Phenols electrochemical oxidation

Relatively simple electrochemical and amperometrical detectors have also been used in combination with reverse-phase LC separation for analysis of environmental water samples . Scrupulous studies of phenols electrochemical oxidation simplified this problem. ... 

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). 

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... 

FIGURE 2.1 Reaction scheme for OPH hydrolysis of methyl parathion and paraoxon (a) followed by the electrochemical oxidation of p-phenol (b). R and R are ethoxy and methoxy and X is O and S in paraoxon and methyl parathion, respectively. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

Itoh et al. (151) employed a tetradentate amine and synthesized the complex [Znn(l)(MeCN)]PF6, where 1 represents the monoanion of 2-methylthio-4-ferf-6-[[bis[2-(2-pyridyl)ethyl]amino]methyl]phenol (see Fig. 31 for 1). This complex was chemically [with (NH4)2[Ce(N03)6]) or electrochemically oxidized yielding the (phenoxyl)zinc species Zn1 (1 )(MeCN )]PF6. It displays phenoxyl K-K tran-... 

Electrochemical detectors, which are based on the electrochemical oxidation or reduction of the analyte, can be applied to the analysis of selected compounds such as phenols. It is physically simple, but is very sensitive for catecholamines. However, the adsorption of reacted molecules on the surface of the electrodes can reduce the conductivity. To overcome this problem a pulsed voltage is applied, which cleans the electrode surface between measurements. This pulsed amperometric detection is also sensitive for carbohydrates. 

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... 

The electrochemical oxidation of 4-(2 -alkenylphenyl)phenols (32) in MeCN/ MeOH affords spirodienones (33) arising from cyclization by the intramolecular addition of the 4-position of the phenol to the olefinic side chain and... 

The electrochemical oxidation of phenols produces quinones that can be used as dienophiles for the Diels-Alder reaction. A typical example is shown in Scheme 14, where a lithium perchlorate/nitromethane system and an electrode coated with a PTFE [poly-(tetrafluoroethylene)] fiber, to create a hydrophobic reaction layer. 

The advantages of using ultrasound in the electrochemistry of organic compounds has been reviewed elsewhere [40]. The combination has been found to be particularly useful in the destruction of chemical pollutants. It has been applied to the destruction of phenols by electrochemical oxidation. Ultrasound (25 kHz, lO W m ) when applied... 

Solutions of the nitrosodisulfonate salts are most stable in weakly alkaline solutions (pH 10) and decompose rapidly when the solution is acidic or strongly alkaline. The solid- dipotassium nitrosodisulfonate (Fremy s salt) has been reported to procedures involving the use of substantial quantities of the dry solid salt may be hazardous. In the present procedure, separation and use of the solid salt is avoided since the disodium nitrosodisulfonate is formed and used in aqueous solution. In this procedure, 2 moles of the preformed nitrosodisulfonate salt are consumed in the oxidation of one mole of the phenol to the benzoquinone huivative. The submitters report that only one molar equivalent of the nitrosodisulfonate salt is required if the electrochemical oxidation is carried out in the presence of a heptane solution of the phenol. 

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... 

Electrochemical oxidation of phenol substituted by a side chain containing an oxime group afforded a spiro-isoxazole in a quantitative yield (equation 18) . 

The direct electrochemical oxidation of phenols generates phenoxonium cations which are able to undergo [3-1-2] cycloaddition in the presence of unactivated alkenes to produce benzofurans <1999JOC7654>. Thus, electrolysis of methyl 2,5-dihydroxybenzoate in a solution of lithium perchlorate in nitromethane in the presence of acetic acid and 2-methyl-2-butene produces the dihydrobenzofuran in excellent yield (Equation 88). 

The electrochemical oxidation of substituted phenols was used on the laboratory scale for the production of specialties. 

Figure 8. A brief outline of the electrochemical oxidation of phenols showing some typical reaction products...

Sharifian H, Kirk DW. Electrochemical oxidation of phenol. J Electrochem Soc 1986 133 921-924. 

Comninellis C, Pulgarin C. Electrochemical oxidation of phenol for waste-water treatment using Sn02 anodes. J Appl Electrochem 1993 23 108-112. 

Iniesta J, Michaud PA, Panizza M, Cerisola G, Aldaz A, Comninellis C (2001) Electrochemical oxidation of phenol at boron-doped diamond electrode. Electrochim Acta 46 3573-3578... 

Morphinandienones have also been synthesized by the oxidation of mono-phenolic benzylisoquinolines with vanadium oxyfluoride in trifluoroacetic acid, other products of the reaction being proaporphines, and similar oxidations have been accomplished with phenethylisoquinolines.183 The same reagent will oxidize the lactam (150) to the dienone (151), which is an analogue of dehydrometatheb-ainone, and the same cyclization may be effected by electrochemical oxidation.184... 

ComnineUis, Ch. and Pulgarin, C. (1993) Electrochemical oxidation of phenol for wastewater treatment using tin dioxide anodes. J. Appl. Electrochem. 23, 108-112. 

Fig. 2.4 Influence of the initial phenol concentration on the EOI and EOD values obtained during electrochemical oxidation at platinum electrodes.

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