Amines anodic processes

Anodic limits on mercury. Mercury is readily oxidized, particularly in the presence of anions that precipitate or complex mercury or mercury ) ions, such as the halides, cyanide, thiosulfate, hydroxide, or thiocyanate. For this reason, mercury is seldom used to study anodic processes except for those subtances that are easily oxidized, for example, Cr(II), Cu(I), and Fe(II). Under carefully controlled conditions, mercury can be coated with a thin layer of mercury chloride such that it does not interfere with electron transfer in the oxidation of a number of organic compounds, particularly amines.66... 

These studies indicate that the selectivity of a-aminoalkyl radical formation is Me > Et /-Pr, which is the opposite of that expected on the basis of radical stability. For example, in diisopropylmethylamine, methyl adducts are formed exclusively. Similar selectivity has been observed for oxidation of unsymmetrical amines by ferricyanide [43, 123] and in anodic processes [124]. This selectivity has been attributed to requirement of overlap between the half-vacant nitrogen p orbital and the a-CH orbital of the a-carbon. From the Newman projections below it can be seen that the conformation necessary for methyl deprotonation (a) is lower in energy than that for the isopropyl deprotonation (b) [5, 122]. 

In unsymmetrical amines this process generally leads to a-cyanation of the less substituted carbon. It was earlier proposed that the regioselectivity of cyanation was decided by the conformation of the amine adsorbed on the anode. However... 

Anodic processes of amines are described in most detail either in 7 or in a special chapter in 5. This chapter (which is more recent) has been written by V. D. Parker who published the most fundamental papers in this field. 

Fluorocarbons are made commercially also by the electrolysis of hydrocarbons in anhydrous hydrogen fluoride (Simons process) (14). Nickel anodes and nickel or steel cathodes are used. Special porous anodes improve the yields. This method is limited to starting materials that are appreciably soluble in hydrogen fluoride, and is most useflil for manufacturing perfluoroalkyl carboxyflc and sulfonic acids, and tertiary amines. For volatile materials with tittle solubility in hydrofluoric acid, a complementary method that uses porous carbon anodes and HF 2KF electrolyte (Phillips process) is useflil (14). 

As already noted (p. 1073), the platinum metals are all isolated from concentrates obtained as anode slimes or converter matte. In the classical process, after ruthenium and osmium have been removed, excess oxidants are removed by boiling, iridium is precipitated as (NH4)2lrCl6 and rhodium as [Rh(NH3)5Cl]Cl2. In alternative solvent extraction processes (p. 1147) [IrClg] " is extracted in organic amines leaving rhodium in the aqueous phase to be precipitated, again, as [Rh(NH3)5Cl]Cl2. In all cases ignition in H2... 

Conversely, the use of elevated temperatures will be most advantageous when the current is determined by the rate of a preceding chemical reaction or when the electron transfer occurs via an indirect route involving a rate-determining chemical process. An example of the latter is the oxidation of amines at a nickel anode where the limiting current shows marked temperature dependence (Fleischmann et al., 1972a). The complete anodic oxidation of organic compounds to carbon dioxide is favoured by an increase in temperature and much fuel cell research has been carried out at temperatures up to 700°C. 

Nickel oxide anodes are another example for a relatively simple oxide electrocatalyst used rather widely in the oxidation of organic substances (alcohols, amines, etc.) in alkaline solutions at relatively low anodic potentials (about +0.6 V RHE). These processes, which occur at an oxidized nickel surface, are rather highly selective. As an example, we mention the industrial oxidation of diacetone-L-sorbose to the corresponding acid in vitamin C synthesis. This reaction occurs at nickel oxide electrodes with chemical yields close to 100%. 

