Ammonium salts electrolytic reduction

In reviewing the intrinsic electrochemical behavior of nonaqueous systems, it is important to describe reactions of the most common and unavoidable contaminants. Some contaminants may be introduced by the salts (e.g., HF in solutions of the MFX salts M = P, B, As, etc.). Other possible examples are alcohols, which can contaminate esters, ethers, or alkyl carbonates. We examined the possible effect of alcoholic contaminants such as CH3OH in MF and 1,2-propylenegly-col at concentrations of hundreds of ppm in PC solutions. It appears that the commonly used ester or alkyl carbonate solvents are sufficiently reactive (as described above), and so their intrinsic reactivity dominates the surface chemistry if the concentration of the alcoholic contaminant is at the ppm level. We have no similar comprehensive data for ethereal solutions. However, the most important contaminants that should be dealt with in this section, and which are common to all of these solutions, are the atmospheric ones that include 02, H20, and C02. The reduction of these species depends on the electrode material, the solvent used, and their concentration, although the cation plays the most important role. When the electrolyte is a tetraalkyl ammonium salt, the reduction products of H20, 02 or C02 are soluble. As expected, reduction of water produces OH and... 

Baizer, working at the Monsanto Company, showed that good yields of adiponitrile are obtained from aqueous solutions by reduction at mercury or lead in the presence of a high concentration of quaternary ammonium salt [62]. Tetraethyl-ammonium toIuene-4-sulphonate was favoured as electrolyte. The first commercial plant operating the process was commissioned in 1965. It used a divided cell system with a lead cathode and aqueous tetraethylammonium ethylsulphate as electrolyte, with the addition of acid to regulate the pH. A lead anode with an anolyte of dilute sulphuric acid was employed. 

Phenylacetamide has been obtained by a wide variety of reactions from benzyl cyanide with water at 250-260° 6 from benzyl cyanide with water and cadmium oxide at 240° 6 from benzyl cyanide with sulfuric acid 7 8 by saturation of an acetone solution of benzyl cyanide with potassium hydrosulfide 9 from benzyl cyanide with sodium peroxide 10 by electrolytic reduction of benzyl cyanide in sodium hydroxide 11 from ethyl phenyl-acetate with alcoholic 12 or aqueous 13 ammonia from phenyl-acetic acid with ammonium acetate 14 or urea 15 from diazoacetophenone with ammoniacal silver solution 16 from phenyl-acetic acid imino ether hydrochloride and water 17 from acetophenone with ammonium poly sulfide at 215° 18 from benzoic acid 19 and by heating the ammonium salt of phenyl-acetic acid.20... 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

Figure 6 presents a scheme of an electrolysis cell for the isolation of reduction and oxidation products of nonaqueous solutions [15]. The electrolyte of the W.E. solution must be an alkyl ammonium salt because the reduction products of most of the commonly used solvents in the presence of metal cations precipitate as insoluble metal salts. The counter- and reference electrode compartments are separated from the working electrode compartment by two frits each. The separating units have pipes which enable the sampling of their solutions in order... 

There has been considerable research into the electrolytic reduction of aromatic carboxylic acids to the corresponding aldehydes. A general procedure has been described in which key elements are the use of the ammonium salt of the acid, careful control of the pH and the presence of an organic phase (benzene) to extract the aldehyde and thus minimize overreduction. The method appears to work best for relatively acidic substrates for example, salicylaldehyde was obtained in 80% yield. Danish workers have shown that, under acidic conditions, controlled electrolytic reductions are possible for certain pyridine-, imidazole- and thiazole-carboxylic acids. In these cases, it is thought that the product aldehydes are protected by geminal diol formation. A chemical method which is closely related to electrolysis is the use of sodium amalgam as reductant. Although not widely used, it was successfully employed in the synthesis of a fluorinated salicylaldehyde. ... 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

The electrochemical reduction of CO in fhe COj-methanol solution was carried out under high COj pressure. The high pressure apparatus was assembled from a SUS-316 sfainless steel tube. A glass inner tube was used to avoid contact of the electrolyte with the metal apparatus. Various metal electrodes were used in this study. The details have been described previously [9]. A Ft counter electrode and a silver quasi-reference electrode were used. Reagent grade methanol was used as the solvent. Tetrabutyl or tetraethyl ammonium salts were used as supporting electrolytes. 

Bockris and his coworkers studied photoelectrochemical reduction of CO2 in DMF based electrolyte solution, and proposed a concept of tetraalkyl amonium cation as a mediator in electron transfer. Saeki et al. showed that CO2 reduction in a methanol based electrolyte proceeds with tetraalkyl ammonium radical working as an electron mediator. The CO2 reduction does not take place when LiCl is used as the supporting electrolyte. Tomita et al. also indicated that CO2 reduction at a Pt electrode in AN requires some tetraalkyl ammonium salts. The signigicance of electron mediator in CO2 reduction was stressed by Geimaro et al. as well. " ... 

The solubility of CO2 is liigh in methanol. Fujishima and his coworkers employed CO2 methanol mixtures imder elevated pressure as the electrolyte solution. Tetraalkyl ammonium salts were used for the supporting electrolytes. They showed that CO2 reduction can proceed with a Cu electrode at 200 to 500 mA cm imder 40 to 60 atm. The major products were CO and methyl formate. ... 

The key to this process [63, 64] was the addition of a quaternary ammonium salt (tetraethy-lammonium p-toluenesulfonate or tetraethylammonium ethylsulphate) to the reaction medium, which increased the solubility of acrylonitrile in the aqueous electrolyte and generated an aprotic organic layer on the cathode surface that inhibited the synthesis of propionitrile by-products (formed by the simple reduction of acrylonitrile). 

This conversion produces as the bye product through direct reduction of H. The p-CdTe and p-InP electrodes have also been Investigated (12) while the reaction scheme Is essentially as indicated above, the net formation of formic acid is demonstrated to be a function of the pH of the medium. Hence using the solutions of supporting electrolytes such as carbonates, sulphates, phosphates and perchlorates of alkali salts or tetra alkyl ammonium salts, the reaction (4) has been effected. 

The choice of organic solvents permits the occurrence of certain reduction reactions of alkyl halides which, if attempted in aqueous solutions, would rapidly be inhibited by the precipitation of insoluble products on the electrode. Under mildly cathodic conditions, the formation of metal alkyls occurs in high yields when alkyl halides are reduced at cathodes of low boiling metals in acetonitrile, DMF or propylene carbonate containing quaternary ammonium supporting electrolytes. " Whilst the formation of lead tetraethyl from ethyl bromide in DMF proceeds irrespective of the substitution of a sodium salt for the quaternary ammonium salt, the same change of electrolyte in propylene carbonate no longer leads to metal alkyl products. ... 

Instead of chemically generated electrons as reducers, the electrolytic reduction of metal salts represents a successful alternative in cormection with the fabrication of tetra-alkylammonium salt-stabilized nanopartides [159-165]. The first step comprises an oxidative anodic dissolution of the corresponding metal, followed by the formation of zerovalent metal atoms at the cathode. Nucleation and particle growth then follow, some of which is stopped by the addition of a tetra-alkylammonium salt. This technique not only prevents the formation of byproducts but also allows a rather good size-selectivity high current densities lead to small particles, while low current densities yield larger spedes. Ammonium salt-protected nanopartides of Ti, Fe, Co, Ni, Pd, Pt, Ag, and Au may also be prepared in this way. 

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