Nitrocompounds, reduction

Excellent yields of the oximes of phenylacetaldehydes are obtained by reduction of 6-nitrostyrenes over Pd-on-C in a pyridine solvent (74,75). The technique gives yields of only about 60% when applied to aliphatic unsaturated nitrocompounds better yields are obtained in acidic media(6 5). Over 5% Rh-on-Al203 in ethanol-acetic acid-ethyl acetate, 2- 6-dinitro-styrenes are converted to 2-nitrophenylacetaldehyde oximes (13). 

Nitroaminocompounds or Nitroamines. Compds containing both —N02 and —NH2 radicals attached to different C atoms, eg, nitroaniline, 02N.C6H4.NH2, etc. Nitroamines can be prepd either by the nitration of amines, or by partial reduction of nitrocompounds contg several —N02 groups. Another method is to treat a chloronitrocompound with ammonia, as in the prepn of dinitroaniiine... 

Aromatic nitrocompounds (see Fig. 21) such as nitrobenzene and the dinitrobenzenes diminish the rate of polymerization of styrene without suppressing it altogether and without introducing an induction periodi.e., they are typical retarders. Larger quantities are required to produce significant reductions in the rate, and the retardation persists throughout the polymerization, suggests... 

It is worth noting, however, that up until fairly recently the reduction of aromatic nitrocompounds to the corresponding amines was commonly performed on an industrial scale, e.g. in the synthesis of azo dyes, with a mixture of iron and hydrochloric acid. This so-called Bechamp reduction has an E factor of ca. 15 compared to 1 for catalytic hydrogenation. 

J. Heyrovsky and K. Holleck and B. Kastening pointed out that the reduction of aromatic nitrocompounds is characterized by a fast one-electron step, e.g. 

Ammonium hydrogen sulphide is also a very suitable reagent for the reduction of nitrocompounds to arylhydroxylamines it is used in alcoholic solution in the cold.1... 

Reduction of nitrocompounds involves a series of one-electron additions and chemical steps and an important reaction intermediate is the nitrosocompound. The redox properties of nitro- and nitroso-compounds will be discussed first. Some values of reduction potentials for nitrocompounds are collected in Table 11.1. 

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... 

Hydroxylamines from reduction of nitrocompounds are trapped by reaction with any adjacent carbonyl function under slightly basic conditions. This reaction forms a nitrone, e.g. 1, which can be reduced in acid solution to a pyrrolidine [20]. 

The effect of temperature on the reduction of nitro-alkanes has been extensively exploited in the synthesis of cp-ephedrine analogues [i5,21,22] starting from 1-aryl-2-nitropropenes which are easily converted to the substrates 2. Reduction of these nitrocompounds below 20° C affords the hydroxylamine. The amine is formed above 55° C. Neutralisation of either reaction mixture causes rapid intramolecular migration of the acetyl group from oxygen to nitrogen, a reaction which implies a... 

Reduction of aromatic nitrocompounds takes place in three overall stages. The first two steps proceed in sequence because nitrosocompounds produced in the first step have a much less negative reduction potential than nitrocompounds. Arylhy-... 

Electrolytic reduction of an emulsion of the nitro compound in 1 M zinc chloride solution at high current density is another proposed method for conversion to the amine. Finely divided zinc is produced and this reduces the nitrocompound. Zinc ions also function as Lewis acid in the reduction of arylhydroxylamines [44]. 

Pb Lead, and particularly underpotentially deposited Pb, exhibits electrocatalytic properties in numerous electrode processes. The model reaction can be oxygen reduction with slow step of peroxide reduction ]374-376] or reduction of nitrobenzene and other nitrocompounds [377, 378]. In the case of... 

Compounds related to aniline, either directly or by oxidation, and to nitrobenzene by reduction, are numerous and important. When nitrobenzene is reduced in the presence of hydrochloric acid by tin or iron, the product is aniline (colorless liquid in die presence of water by zinc, the product is phenylhydroxylamine (white solid) in the presence of methyl alcohol by sodium alcoholate 01 by magnesium plus ammonium chloride solution, the product is azoxybenzene (pale yellow solid) by sodium stannitc, or by water plus sodium amalgam, the product is azobcnzcnc (red solid) in the presence of sodium hydroxide solution by zinc, the product is hydrazobenzene (pale yellow solid). The behavior of other nitrocompounds is similar to that of nitrobenzene. 

One unusual case of anion radical preparation is 4- and 3-nitrocatechol or nitrohy-droquinone. Reduction of these nitrocompounds in aqueous solutions with sodium boro-hydride in air leads to the corresponding anion radicals, which are stable at pH 9-12 despite the presence of water (Grenier et al. 1995). 

The alkylation of ammonia, Gabriel synthesis, reduction of nitriles, reduction of amides, reduction of nitrocompounds, and reductive amination of aldehydes and ketones are methods commonly used for preparing amines. 

Aromatic amines are normally prepared by reduction of the corresponding aromatic nitrocompound. 

However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes or acides, and also the electro reduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (8), whereas it inhibits the adsorption of hydrogen on platinum (9,10),... 

For the quantitative estimation the nitrometer method is not in general applicable, though the nitramine group in tetiyl responds to this test. The Dumas method may be used under precautions, and the Kjeldahl method is sometimes used for the lower nitrocompounds. Titration methods depending on reduction with stannous chloride are described by Limpricht,... 

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