Solvents for electrolysis

The potential window is one of the most important physical properties for the selection of a solvent for electrolysis. However, it should also be noted that the surface layer on the electrode, which is formed by chemical or electrochemical deposition, often stabilizes the system. For example, Katayama reported that a lithium ion conductive passivated layer, which is formed on the tungsten electrode as a result of reductive decomposition of the cation of the IL during the first sweeps, enables the reversible deposition and dissociation of Li metal [104], Howlett et al. have also discussed this extensively [112a-c]. 

Other Acids. Methanesulfonic acid [325] and formic acid have also been used as solvent for electrolysis superacids offer possibilities for unusual electrode reactions. 

Other nitriles, aliphatic and aromatic, have been investigated as solvents for electrolysis. Butyronitrile has been suggested as the solvent of choice for low-temperature electrochemistry [86], and propionitrile has in some cases been employed, as it is less... 

DMSO is an excellent solvent for many inorganic salts and organic compounds. It is difficult to reduce and fairly resistant to electrolytic oxidation. Its dielectric constant is high (s = 47). It thus has many of the qualities desirable for a solvent for electrolysis, and it shows promise of being one of the most important electrochemical media [387]. The liquid range is from 18 to 189°C, which makes it somewhat inconvenient to get rid of DMSO in the workup. When used as solvent for electrolysis it must be considered that DMSO is not always inert but has a fair reactivity in certain reactions. DMSO is unfit for UV spectroscopy. Its autoprotolysis constant is 31.8. 

Water is one of the most ideal solvents for electrolysis because water has a high dielectric... 

The role of moisture in corrosion of metals and other surfaces is twofold surface wetness acts as a solvent for containments and for metals is a medium for electrolysis. The presence of sulfate and chloride ions acceler-... 

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

This reaction is analogous to 10-7. It may be acid (including Lewis acids),base, or alumina catalyzed, occur with electrolysis, and may occur by either an SnI or Sn2 mechanism. Many of the P-hydroxy ethers produced in this way are valuable solvents, for example, diethylene glycol, Cellosolve, and so on. Reaction with thiols leads to hydroxy thioethers. Aziridines can similarly be converted to P-amino ethers. 

Higher alcohols, however, cannot be used as neat liquids in electrolysis. For anodic oxidation those alcohols must be dissolved in appropriate solvents. Acetonitrile is the most frequently used solvent for that purpose. Electrochemical oxidation of n-butyl alcohol to n-butyraldehyde was achieved in moderately dilute acetonitrile solution in a current yield of 77% [9]. 

Phenoxy cations (35) electrochemicaUy generated from phenols (34) react with olefins to afford two types of bicycKc compounds (36) and (37), depending on the substituents (X, Y, Z), the solvent used for electrolysis, and the structure of the olefin (Scheme 13) [72]. 

It has been demonstrated that electrochemical deposition of fullerene-containing films from the TFE solutions on the nickel electrodes is possible in principle. It has been found that the thickness, structure, density and chemical composition of the films depend heavily on the type of hydrocarbon solvents for fullerenes, the chemical composition of a base electrolyte and conditions for electrolysis. 

The course of electrical reduction, like that of purely chemical reduction, depends decisively upon whether the reduction is carried out in an alkaline or acid solution. But these relations are of a positive nature in electrolysis only so long as they are not compensated by the electrical factors, such as cathode material and potential. To avoid a complication, it is necessary to limit the considerations primarily to unattackable cathodes and to take no account of an adjustment to certain and constant cathode potentials, and to exclude a secondary interference of the solvent, for instance by molecular rearrangements. In this general comprehension of the problem it can be said that the well-known chemical rule reoccurs in electrolytical... 

The solvent may influence the electrolytic reaction in different respects. The solubility of the substrate in the solvent is important for the attainment of a high initial current the solubility of a supporting electrolyte and the dielectric constant of the medium are reflected in the ohmic resistance. The adsorption of substrate on the electrode depends on the medium, and so does the availability of protons. Some solvents are, themselves, oxidized or reduced too easily to be useful. A review on the solvents of interest for electrolysis has recently been published 36 it includes information on the solubility of supporting electrolytes, useful reference electrodes, and attainable potential range. 

Iron has also been obtained from solutions of its salts in organic solvents by electrolysis between platinum electrodes. A solution of ferric chloride in methyl chloride conducts electricity well, and may be used for the purpose.9 Produced in this way, however, the metal is particularly liable to be contaminated with carbon.10... 

The supporting electrolyte is essential for the electroorganic reaction. The foUowing points are important for the selection of the supporting electrolyte (i) solubility to the solvent commonly used for electrolysis (ii) electrochemical stability (iii) interaction with reaction inteimediate and (iv) relative difficulty of preparation. 

For electrolysis with the hydrogen anode we use a mixture of THF and HMPA (hexamethyl-phosphoric triamide) with 0.02 M Et4NBF4 as solvent/electrolyte system. Trying to find a substitute for HMPA, we tested several rotic solvents, known to replace HMPA very effectively in organic synthesis (see Table 1). 

Except THF alone, all of the chosen mixtures showed a sufficient conductivity. But when using them for electrolysis of trimethylchlorosilane, either the solvent or the supporting electrolyte is not stable, thus leading to the formation of hexamethyldisiloxane instead of hexamethyldisilane. With TMEDA MezSiCl reacts to an insoluble, white complex that can not be electrolyzed. 

Redox processes involving 178 have also been studied.Anodic oxidation of thianthrene has been eifected in a wide variety of solvents. Use of trifluoracetic acid gives stable solutions of 178 and, if perchloric acid is included, the solid perchlorate salt may be isolated on evaporation of the solvent after electrolysis. Dichloromethane at low temperatures has been used and, at the opposite extreme, fused aluminum chloride-sodium chloride mixtures. " Propylene carbonate permits the ready formation of 178, whereas the inclusion of water in solvent mixtures gives an electrochemical means of sulfoxidizing thianthrene. Reversible oxidation of 178 to thianthrenium dication may be brought about in customary solvents such as nitriles, nitro compounds, and dichloromethane if the solvent is treated with neutral alumina immediately before voltammetry addition of trifluoracetic anhydride to trifluoracetic acid equally ensures a water-free medium. The availability of anhydrous solvent systems which permit the reversible oxidation and reduction of 178 has enabled the determination of the equilibrium constants for the disproportionation of the radical and for its equilibria with other aromatic materials. ... 

The viscosity of the medium influences not only the mass transfer, but also the rate constant of the heterogeneous electron transfer [285]. A low viscosity is preferable both from the point of view of diffusion and from considerations of pumping in flow cells. For some kind of electroanalytical work, however, a high viscosity is preferable. Diffusion coefficients in some solvents useful for electrolysis have been published [286]. 

Nitrobenzene has been used for electrolysis [402] it was found that certain radicals were rather stable in this solvent. Nitrobenzene has a liquid range from 5.7 to 210.9°C BU4NCIO4 may be used as supporting electrolyte. An aqueous SCE separated from the solution by a suitable bridge with porous glass has been used as reference electrode. Nitrobenzene may be purified by passing it through a column of alumina followed by a distillation in vacuo. 

The properties of superoxide anion have been well reviewed [45,46]. In water it is a relatively weak nucleophile, whereas in aprotic solvents, such as those commonly used for electrolysis, it is a powerful nucleophile. Its basic properties stem from the driving force of the disproportionation [45,46] depicted as Eq. (5), similar to the disproportionation discussed for organic radical anions in Sec. II.B. Thus, although superoxide anion itself is a relatively weak Bronsted base— pA a(H02) 12 in DMF [46]—the overall equilibrium... 

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