Aniline from nitrobenzene reduction

Reduction of nitrobenzene Aniline can be prepared from nitrobenzene by either chemical reduction using acid and metal or catalytic hydrogenation using molecular hydrogen. 

On the other hand, the production of desired compounds through reduction of starting material requires the electron donors to be oxidized (reductant). Alcohols are often used not only as a solvent but as the donor to produce useful compounds, e.g., anilines from nitrobenzenes,22) alcohols from aldehydes,23) and secondary amines from the corresponding Schiff bases.24) From the organic synthetic point of view, however, the separation of undesired products, aldehydes or ketones, from the alcohols is necessary unless subsequent reaction processes consume them25,26) or they are easily removed by distillation or other procedures. A recent report has shown that water acts as the electron donor and is converted into 02 in the photocatalytic regio-selective reduction of terpenes mixed with aqueous suspension of Ti02.27,28) It is notable that isolation of the desired product from the reaction mixture is simple in this type of photocatalytic reduction. 

Aniline (also know as aminobenzene and benzamine) was first produced in 1826 by Unverdorben by the dry distillation of indigo, the oldest known vat dye. Fritsche also obtained aniline from indigo by heating it with potash (K2O), and he named it aniline. Hofmann obtained aniline by the reduction of nitrobenzene in 1843 and was able to prove the structure. Aniline is a colorless, oily, flammable liquid that is slightly soluble in cold water and infinitely soluble in alcohol and ether. Its physical properties are summarized in Table 20.1. 

After the reduction of nitrobenzene is completed, it is necessary to neutralize any excess of acid present and decompose the aniline salts. When tin is employed as a reducing agent, it is necessary to use about 7-8 moles of sodium hydroxide for each mole of aniline. When iron is employed, a small amount of sodium carbonate is used to neutralize the excess of acid and decompose the anihne salts. After this operation the reaction mixture is subjected to steam distillation to separate the aniline from the mixture. 

Despite the availability of methods for extracting aniline from coal tar, this source hardly provided an abundant supply of the aromatic amine. Some chemists worked on the development of the two-step synthesis from coal-tar benzene. They included Hofmann s assistant Charles Blachford Mansfield, who pioneered the separation by distillation of coal-tar hydrocarbons, undertook nitration of benzene, and reduction of the nitrobenzene, probably by the method of Zinin. Mansfield s experiments came to an abrupt end early in 1855 when, while preparing samples for the Paris International Exhibition, a still in his laboratory caught fire. He was badly burned and died in hospital a few days later. 

The synthesis of aminobenzenes from reduction of nitrobenzenes was beyond any doubt a pivotal discovery, and the original preparation of aniline by Nikolai Zinin is still kept in the Museum of Kazan School of Chemistry (Russia) http //www.kazan.ru/tat ru/universitet/ museums/chmku/eng/s2.php... 

Vapor-phase reductions are sometimes fraught with a number of technological problems (1) limited per-pass conversion, thus necessitating separation of aniline from nitrobenzene (unless the heat of reaction is gainfully employed, this will constitute a significant item of expense), and (2) sensitivity of catalytic operations which may result in overreduction and Pharm. Soc. Japatif 74, 889 (1954). 

Working under favorable conditions with a nickel catalyst. Brown and Henke obtained a 95 per cent yield of aniline from nitrobenzene. With a constant rate of flow of nitrobenzene, the yields drop off with too much or too little hydrogen. This fluctuation has been ascribed to over- or underreduction. Since nickel is such an active catalyst for this purpose, reduction of the aniline to cyclohexane and ammonia is known to take place. 

It is economically advantageous to conduct the manufacture of aniline from chlorobenzene in conjunction with the large-scale production of chlorine and chlorinated products, to permit the introduction of cheap chlorobenzene into the aniline plant. With such a setup, this process competes favorably with the older method involving the iron-acid reduction of nitrobenzene. 

In order to liberate the aniline formed as the result of the reduction of nitrobenzene, the product of the reaction is treated with an excess of sodium hydroxide, and distilled with steam. Enough of the alkali must be added to precipitate the tin and to set free the aniline from its hydrochloride. 

