (6) were used as acid-base indicators for establishment of such scales, but whether an acidity scale based on one type of indicator can be rigorously applied to acid-base equilibria involving structurally different acidic groups for reactions in strongly alkaline media remains questionable. For substituted anilines, behavior both parallel (7) and nonparallel (8) to the H scale based on indole derivatives has been reported. The limited solubility of anilines in aqueous solutions of alkali metal hydroxides, the reactions of the aniline derivative with more than one hydroxide ion, irreversible substitution reactions (9), and the possibility of hydroxide ion addition rather than... 

A mixture of arene oxides and quinones is reported to have been formed by the electrochemical oxidation of aromatic hydrocarbons in an atmosphere of oxygen or oxygen-containing gaseous mixtures. The method is patented and details have not been disclosed.37... 

Catechols and Hydroquinones. Just as quinones are ideal examples of electrophilic substrates, their fully reduced form (catechols and hydroquinones) illustrates the electrochemical oxidation of aromatic nucleophilic substrates (Lewis bases). Figure 12.3a, b illustrates the oxidation of 3,5-di+m-butyl-catechol (DTBCH2) via an irreversible two-electron process (ECEC) to give the o-quinone (DTBQ) 12... 

Panizza, M. and Cerisola, G. (2006a) Electrochemical oxidation of aromatic sulphonated acids on a boron-doped diamond electrode. Int. J. Environ. Pollut. 27, 64-74. 

Fenton reagent generated in situ — Indirect electrochemical oxidation of aromatic compounds (e.g., benzene to phenol) proceeds with the Fenton reagent generated in situ electrochemically at the cathode by the reduction of ferric to ferrous salt and by the reduction of oxygen to hydrogen peroxide... 

The electrochemical oxidation of aromatic compounds in the presence of ammonium nitrate or N2O4 results in the nuclear nitration shown in equation (34). ... 

Aromatic Nuclear Substitution. Electron-rich aromatic derivatives react with this reagent in acetic acid containing 0.5 M KOAc to give aryl acetates. Some examples of this stoichiometric reaction are given in Table 1. The reaction closely resembles the electrochemical oxidation of aromatic compounds isomer distributions are quite similar. Of particular interest is the fact that, in many cases, yields higher than 100% were observed. This was explained by assuming Ag as the primary oxidant of the aromatic compound followed by reoxidation of the formed Ag hy the counterion peroxydisulfate. ... 

Pagliaro and coworkers [87] mixed APTMS with 4-oxo-2,2,6,6-tetramethylpi-peridine-l-oxyl (4-oxo-TEMPO) radical in a methanol solution of NaBHsCN forming TEMPO-fimctionalized silane. The latter was co-electrodeposited with MTMS on ITO. The obtained films were observed to be efficient in electrochemical oxidation of aromatic alcohols in water. Figure 12.32 shows the degradation of benzyl alcohol by electrolysis with the TEMPO-functionalized silane-modified ITO electrode at 1.4 V versus Ag/AgQ. It is seen that after 200 h of electrolysis the concentration of benzyl alcohol decreases to about 10% of its initial value. 

Yoshida and coworkers developed a method for C-H nitrogenation of aromatics based on electrochemical oxidation of aromatic compounds in the presence of pyridine followed by the reaction of the resulting A -arylpyridinium ions with piperidine to selectively give aromatic primary amines as products [108]. This transformation provides a metal-free protocol for one-pot synthesis aromatic amines with broad functional groups compatibility (Scheme 2.10). 

Since 19S7, studies of electrochemical oxidation of aromatic monomers, now widely used as one method of synthesis of CPs, have been reported under various descriptions such as "electro-organic preparations" and "electro-oxidations" [9-11]. More recently, in 1967, electrically conducting polymers from pyrrole, thiophene and fimm were characterized [12] and the electrical conductivity of poly(anilines) noted [13]. As early as 1968, dall Ollio [14] described electropolymerization of poly(pyrrole). In many a sense, thus, CPs are "rediscovered" materials. 

The capability of forming these chemically reactive hydroxyl radicals on the BDD surface has made possible an important new electrochemical technology. The main applications are the electrochemical oxidation of aromatic compounds for synthetic applications, the production of strong oxidants, and the electro-incineration of organic pollutants for wastewater treatment. 

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