An electroreductive Barbier-type allyla-tion of imines (434) with allyl bromide (429) also occurs inaTHF-PbBr2/Bu4NBr-(Al/Pt) system to give homoallyl amine (436) (Scheme 151) [533]. The combination of Pb(II)/Pb(0) redox and a sacrificial metal anode in the electrolysis system plays a role as a mediator for both cathodic and anodic electron-transfer processes. The metals used in the anode must have a less positive anodic dissolution potential than the oxidation potentials of the organic materials in order to be present or to be formed in situ. In addition, the metal ion plays the role of a Lewis acid to form the iminium ion (437) by associating with imine (435) (Scheme 151). 

Goto et al. (2004) measured the reaction kinetics of one-electron oxidation of A -methyl-p-anisidine in AN. In the electrode process, oxidation was performed at the platinum disk-shaped anode, in the chemical process, by means of the tris(p-bromophenyl)amine cation-radical. In both the cases, after one-electron oxidation, dimerization took place leading to the formation of the dye variamine blue. According to the kinetic data, the mechanism of this dye formation is different in the electrode and chemical processes (see Scheme 2.34). Namely, in the electrode oxidation, the cation-radical appears to be surrounded by a huge amount of the initial (nonoxidized) A-methyl-p-anisidine... 

The a-aminoalkyl radical intermediates from electrochemical oxidation of amines show a strong tendency to lose a further electron and form an immonium ion. This process shows an anodic polarographic wave at negative electrode poten-... 

Anodic oxidation of enamine ketones or esters in CH30H-NaC104 at a graphite anode gives substituted pyrroles in 15-45% yield.101 Formation of the symmetrically substituted pyrroles 47 indicated radical dimerization of radical-cations formed as primary products from 46. This process leads to dications from which the pyrroles can be formed by cyclization and elimination of an amine [Eq. (44)]. 

Adipodinitrile is an intermediate for hexamethylenediamine, the amine component of nylon 66. The electrochemical process is economically superior to the synthesis of adipodinitrile from cyclohexanone. Today, it essentially competes with the addition reaction of HCN to butadiene. The total capacity of the electrochemical ADN synthesis is currently about 250,000 tonnes/year. The process is industrially fully developed. Recent work 34 347) is aimed at reducing the oxygen evolution potential at the anode in order to save further energy. 

Experiments designed to elucidate the role of S in cathodic reduction tend to be just as ambiguous as their anodic counterparts, unless certain precautions are taken. The possible intervention of S in the reduction of aromatic hydrocarbons (Asahara et al., 1968 Benkeser and Kaiser, 1963 Benkeser et al., 1964 Sternberg et al., 1963, 1966, 1967, 1969) in SSEs made up of amines or HMPA (to which up to 65% ethanol can be added without impairing the stability of HMPA too much) as compared to the possible direct processes taking part in protic solvents illustrates the problem. 

While the process at the cathode always ends finally in withdrawal of oxygen or in taking up of hydrogen, the number of possible reactions at the anode—aside from solution-phenomena, which are without interest here—is a much greater one. For, each ion which is capable of substituting can pass into the reactive state at the anode and produce reactions which cannot be numbered with the real oxidations. In the first place numerous substitutions can occur in difficultly oxidizable bodies, especially aromatic compounds, for instance the chlorination of phenols and phthale ins, nitration of acids, diazotizing of amines, etc. Substitution and oxidation processes often occur simultaneously, as in the electrolytic formation of iodoform from alcohol. 

However, if an amine alone is subjected to the anodic current action in the presence of the nitrite, complicated products result besides the action of the NQ2 ions upon the amido-group and the typical decomposition of the diazo-body by the electrolyte, substitution and oxidation processes seem to occur. 

Simons process — Electrochemical polyfluorination reactions of organic compounds are the only efficient way to industrial production of perfluorinated compounds. The reaction proceeds in the solution of KF in liquid HF (b.p. 19.5 °C), where the starting substances as alcohols, amines, ethers, esters, aliphatic hydrocarbons and halo-hydrocarbons, aromatic and heterocyclic compounds, sulfo- or carboxylic acids are dissolved. During anodic oxidation, splitting of the C-H bonds and saturation of the C=C bonds occur and fluorine atoms are introduced. 

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