The reduction of nitro compounds

Arylamines are generally prepared by the reduction of nitro compounds. When only small quantities are to be reduced and the time element is important and cost is a secondary consideration, tin and hydrochloric acid may be employed, for example ... 

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. 

In the reduction of nitro compounds to amines, several of the iatermediate species are stable and under the right conditions, it is possible to stop the reduction at these iatermediate stages and isolate the products (see Figure 1, where R = CgH ). Nitrosoben2ene [586-96-9] C H NO, can be obtained by electrochemical reduction of nitrobenzene [98-95-3]. Phenylhydroxylamine, C H NHOH, is obtained when nitrobenzene reacts with ziac dust and calcium chloride ia an alcohoHc solution. When a similar reaction is carried out with iron or ziac ia an acidic solution, aniline is the reduction product. Hydrazobenzene [122-66-7] formed when nitrobenzene reacts with ziac dust ia an alkaline solution. Azoxybenzene [495-48-7], C22H2QN2O, is... 

Hydrogen gas can be replaced by ammonium formate for the reduction of nitro compounds to amines. The ammonium formate method is efficient, and the rapid workup procedure by simple filtratkin makes it widely used for converting the NO to the NH. ° For example, ct-nitro esters are reduced to ct-amino esters in excellent yields on treatment v/ith HCO NH and PdAZ in methanol. ... 

Reductive carbonylation of nitro compounds (in particular aromatic dinitro compounds) is an important target in industry for making diisocyanates, one of the starting materials for polycarbamates. At present diisocyanates are made from diamines and phosgene. Direct synthesis of isocyanates from nitro compounds would avoid the reduction of nitro compounds to anilines, the... 

Two basic methods are used for the manufacture of sulphur dyes. In one, the starting materials are baked either with sulphur alone or with sulphur and sodium sulphide at a temperature between 180 and 350 °C. Alternatively the intermediates are heated under reflux in aqueous or alcoholic sodium polysulphide this process may also be carried out under pressure at temperatures up to about 130 °C. Following sulphurisation the dye is precipitated by means of air or chemical oxidation, acidification or a combination of these methods. The sulphurisation process results in the evolution of hydrogen sulphide, which is usually absorbed in aqueous sodium hydroxide for use elsewhere - in the reduction of nitro compounds, for example. 

For the reduction of nitro-compounds on a small scale, tin or stannous chloride and concentrated hydrochloric acid are the most suitable reagents. Solids are often difficult to reduce in the absence of a solvent and call for the addition of alcohol or glacial acetic acid. When the addition of water to a sample of the reaction mixture produces a clear solution it is known that the reduction is complete. The base will, of course, be present as hydrochloride, and hydrochlorides are, almost without exception, soluble in water. It should be noted, however, that sparingly soluble double salts with stannous chloride are often formed, but these double salts are usually soluble in boiling water. 

For the reduction of nitro-compounds containing a group which may be attacked by nascent hydrogen, as, for example, an aldehyde group, an unsaturated side chain, and so on, special methods must he applied. In such cases ferrous hydroxide or iron powder (cf. Chap. VII. 5, arsanilic acid) are often used. The reduction is carried out thus a weighed amount of ferrous sulphate is caused to act, in the presence of alkali (potassium or sodium hydroxide, baryta), on the substance to he reduced. In this way it is possible to reduce, for example, o-nitro-benzaldehyde to aminobenzaldehyde, and o-nitrocinnamic acid to amino-cinnamic acid. 

In a manner analogous to the formation of the other hydridometal complexes, the tricarbonylhydridovanadate anion is easily produced from r)5-cyclopentadienyl-vanadiumtetracarbonyl under basic phase-transfer catalytic conditions and it has been used in the reduction of nitro compounds and the reductive dehalogenation of a wide range of halides [ 12]. 

Stannous chloride is used most frequently for the reduction of nitro compounds [177, 178, 179] and of quinones [180, 181], It is also suitable for conversion of imidoyl chlorides [182] and of nitriles [183] to aldehydes, for transformations of diazonium salts to hydrazines [184], for reduction of oximes [f[Pg.30]

With the exception of the nitroso stage, all the intermediate stages of the reduction of nitro compounds can be obtained by controlled catalytic hydrogenation [572], and all reduction intermediates were prepared by reduction with appropriate hydrides or complex hydrides. However, the outcome of many hydride reductions is difficult to predict. Therefore more specific reagents are preferred for partial reductions of nitro compounds. 

In this paper, the selectivity of the ECH method for the reduction of nitro compounds to the corresponding amines on RCu electrodes will be compared with that of reduction by RCu alloy powder in alkaline aqueous ethanol. In the latter method (termed chemical catalytic hydrogenation (CCH)), chemisorbed hydrogen is generated in situ but by reduction of water by aluminium (by leaching of the alloy) (equation [12]). The reductions by in situ leaching must be carried out in a basic medium in order to ensure the conversion of insoluble Al(OH)3 into soluble aluminate (equation [12]). The selectivity and efficiency of the electrochemical reduction of 5-nitro-indoles, -benzofurane, and -benzothiophene at RCu electrodes in neutral and alkaline aqueous ethanol will also be compared with that of the classical reduction with zinc in acidic medium. 

R.A. Sanchez-Delgado u. B.A. Oramas, J. Mol. Catal. 36, 283-291 (1986) A Comparative Study of the Reduction of Nitro Compounds under Hydrogen, Syngas, and Water Gas". 

It is natural to presuppose that the reduction of nitro compounds should lead to the nitroso compounds, at least as an intermediate stage. Until quite recently, no reductive processes for the formation of nitrosoalkanes were known [3], More recently, some indirect evidence is said to show that, on electrolytic reduction of tertiary aliphatic nitro compounds, the final t-alkyl-hydroxylamines are produced by the intermediate formation of nitroso compounds which were not isolated [99]. 

The utility of all these transformations may become clearer if you work Exercise 23-34. It will give you practice in seeing how various benzene derivatives can be prepared from primary benzenamines. Later in the chapter we shall see that amines can be prepared by the reduction of nitro compounds, which permits the following sequence of reactions ... 

Some care must be exercised in the reduction of nitro compounds because such reductions can be highly exothermic. For example, the reaction of 1 mole (61 g) of nitromethane with hydrogen to give methanamine liberates sufficient heat to increase the temperature of a 25-lb iron bomb 100° ... 

All of these substances can be reduced to benzenamine with tin and hydrochloric acid. As a result, each could be, but not necessarily is, an intermediate in the reduction of nitro compounds to amines. Formation of the bimolecular reduction products is the result of base-induced reactions between nitroso compounds and azanols or amines and possibly further reduction of the initially produced substances (see Exercise 24-18). 

Although the reduction of nitro compounds is more widely applied in the aromatic series using procedures discussed in Section 6.5.1, p. 890, these are equally applicable to the reduction of primary or secondary nitroalkanes when these are readily available. a,/ -Unsaturated nitro compounds (arising for example from the Claisen-Schmidt reaction, Section 6.12.2, p. 1032) may be reduced to the saturated amine very effectively with sodium dihydrobis(2-methoxyethoxy)-aluminate ( Red-Al ). 

Intermediate products in the reduction of nitro compounds (Expts 6.87 to 6.91). 

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