Hydroxylamines cathodic reduction

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

The cathodic reductions of nitro compounds are among the most thoroughly investigated reactions of organic electrochemistry. At least on the laboratory scale, the reaction permits the synthesis of many intermediates with different oxidation states. However, most syntheses can now also be carried out more economically by catalytic reactions. Therefore, only a few electrochemical reactions are still of industrial interest, i.e. the single-step syntheses of hydroxylamines, aminophenols, or anisidines. 

More recent work has shown that the cathodic reduction of aliphatic nitro compounds in acidic medium proceeds through the nitroso stage 149 to give the hydroxylamine in a four-electron reaction ... 

Non-Reversible Processes. —Reactions of the non-reversible type, i.e., with systems which do not give reversible equilibrium potentials, occur most frequently with un-ionized organic compounds the cathodic reduction of nitrobenzene to aniline and the anodic oxidation of alcohol to acetic acid are instances of this type of process. A number of inorganic reactions, such as the electrolytic reduction of nitric acid and nitrates to hydroxylamine and ammonia, and the anodic oxidation of chromic ions to chromate, are also probably irreversible in character. Although the problems of electrolytic oxidation and reduction have been the subject of much experimental investigation, the exact mechanisms of the reactions involved are still in dispute. For example, the electrolytic reduction of the compound RO to R may be represented by... 

The cathodic reduction of nitroalkanes (primary and secondary) affords [50] (four-electron reduction in acidic or neutral media) the corresponding hydroxylamines that are often quite difficult to be converted into amines. 

The next step in the Purex process after primary decontamination is separation of plutonium from uranium. This is done by reducing plutonium to the trivalent state, in which it is inextractable by TBP, while leaving the uranium in the extractable hexavalent condition. Reductants that have been used for this purpose include Fe, U, hydroxylamine, or cathodic reduction. 

With ferrous ion or cathodic reduction, conversion of plutonium from Pu to Pu is so rapid that back extraction of plutonium to the aqueous phase and reduction there to Pu can be carried out simultaneously in a single multistage contactor. With tetravalent uranium, reduction of plutonium is slower, so that additional contactor volume is desirable to complete back extraction. With hydroxylamine, reduction of plutonium is so much slower that it is preferable first to return both uranium and plutonium to the aqueous phase by stripping with dilute nitric acid and then to reduce the plutonium in equipment providing sufficient residence time for reduction to proceed to completion. Finally, the uranium is reextracted by TBP. 

Hydroxylamine is used for plutonium reduction instead of cathodic reduction as in the Barnwell flow sheet Fig. 10.11, because the plutonium/uranium ratio in this LMFBR fuel is 10 times that in LWR fuel and because electrolytic reduction has not been demonstrated for this high plutonium content. 

Hydroxylamine salts are made commercially in two ways the hydrolysis of primary nitroparaffins and the cathodic reduction of nitric acid. In the laboratory it is easiest to reduce nitrous acid with a bisulfite and then hydrolyze the resulting salt of hydroxylamine disulfonic acid. The reactions are as follows ... 

A typical case is the growth of silver whiskers by cathodic reduction from concentrated silver nitrate solutions (0.3 M) containing certain organic additives such as oleic acid, gelatine, albumin, or n-heptyl, n-octyl, and -nonyl alcohols, or, from silver nitrate solutions aged for several weeks. Copper whiskers can be grown from solutions containing a variety of additives or hydroxylamine ions in the presence of chlorides. 

Hydroxylamine is derived from ammonia by replacing one hydrogen atom by a hydroxyl group. It is prepared by the electrolytic reduction of nitric acid, using a lead cathode ... 

