Benzene, anodic processes

As for the anode process at comparable conditions, the yield of 2,5-dimethoxy nitro benzene depends distinctly on the electrical nature of the micelle. Namely, the yields are equal to 30% for the positively charged micelle, 40% for the negatively charged micelle, and 70% for the neutral charged micelle. The observed micellar effect corroborates the mechanism, including the dimethoxy benzene cation radical and the nitrogen dioxide radical as reacting species. 

The anodic oxidation of organic substances is a complex multistep process. The question as to the depth of oxidation required (and sufficient) lias to be answered in each case. Where intermediate oxidation products pose no ecological risk, one can stop at incomplete oxidation. However, in the anodic oxidation of many aromatic substances, the corresponding quinones are formed in the first step, and these are more harmful than the original substances. Upon more profound oxidation, the benzene rings are broken and aliphatic substances are formed that are almost as harmless as carbon dioxide. 

Direct production of benzoquinone (BQ) from benzene is one of the targets in industrial chemistry. Considerable efforts have been made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process [40]. Benzene in aqueous emulsions containing sulfuric acid (1 1 mixture of benzene and 10% aqueous H2S04) forms, at the anode, p-benzoquinone which can be reduced cathodically to yield hydroquinone in a paired synthesis. A divided cell with Pb02 anodes is used. 

Anodic oxidation of 1,2,3-trimethoxybenzene in acetone containing dilute sulphuric acid gives 2,6-dimethoxybenzoquinone but in contrast 1,2,4-trimethoxy-benzene affords the dehydrodimer 11 in good yield [82]. Dehydrodimerization becomes an important process in the oxidation of methoxybenzenes in dichlo-... 

The electrofluorination of acetophenone and benzophenone takes place in anhydrous HF and in the presence of solvents such as chloroform and acetonitrile [38]. The fluorination of the aromatic rings occurred to various extent. Further uses of anhydrous hydrogen fluoride as a liquid environment for electrofluorination processes have been reported, for example, by Matalin etal. [39]. In particular, systems with low conductivity in liquid hydrogen fluoride and nonselective processes have been studied and optimized. The fluorination of benzene and halobenzenes in the presence of Et4NF—(HF) in an undivided cell has been studied by Horio et al. [40] Cathodic dehalogenation is observed to accompany the anodic fluorination process. 

Asahi Chemical Industries KK have patented [25] a process for the fluorina-tion of benzene using a nitrile solvent, a base salt, typically (n-C4H9)4NF 3HF, at a rhodium anode, said to be superior to platinum in corrosion resistance and performance. 

Recently, Fleszar and Ploszynska [546] re-examined the kinetics of electro-oxidation of benzene and phenol at Pb02 anodes. These authors concluded that the oxidation process does follow an E.C.E.-type mechanism, there not being enough evidence to indicate direct participation of Pb02 in the oxidation of these molecules. They observed a linear dependence between oxidation rate and the rate of water electrolysis. Fleszar and Ploszynska [546] advanced the hypothesis that hydroxyl radicals formed on the anode surface caused direct hydroxylation of the benzene and phenol compounds as shown in the following scheme. In a rationalization of this mechanism, the authors invoked semiconductor-based arguments, viz. 

If production of an oxidizing hole in the da orbital is the important factor in the photochemical reaction, then electrochemical veneration of such a hole should produce a highly reactive intermediate mat would mimic the initial step in the 3(da po) photoreaction. Several of the binuclear complexes undergo reversible one-electron oxidations in noncoordinating solvents (22-24). The complex Rh2(TMB)42+ possesses a quasireversible one-electron oxidation at 0.74 V (Electrochemical measurements for [Rh2(TMB)4](PF6)2 CH2CI2/TBAPF6 (0.1 M), glassy carbon electrode, 25°C, SSCE reference electrode). Electrochemical oxidation of Rh2(TMB)42+ in the presence of 1,4-cyclohexadiene exhibits an enhanced anodic current with loss of the cathodic wave, behavior indicative of an electrocatalytic process (25). Bulk electrolysis of Rh2(TMB)42+ in an excess of 1,4-cyclohexadiene results in the formation of benzene and two protons (Equation 4). 

The isomer distribution for anodic acetoxylation of a number of monosubstituted benzenes has been determined [122]. The reaction closely resembles ordinary electrophilic aromatic substitution processes, perhaps on the side of low-selectivity reactions. The isotope effect, A h//cd, for nuclear acetoxylation in anisole was found to be 1.0, whereas for a-substitution in ethylbenzene a value of 2.6 was observed. The interpretation of these values is not straightforward [126]. 

The introduction of hydroxyl-groups into azo-benzene, which Heilpern succeeded in accomplishing, can also be regarded as an oxidation process. If azo-benzene be dissolved in as small a quantity of cone, sulphuric acid as possible and this solution be subjected to electrolytic action at the anode, chiefly tetra-oxy-azo-benzene is formed ... 

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