The procedure for isolating and purifying the aniline formed in this experiment represents an excellent example of how the physical and chemical properties of a component in a mixture of organic substances can be exploited to isolate it without using chromatographic techniques. For example, after completing the procedure for the chemical reduction of nitrobenzene, it is necessary to remove aniline from its principal impurities, which are unchanged nitrobenzene and two by-products, benzidine (12), and 4-aminophenol (13). 

Reductive carbonylation of nitro compounds is catalyzed by various Pd catalysts. Phenyl isocyanate (93) is produced by the PdCl2-catalyzed reductive carbonylation (deoxygenation) of nitrobenzene with CO, probably via nitrene formation. Extensive studies have been carried out to develop the phosgene-free commercial process for phenyl isocyanate production from nitroben-zene[76]. Effects of various additives such as phenanthroline have been stu-died[77-79]. The co-catalysts of montmorillonite-bipyridylpalladium acetate and Ru3(CO) 2 are used for the reductive carbonylation oLnitroarenes[80,81]. Extensive studies on the reaction in alcohol to form the A -phenylurethane 94 have also been carried out[82-87]. Reaction of nitrobenzene with CO in the presence of aniline affords diphenylurea (95)[88]. 

The chemical production of aminophenols via the reduction of nitrobenzene occurs in two stages. Nitrobenzene [98-95-3] is first selectively reduced with hydrogen in the presence of Raney copper to phenylhydroxylamine in an organic solvent such as 2-propanol (37). With the addition of dilute sulfuric acid, nucleophilic attack by water on the aromatic ring of /V-phenylhydroxylamine [100-65-2] takes place to form 2- and 4-aminophenol. The by-product, 4,4 -diaminodiphenyl ether [13174-32-8] presumably arises in a similar manner from attack on the ring by a molecule of 4-aminophenol (38,39). Aniline [62-53-3] is produced via further reduction (40,41). 

Chiral recognition of A-[Co(phen)3]3+ has been observed in a modified /3-cyclodextrin.772 Chiral discrimination has also been seen in photoinduced energy transfer from luminescent chiral lanthanoid complexes773 to [Co(phen)3]3+ and between photoexcited [Ru(bpy)3]2+ and [Co(phen)3]3+ co-adsorbed on smectite clays.774 The [Co(bpy)3]3+ ion has been incorporated into clays to generate ordered assemblies and also functional catalysts. When adsorbed onto hectorite, [Co(bpy)3]3+ catalyzes the reduction of nitrobenzene to aniline.775 The ability of [Co(phen)3]3+ to bind to DNA has been intensively studied, and discussion of this feature is deferred until Section 6.1.3.1.4. 

The reduction of nitrobenzene to aniline is a major industrial process at the heart of the production of polyurethanes, and it is also often used as a marker reaction to compare activities of catalysts [1,2], It can be performed over a variety of catalysts and in a variety of solvents. As well as its main use in polymethanes, aniline is used in a wide range of industries such as dyes, agrochemicals, by further reaction and functionalisation. Reductive alkylation is one such way of functionalising aromatic amines [3, 4], The reaction usually takes place between an amine and a ketone, aldehyde or alcohol. However it is possible to reductively alkylate direct from the nitro precursor to the amine and in this way remove a processing step. In this study we examined the reductive alkylation of nitrobenzene and aniline by 1-hexanol. 

In the sulphonation of aniline small amounts of the o-compound are produced along with sulphanilic acid. Aniline o-sulphonic acid, however, is of no further interest. Metanilic acid, on the other hand, is also manufactured as an intermediate in the azo-dye industry. It is obtained from nitrobenzene-m-sulphonic acid by reduction. The amino-(iand hydroxy-) sulphonic adds of the naphthalene series are of the greatest technical importance. They are either diazotised themselves or serve for coupling with other diazo-compounds. In this way the most important azo-dyes are produced. 

Let us now turn to some kinetic considerations of NAC reduction. As an example, consider the time courses of nitrobenzene (NB) concentration in 5 mM aqueous hydrogen sulfide (H2S) solution in the absence and presence of natural organic matter (Fig. 14.7). As is evident, although reduction of NB by H2S to nitrosobenzene and further to aniline (Eq. 14-31) is very favorable from a thermodynamic point of view (see Fig. 14.4), it seems to be an extremely slow process. However, when DOM is added to the solution, reduction occurs at an appreciable rate (Fig. 14.7). In order to understand these findings, some general kinetic aspects of redox reactions involving NACs should be recognized. 

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