Aromatic compounds electrochemical methods

Related pyrazole-containing compounds such as 500 and 161 (Figure 33) have shown similar properties in 1 M H2SC>4 using electrochemical methods the choice of these molecules is based on the presence of an electron cloud on the aromatic rings, the presence of 71 electrons of N = N, C = N, C = O, and C = S is expected to affect the corrosion of carbon steel <2003MI1>. 

Barek et al. have reported on the determination of AT-nitroso compounds, azo compounds, heterocychcs, aromatic nitro compounds, heterocychc amines and even benzyl chloride using electrochemical methods such as voltammetry and polarog-raphy. The nitro and AT-nitroso compounds work particularly well in reductive mode [47, 48]. For appropriate analytes, adsorptive stripping voltammetry and anodic stripping voltammetry can give orders of magnitude lower detection hmits than are available from HPLC with electrochemical detection [48]. 

The intercalation of polycyclic aromatic compounds into duplex DNA structures was used to develop nucleic acid-based electrochemical sensors.66 For example, the bis-ferrocene-tethered naphthalene diimide (16) was used as a redox-active intercalator to probe DNA hybridization.67 The thiolated probe was assembled on a Au electrode, and the formation of the duplex DNA with the complementary analyte nucleic acid was probed by the intercalation of (16) into the double-stranded nucleic acid structure and by following the voltammetric response of the ferrocene units (Fig. 12.17a). The method enabled the analysis of the target DNA with a sensitivity that corresponded to ca. 1 x 10-20mol. 

Oxidation to Quinones. Direct oxidation of arenes to quinones can be accom-plished by a number of reagents. Very little is known, however, about the mechanism of these oxidations. Benzene exhibits very low reactivity, and its alkyl-substituted derivatives undergo benzylic oxidation. Electrochemical methods appear to be promising in the production of p-benzoquinone.797 In contrast, polynuclear aromatic compounds are readily converted to the corresponding quinones. 

The electrochemical formation of a radical ion from an aromatic compound or other highly conjugated species is, generally, fast and, therefore, the kinetics of the heterogeneous electron transfer process usually do not interfere with the kinetics of the follow-up reactions to be studied. For species with only one or two double bonds, the initial electron transfer process is often slow and may even be rate determining. In such cases, the kinetics of the follow-up reactions may be studied only with some difficulty. One method is to use a so-called mediator (Med) which serves to shuttle electrons between the substrate and the electrode. Thus, the slow electron transfer between the substrate and the electrode is replaced by two fast electron transfer processes, between the mediator and the electrode, and between the oxidised or reduced mediator and the substrate. In this event, the single reaction of Equation 6.4 is replaced by the two reactions in Scheme 6.8 [32 ]. It is seen that the mediator is recycled and consequently needs be present in only small, non-stoichiometric amounts. 

The possibility to functionalize aromatic compounds by electrochemical methods is of great interest to chemical industry. Therefore, considerable efforts were made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process. The electrochemical oxidation of benzene in aqueous emulsions containing sulfuric acid using divided cells and Pb02 anodes formsp-benzoquinone. The product can then be reduced cathodically to yield hydroquinone in a paired synthesis. 

The results given indicate the electrochemical method for the synthesis of nanostructured carbon materials to be promising. In the reaction space, dissipative self-assembly of carbon compounds takes place by the action of electric discharges. In this case, structures of new type can be formed, which are transitional between polycyclic aromatic hydrocarbons and fullerenes, nanotubes. 

Electron spin resonance (ESR, also known as electron paramagnetic resonance, EPR) is used for the detection and identification of electrogenerated products or intermediates that contain an odd number of electrons that is, radicals, radical ions, and certain transition metal species. Because ESR spectroscopy is a very sensitive technique, allowing detection of radical ions at about the 10 M level under favorable circumstances, and because it produces information-rich, distinctive, and easily interpretable spectra, it has found extensive application to electrochemistry, especially in studies of aromatic compounds in nonaqueous solutions. Also, electrochemical methods are particularly convenient for the generation of radical ions thus they have been used frequently by ESR spectroscopists for the preparation of samples for study. Several reviews dealing with the principles of ESR and the application to electrochemical investigations have appeared (134-138). 

