Electrochemical synthesis examples

Electrochemistry is widely used in industry, for example in effluent treatment, corrosion prevention and electroplating as well as in electrochemical synthesis. Electrochemical synthesis is a well-established technology for major processes such as aluminium and chlorine production there is, however, increased interest in the use of electrochemistry for clean synthesis of fine chemicals. The possible green benefits of using electrochemical synthesis include ... 

As an illustrative example of this method for electrochemical synthesis of sulfide compounds consisfed in utilizing a sulfur-modified mefal surface as a template for fhe elecfrodeposition of mefal sulfide films, Tacconi and Rajeshwar described fhe firsf aflempl fo obfain all-elecfrodeposifed indium sulfide tiiin films, by a dual batii procedure [95] (cf conventional deposition of indium chalcogenides). The... 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

A typical example is the synthesis of oxalic acid. Electrochemical synthesis of oxalic acid by reduction of C02 in aprotic media with a Zn sacrificial anode was brought to pilot scale by the Dechema Institute some years ago (1981) [177]... 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

It should be mentioned that by changing the conditions of electrochemical synthesis — such as nature of the alcohol, the purity of the metal used as anode, the nature and concentration of the conductive additive, the voltage (usually 30 — 110 V DC is applied), the temperature, and even the construction of the cell — one can significantly effect the process of the anodic dissolution or even change its mechanism. As it has already been mentioned, the electrochemical dissolution of iron in alifatic alcohols gives insoluble iron (II) alkoxides [1005], while in 2-methoxyethanol a soluble iron (HI) complex is obtained [1514], Another example is provided by tantalum dissolution in isopropanol while high-purity metal is rapidly dissolved anodically [1639], the one containing impurities is passivated. Therefore, it is quite clear that each synthesis requires careful study, optimization of parameters of electrochemical synthesis, and isolation and purification of final products. 

A combination of different techniques can frequently improve yields of final compounds or synthetic conditions, for example a reunion of direct electrochemical synthesis and simultaneous ultrasonic treatment of the reaction system [715]. Reunion of microwave and ultrasonic treatment was an aim to construct an original microwave-ultrasound reactor suitable for organic synthesis (pyrolysis and esterification) (Fig. 3.7) [716], The US system is a cup horn type the emission of ultrasound waves occurs at the bottom of the reactor. The US probe is not in direct contact with the reactive mixture. It is placed a distance from the electromagnetic field in order to avoid interactions and short circuits. The propagation of the US waves into the reactor occurs by means of decalin introduced into the double jacket. This liquid was chosen by the authors of Ref. 716 because of its low viscosity that induces good propagation of ultrasonic waves and inertia towards microwaves. 

Di-iminoisoindoline was used as a precursor for Pc in different protic and aprotic systems, without catalysts or promoters, to study the solvent effect on the possibility of phthalocyanine formation [32], As can be observed (Example 13), it is possible to carry out the chemical and electrochemical synthesis of metal-free Pc in aprotic solvents, such as DMF or DMSO, in contrast to the results with PN. It is surprising that the yields of Pc in ROH are comparatively small. The N,N-dimethyletanolamine is characterized by the best yields, as in the case when PN was used as precursor. 

Examples of the reaction arc given in Table 12. Similar yields are obtained when l-(difluoro-iodo)-4-nitrobenzene is used as the fluorinating agent. Only catalytic amounts of the reagent are required if an electrochemical synthesis is used where the (difluoroiodo)arene serves as an in-cell mediator for the fluorination of the dithiolane. 

Those interested in pursuing sonoelectrochemistry should note that ultrasound is sometimes employed in electrochemical systems without this fact being obvious from the title and abstracts of a published paper. Often, in these reports, the purpose for the ultrasound and the significance of its use are not discussed in detail. One such example is, Competitive Electrochemical Synthesis of Polydimethylsilane without Solvent, where sonication was found to increase the yield, although without explanation [254], The reader is advised that there may be more such reports in the literature, and the reviewers apologize for inadvertent omission of work in the field. 

This chapter has provided some examples of the ways in which conjugated polymers can be prepared. While the account is not of course exhaustive, and indeed many extremely important synthetic routes have not been included, such as the formation of polyacetylene by the Durham route,it does serve to illustrate that the range of synthetic techniques vary from the simple to the extremely sophisticated. Electrochemical synthesis is largely in the former classihcation, however, it does have considerable potential in the design of materials for molecular electronics since it will allow patterns to be formed on the electrode surface. With the continuing demand for new materials both for electronic and power distribution needs, it is to be expected that this area will continue to develop in the foreseeable future. 

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