Benzene electrochemical oxidation

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Reactions of partial electrochemical oxidation are of considerable interest in the electrosynthesis of various organic compounds. Thus, at gold electrodes in acidic solutions, olefins can be oxidized to aldehydes, acids, oxides, and other compounds. A good deal of work was invested in the oxidation of aromatic compounds (benzene, anthracene, etc.) to the corresponding quinones. To this end, various mediating redox systems (e.g., the Ce /Ce system) are employed (see Section 13.6). 

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. 

In this context see also Refs. [83a, 83b]. Comninellis and Plattner [287,287a, 288] have developed a simple method for estimating the facility of the electrochemical oxidation of organic species based on a newly defined electrochemical oxidizability index (EOI) and the degree of oxidation using the electrochemical oxygen demand (EOD). Electrochemical oxidizability index for various benzene derivatives obtained at Pt/Ti and Sn02-ABB-anodes are listed in Table 23. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

Electrochemical oxidation affords a simple route for the conversion of benzene derivatives to the corresponding phenol via the phenyl acetate. In practice however high yields are difficult to achieve because the product readily undergoes further... 

Phenoxanthin, 68 X = S Y = O, is prepared by the electrochemical oxidation of diphenyl ether in dichloromethane and trichloroacetic acid containing tetraethyl-ammonium perchlorate at a composite anode of carbon and sulphur. The anode generates sulphur cations, which carry out electrophilic substitution on the benzene ring [237], Phenoxathiin radical-cation, formed at the potential of the fust oxidation wave, has been characertised by esr spectroscopy [238],... 

In one study of the effects of additives,9 it was found that on electrochemical oxidation of rubrene, emission was seen in dimethylforma-mide, but not in acetonitrile. When water, n-butylamine, triethylamine, or dimethylformamide was added to the rubrene solution in acetonitrile, emission could be detected on simply generating the rubrene cation.9 This seems to imply that this emission involves some donor or donor function present in all but the uncontaminated acetonitrile system. The solvent is not the only source of impurity. Rubrene, which has been most extensively employed for these emission studies, is usually found in an impure condition. Because of its relative insolubility and its tendency to undergo reaction when subjected to certain purification procedures, and because the impurities are electroinactive and have relatively weak ultraviolet absorptions, their presence has apparently been overlooked, They became evident, however, when quantitative spectroscopic work was attempted.70 It was found, for example, that the molar extinction coefficient of rubrene in benzene at 528 mjj. rose from 11,344 in an apparently pure commercial sample to 11,980 (> 5% increase) after repeated further recrystallizations. In addition, weak absorption bands at 287 and 367 m, previously present in rubrene spectra, disappeared. 

However, because of the very similar ionisation potentials (and electrochemical oxidation potentials) of the precursors and products (e. g., benzene, 9.25 eV fluorobenzene, 9.21 eV p-difluorobenzene, 9.15 eV) selective mono-fluorination is often difficult to achieve. 

It is well known that U.S. space vehicles obtain their auxiliaiy power in space by the use of fuel cells (Chapter 13), electrochemical devices in which the spontaneous tendency of hydrogen to combine with oxygen drives the cell and produces electricity, with water as a by-product (pure enough to drink). It stands to reason then, that one might think of producing substances more economically valuable than water in this electrogenerative way. Such work is into its first decade and Fig. 7.190 shows an example benzene is oxidized to phenol with electricity as a by-product Clearly, the economics of such a process depend on the cost of the H2 and whether one can sell the electricity. This gives rise to a speculation. 

Stable disilenes are usually highly reactive toward oxidation and reduction because the 7i and n levels of disilene are much higher and lower than those of ethylene, as shown theoretically (Section III.A) and by the electrochemical oxidation and reduction potentials (Section III.E). Several stable disilenes are however stable in air for a long period. Disilene 27 undergoes very slow decomposition (half-life ca. 84 h) in wet THF solution at rt,60,61 and air oxidation of disilene 63 is completed in benzene at rt for a week.35 Disilene 27 survives for 4 months in the solid state. Since the oxidations and reductions of disilenes and their mechanisms have been extensively discussed in review OW, we mostly show herein the results of recent studies. 

Bard, A. J., Flarshein, W. M. and Johnston, K. P. (1988) High-pressure electrochemical oxidation of benzene at a lead dioxide electrode in aqueous bisulfate solution at 25°C to 250°C. J. Electrochem. Soc. 135, 1939-1944. 

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 first example of a Friedel-Crafts type reaction in an ionic liquid medium dates back to 1976 when the electrochemical oxidation of hexamethylbenzene in [C2py]Br-AlCl3 afforded a mixture of alkylated polyphenyl compounds.[69] Other early examples include the alkylation of benzene in C2Ciim C1-A1C137 and the acylation of ferrocene in [C2Ciim]I-AlClJ71 There are now numerous examples of Lewis- or Bronsted acid-catalysed Friedel-Crafts type reactions in ionic liquids. These include alkylation,[72 76] acylation,[71,77"83] arylation,[77 841 sulfonylation,[851 sulfoamylation[86] and O-acetylation of alcohols.[87,881... 

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). 

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