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

The reduction of aliphatic nitrocompounds in acid solution proceeds in two steps. First the nitrosocompound is formed. A low steady state concen ation of 2-methyl-2-nitrosopropane has been detected during the reduction of 2-methyl-2-nitropropane [13]. At the cathode potential necessary to attach the first electron to a nitro group, the nitroso intermediate undergoes further reduction to the hydroxyla-mine. When the nitrocompound has one a-hydrogen substituent, tautomerism of the nitroso intermediate to an oxime is in competition with further reduction. Both temperature and proton availability affect the rate of this isomerisation. Reduction of aliphatic nitrocompounds to the hydroxylamine is usually carried out in acid solution at 0-5° C to minimise oxime formation [14, 15], The hydroxylamine is stable towards further reduction in acid solution. Oximes in acid solution are reduced... 

Conversion of substituted nitrobenzenes to the arylhydroxylamine is easily achieved by reduction in neutral or slightly acid solution. In the first classical experiments, Haber [35] used a platinum cathode and ammonia ammonium chloride buffer and die process was improved by Brand [57] using either a nickel or silvered copper cathode in an acetate buffer. The hydroxylamine can also be obtained from reduction in dilute sulphuric acid provided tire temperature is kept below 15° C to suppress furtlier reduction [58]. This electrochemical route to arylhydroxylamines due to Brand is superior to the chemical reduction using zinc dust and ammonium chloride solution. The latter process is known to give variable yields depending on... 

Sodium hypnnitrite Na N 0 is formed (I) by reaction of sodium nitrate or nitrite solution with sodium amalgam (sodium dissolved in incrcuryl, alter which acetic acid is added to neutralize the alkali. Sodium stannite ferrous hydroxide, or electrolytic reduction w ith mercury cathode may also be utilized. (2) by reaclion of hydroxylamine sulfonic acid and sodium hydroxide. Silver hyponitrite is formed by reaclion of silver nitrate solution and sodium hyponitrite. 

Very rapid and efficient stirring is required to strip the intermediate o-chlorophenylhydroxylamine from the cathode so that the acid-catalyzed rearrangement to 4-amino-3-chlorophenol may occur rather than reduction of the substituted hydroxylamine to o-chloroaniline. 

J. Tafel electrolyzed a soln. of 0-4 grm. of nitric acid and 20 c.c. of 50 per cent, sulphuric acid, using 10 sq. ems. of cathode surface and 2-4 amps, at 0°. The product of the reduction is largely dependent on the nature of the metal used as electrode. Some results are indicated in Table XXVII. With platinum, no ammonia or hydroxylamine was formed, and with palladium the reduction is extremely slow. Hie chief products of the reduction are hydroxylamine and ammonia. The largest proportion of the hydroxylamine is formed when mercury is used as cathode, and the conversion of the nitric acid into this can be carried out almost quantitatively. With lead electrodes, about 40 per cent, of the nitric acid is converted into hydroxylamine, and with copper electrodes only about 15 per cent. if the copper be in the form of a spongy mass, only about one per cent, of the acid is transformed into hydroxylamine, the remainder being reduced to ammonia. When... 

Thus, copper and platinum give similar results the nitrite formation is greater with iron, nickel, and cobalt, and the nitrate formation less. The gases were mainly nitrous oxide and nitrogen with a small proportion of oxygen. N. D. Zelinsky and S. G. Krapiwin showed that the decomposition of hydroxylamine into acid and base does not occur in soln. with methyl alcohol as solvent. J. Tafel showed that an aq. soln. of hydroxylamine sulphate in presence of 20-50 per cent, of sulphuric acid is not reduced at a copper cathode. O. Flaschner observed some reduction in dil. sulphuric acid soln. J. Tafel and H. Hahl found that reduction always takes place when the sulphuric acid cone, in the layer of electrolyte in contact with the cathode is reduced beyond a certain point, and when there is no excess of acid in other words, when hydroxylamine sulphate itself is electrolyzed, the reduction is quantitative. These results are most readily accounted for on the view that only free hydroxylamine (produced in this case by partial hydrolysis of the sulphate), but not the hydroxylammonium ion, NH3OH, is reduced at a copper... 