Composites of PANI-NFs, synthesized using a rapid mixing method, with amines have recently been presented as novel materials for phosgene detection [472]. Chemiresistor sensors with nanofibrous PANI films as a sensitive layer, prepared by chemical oxidative polymerization of aniline on Si substrates, which were surface-modified by amino-silane self-assembled monolayers, showed sensitivity to very low concentration (0.5 ppm) of ammonia gas [297]. Ultrafast sensor responses to ammonia gas of the dispersed PANI-CSA nanorods [303] and patterned PANI nanobowl monolayers containing Au nanoparticles [473] have recently been demonstrated. The gas response of the PANI-NTs to a series of chemical vapors such as ammonia, hydrazine, and triethylamine was studied [319,323]. The results indicated that the PANI-NTs show superior performance as chemical sensors. Electrospun isolated PANI-CSA nanofiber sensors of various aliphatic alcohol vapors have been proven to be comparable to or faster than those prepared from PANI-NF mats [474]. An electrochemical method for the detection of ultratrace amount of 2,4,6-trinitrotoluene with synthetic copolypeptide-doped PANI-NFs has recently been reported [475]. PANI-NFs, prepared through the in situ oxidative polymerization method, were used for the detection of aromatic organic compounds [476]. 

In the book by Keith Identification and Analysis of Organic Pollutants in Air out of the 473 pages only a short chapter of 13 pages, reporting a preliminary study of the potential of electrochemical methods in the analysis of the mixtures of polycyclic aromatic compounds (PAG), is devoted to polarography and voltammetry. 

Besides that, electrochemical methods can be used to activate aromatic substrates by means of redox-umpolung procedure, which makes possible to cOTivert imreactive arenes, azoles, phenols, and other aromatic compounds, bearing electron-donating groups, into the intermediate radical cation species, which are able to react with nucleophiles (Scheme 61) [47, 187]. 

Yields of products obtained by electrochemical methods are comparable with those reported in the literature for similar alkylations of aromatic compounds in the presence of chemical oxidants. In spite of experimental difficulties, we have succeeded to alkylate dinitrobenzene. The reaction has to be carried out in THF (highly resistant medium) because DMF undergoes decomposition in the presence of butyl lithium. Use of butyl lithium provides higher yields of the target alkylation products, than butylmagnesium chloride. Also equivalent amounts of reactants, nitroarenes and organometallic reagents, have been found to be essential to provide the best yields of the reaction products. 

Finally, the chapter Electrochemical C-H Eunctionalization of Arenes by lluminada Gallardo and Gonzalo Guirado (University of Barcelona, Spain) provide us with an excellent review showing a practical importance of electrochemical methods for the direct C-H functionalization of aromatic and heteroaromatic compounds - a very promising area that has been advanced significantly by these authors. 

By the electrochemical method, the oxidation potential of the polymerization reaction can be controlled and the quality of the polymer optimized moreover, the polymers are obtained in the doped state as films suitable for further electrochemical modifications. Therefore, in this field, electrochemistry plays both a synthetic and an analytical role. Soon the electrochemical polymerization was extended to other aromatic compounds such as furan, indole [12], carbazole, azulene, pyrene [13] benzene [14], and fluorene [15], and electrochemical synthesis is now the most widely used technique for the preparation of conducting PHCs. 

The contrast between the electrochemical method and the method in the literature is very striking in one particular case, namely the formation of the tin(II) compounds by the electrochemical oxidation of the metal in solutions of 1,2-aromatic diols in acetonitrile (R(OH)2 = catechol, 2,3-dihydroxynaphthalene, Br4C5(OH)2, 2,2 -dihydroxybiphenyl). The room temperature, high yield, electrochemical synthesis of Sn(02R) compounds is a great improvement over the high temperature methods used in the earlier syntheses of such compounds [64]. The ready accessibility of the Sn(02R) materials lead to a study of their redox reactions, and their coordination chemistry. Not surprisingly, the direct synthesis of Pb(02R), and its redox chemistry, follow from the tin(II) system [65]. In each case the Ep value of 0.5 mol F can be understood by the sequence... 

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