Solvent-Supporting Electrolytes

As can be seen by inspection of Table 1, the anodic and cathodic limit of a particular SSE depends on an electrochemical process involving either solvent or supporting electrolyte. In for example acetonitrile the anodic limit is dependent on the nature of the anion and the cathodic limit on the nature of the cation, whereas in dimethyl sulfoxide (see Table 2) the anodic limit is due to oxidation of solvent in cases where an oxidation-resistant anion is present, and the cathodic limit is dependent on a process involving reduction of the cation. Thus, one can order anions in a series of increasing resistance towards anodic oxidation  

Solvent Electrolyte Cathodic limit Anodic limit 

The data in Tables 1 and 2 are given for platinum as both anode and cathode material. For mercury, which is the most commonly used cathode metal, the cathodic limits are normally displaced to somewhat more cathodic potentials than on platinum. Mercury is seldom useful as anode material, since it is oxidized at potentials above +0.4 V (SCE) and goes into solution. 

The last entry of table 1, tetrahexylammonium benzoate, is an example of the use of molten salts as electrolytes. In this particular case, the salt is liquid at room temperature, but it has been reported that tetrabutylammonium nitrate at 150° can be used for polarographic and preparative work 7Sa- (oxidation of polycyclic aromatic hydrocarbons). The use of molten salts as SSE s is of great interest because of the high conductivities of such media as compared to conventional SSE s and deserves further studies. 

In aqueous or aqueous-organic SSE s the accessible potential range is dependent on the electrochemical oxidation and reduction of water (or hydroxyl ions and protons in acid or alkaline media) with formation of oxygen and hydrogen, respectively. The potentials at which these processes take place are different for different electrode materials 9 in that the anodic limit for aqueous systems 

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Cyclic sulphone Solvent Supporting electrolyte Alkylation agent after cleavage reaction Products Isolated yield... 

Fig. 14. CV scan with solvent, supporting electrolyte and carrier (oxygen-free).

The electrosynthesized (0EP)Ge(CgHs)C10, was characterized in situ by thin-layer spectroelectrochemistry. The final product of electrosynthesis was spectrally compared with the same compounds which were synthesized using chemical and photochemical methods(35). (0EP)Ge(C6H5)Ci and (0EP)Ge(CsHs)0H were also electrochemically generated by the use of specific solvent/supporting electrolyte systems(35). 

There are many publications which discuss not only the principles of electrochemistry, but also its practical aspects (electrodes, cells, solvents, supporting electrolytes, and so on).1-8 In addition, all the electrochemical instrumentation manufacturers have catalogues of commercial products which allow proper experiments to be carried out. Therefore, the present discussion will be limited to point out the minimal basic knowledge required to set up an electrochemical experiment. 

Solvent Supporting electrolyte Potential region Electrode... 

Both inter- and intramolecular [5 + 2] cycloaddition modes have been utilized in the synthesis of natural products. Successful intermolecular cycloaddition depends on making an appropriate selection of solvent, supporting electrolyte, oxidation potential, and current density. This is nicely illustrated in Schemes 23 to 25. For example, in methanol the controlled potential oxidation of phenol (101) affords a high yield (87%) of (102), the adduct wherein methanol has intercepted the reactive intermediate [51]. In contrast, a constant current electrolysis conducted in acetonitrile rather than methanol, led to an 83% yield of quinone (103). 

In order to find the best reaction conditions for the electrosynthesis of furoxanes, Nyazymbetov et al. [141] have extensively studied all parameters which affect the yield of furoxane such as solvent, supporting electrolyte. 

Also included in Table 2 are the reduction potentials of Cgo using different combinations of solvent, supporting electrolyte, and temperature. It is evident from these results that the above factors have a notable effect on the cathodic electrochemistry of Cgo [30-37]. 

Tab. 2 Half-wave reduction potentials (in V vs. Fc/Fc ) of C q using various solvents, supporting electrolytes, and temperatures...

In a recent review, Tao etal. [34] describe the partial fluorination and the perfluorination of organics with particular emphasis on medically important compounds and pharmaceuticals. The selective electrofluorination (SEF) of olefins and active methylene groups is reviewed by Noel et al. [35] In the case of heterocycles, nuclear fluorination is known to be the predominant process. However, in aromatic compounds, nuclear substitution as well as addition proceeds simultaneously, leading to the formation of a mixture of products. The influence of solvents, supporting electrolytes, and adsorption on product yield and selectivity is summarized and evaluated. Dimethoxyethane is found to be a superior solvent for SEF processes. Redox mediators have been employed to minimize anode passivation and to achieve better current efficiencies. 

Solvent/supporting electrolyte E°, V versus NHE Refe- rences... 

Re 111,111) complex Solvent/supporting electrolyte t, Vversus NHE References... 

The vast majority of electrochemical data on americium ions has heen obtained in aqueous solutions. Americium can exist in aqueous solutions in the oxidation states III, IV, V, and VI. The divalent state is difficult to attain in aqueous solutions because of the proximity of the standard potential of the Am(III)/Am(II) couple to the solvent/supporting electrolyte breakdown potential. Previous reviews have presented the formal and standard potentials for the various americium couples and these reviews should be consulted by the interested reader for more detailed discussion [133, 134]. Table 3 contains a summary of selected formal potentials Ef from these reviews in 1 M HCIO4 for convenience. AU values are calculated from various measurement techniques except for the Am(VI)/Am(V) couple (Am02 /Am02" "), which was determined directly. 

The reductions of halogenated organic compounds (RX) involve the cleavage of carbon-halogen bonds [62]. Depending on the solvent, supporting electrolyte, electrode material and potential, it is possible to electrogenerate either alkyl radicals (R ) or carbanions (R ), which then can lead to the fonnation of dimers (R-R), alkanes (RH) and olefins [R(-H)] ... 

Compounds Solvent Supporting electrolyte Potential Reference Potential... 

Tab. 8.10 Solvent-supporting electrolyte couples for low-temperature electrochemistry1 ...

The method of complete electrolysis is also important in elucidating the mechanism of an electrode reaction. Usually, the substance under study is completely electrolyzed at a controlled potential and the products are identified and determined by appropriate methods, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis. In the GC method, the products are often identified and determined by the standard addition method. If the standard addition method is not applicable, however, other identification/determination techniques such as GC-MS should be used. The HPLC method is convenient when the product is thermally unstable or difficult to vaporize. HPLC instruments equipped with a high-sensitivity UV detector are the most popular, but a more sophisticated system like LC-MS may also be employed. In some cases, the products are separated from the solvent-supporting electrolyte system by such processes as vaporization, extraction and precipitation. If the products need to be collected separately, a preparative chromatographic method is use-... 

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