J. Tafel found that while nitric acid is reduced only to hydroxylamine q.v.) by mercury or well-amalgamated electrodes, a copper cathode reduces it to ammonia and at the same time has no action on hydroxylamine. A. Brochet and J. Petit studied the electro-reduction of nitric acid by an alternating current. T. H. Jeffery described the electrolysis of nitric acid with a gold anode, and obtained from the anode liquor crystals of aurinitric acid, HAu(N03)4.3H20. R. Ihle s observations on the oxidation-potential of nitric acid have been discussed in connection with nitrous acid (q.v.). He found that if the cone, of the nitric acid be expressed by... 

Tafel1 showed that a mercury cathode or one of amalgamated lead gives the best results. At a platinum cathode very little reduction takes place, and the products are ammonia and hydroxylamine. According to Tafel, a lead cathode gives a 40 per cent, conversion of nitric acid to hydroxylamine, but with a copper ielectrode only 15 per cent, reduction to this substance takes place, whilst much ammonia is formed. 

Since hydroxylamine is not reduced to ammonia by a copper cathode, it follows that the reduction of nitric acid... 

The catalytic effect of copper is shown in the reduction of nitrobenzene, which at a copper cathode is reduced to aniline, but while copper sponge under ordinary chemical conditions will reduce phenyl-hydroxylamine to aniline it has no effect upon nitrobenzene, and the inference is that in electrolytic reduction phenylhydroxylamine may be first formed by electrolysis, and this substance is then converted to aniline largely by the catalytic effect of the copper cathode. 

Nitrosobenzene.—It is natural that so good a depolarizer as nitrosobenzene is at the cathode cannot be separated as such under the conditions of a continuous reduction. Haber,5, by adding a-naphthol and hydroxylamine to the electrolyte in alkaline solution, could, however, prove the presence of nitrosobenzene in the form of its characteristic condensation product,... 

The influence of the cathode metal is much more manifest when acid electrolytes are employed than in alkaline reduction. In alkaline solution at copper electrodes, if we except the last-mentioned process, the rapidly occurring condensation of the first reduction phases—of the nitroso- and hydroxylamine body—always leads immediately to the azoxy-body and makes this appear to be the typical product of the alkaline reduction, which can in turn be further reduced. In acid solution this condensation takes place so slowly that the molecular rearrangement of the hydroxylamine and its further reduction to amine has time to take place alongside the formation of the azoxy-body and the reduction of the latter to the hydrozo-compound or benzidine.4... 

A reductive ring closure of a (8-dioxime (72) to a pyrazolidine (73) was reported00 to occur in 30% sulfuric acid at a lead cathode. The following mechanism (Scheme 7) may operate in which the loss of water from the intermediate hydroxylamine may be assisted by the acid.89... 

Among the azomethine derivatives of hydroxylamine, the ordinary oximes are the most important, and many polarographic and electrolytic investigations of oximes have been made [1,85,86]. Most oximes are reducible in acid solution, and only few in alkaline solution polarographic data prove that it is the protonated oxime that is the reducible species in acid and neutral solutions. Classic reductions in sulfuric acid at lead cathodes or controlled potential reductions in acid solution at mercury cathodes yield the amine in a four-electron reduction. As the hydroxylamine corresponding to the oxime is not reducible under the conditions employed, it has been suggested [1] that the reduction proceeds as in ... 

Aromatic nitro compounds may generally be reduced in a four-electron reduction to hydroxylamines, which in acid solution at a more negative potential can be reduced further to amines. By choosing a suitable cathode potential it is thus possible to avoid the further reduction of the hydroxylamine to the amine. When a hydroxyl or amino group is ortho or para to the nitro group, it is not feasible to isolate the hydroxylamino compound, as it is too easily dehydrated to the reducible quinone mono- or diimine. It may, however, be possible to trap the intermediate. 

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