Nitro-hydrocarbons aromatic, preparation

Laurent summarised the experiments which had been made on the action of nitric acid on aromatic hydrocarbons, and prepared nitro-derivatives of naphthalene, anthracene, chrysene and pyrene. In a table he arranged them in decreasing order of the numbers of equivalents of carbon, beginning with anthracene and ending with phene (benzene) He continued to... 

The reduction of aromatic nitro-compounds is of exceptionally great interest, not only scientifically, hut also technically. The conversion of the hydrocarbons of coal tar into useful products began with the discovery of the nitration process the conversion, on the technical scale, of the nitro-group of nitrobenzene into the amino-group gave aniline, the starting material for the preparation of innumerable dyes and pharmaceutical products to aniline were added the homologous toluidines, xylidines, naphthylamines, and so on. 

Many polycyclic aromatic amines and aldehydes are commercially available, but their supply is very limited. Preparation of these starting materials is necessary for studying the (3-lactam formation reaction [93]. Nitro compounds are the precursors for the amines. An important task was to prepare polycyclic aromatic nitro compounds, particularly those of chrysene, phenanthrene, pyrene, and dibenzofluorene in good yield. Nitration of these hydrocarbons with concentrated nitric acid in sulfuric acid is a widely used reaction for this purpose. Our research culminated in facile synthesis of polyaromatic nitro derivative 9 starting from polyaromatic hydrocarbons (PAHs) 8 through the use of bismuth nitrate impregnated with clay (Scheme 1) ([94, 95] for some examples of bismuth nitrate-catalyzed reactions... 

The preparation of phenols by the hydrolysis of diazonium salts with hot aqueous acid, and by a recent milder procedure suitable for diazonium salts having additional acid-sensitive groups, is discussed in Section 6.7.1, p. 922, and illustrated in Expt 6.69. Although these methods enable an aromatic hydrocarbon system to be converted in good yield into a phenol via the corresponding nitro and amino derivatives, the shorter route involving the alkaline fusion of the sulphonic acid discussed above may often be preferred. 

By means of this method, o-nitrophenylarsonic acid was prepared in a yield of 86% as compared with a yield of only 60% by the original Bart process. According to Schmidt, the proportion of by-products, especially the parent hydrocarbon (benzene in the illustration), increases with higher alkalinity. In acid solution the yields are low unless the aromatic diazo compounds contain strongly negative substituents, such as nitro groups, in the ortho and para positions (as in 2,4-dinitrobenzenediazo-nium chloride, for example). 

SAFETY PROFILE An explosive. A hazard in preparation. Reacts with heavy metals to form heat- or impact-sensitive explosive salts. When heated to decomposition it emits toxic fumes of NOx. See also NITRO COMPOUNDS of AROMATIC HYDROCARBONS and EXPLOSIVES, HIGH. 

Three patents issued in 1924 and 1925 are conspicuous as placing special emphasis upon the preparation of particular catalysts as suitable for reactions involving the partial oxidation of side chains present in aromatic nuclei. The first00 of these relates to the oxidation of aromatic hydrocarbons and their derivatives, viz., toluene, xylene, cymene, cumene, mesitylene, cresols, etc., together with their derivatives including nitro-... 

The nitro derivatives of the aromatic compounds are prepared by the action of nitric acid on the hydrocarbons or their substitution-products. The process is called nitration. The ease with which reaction takes place is determined by the nature of the element or group in combination with the benzene ring. In general, it is more diflScult to nitrate a compound which contains a strongly negative substituent than one which contains alkyl, hydroxyl, or amino groups. For example, benzoic acid,... 

Nitro derivatives of certain aromatic hydrocarbons are manufactured commercially in large quantities, and are used in the preparation of amines which are converted into a great variety of dyes. 

Diamines.—Derivatives of aromatic hydrocarbons which contain two or more amino groups are known. They are prepared by reducing nitro compounds which contain two or more nitro groups. The diamines are colorless compounds which rapidly turn brown in the air, and are oxidized readily. When they are treated with an aqueous solution of ferric chloride characteristic colors are formed. 

In this way aniline, C6H5NH2, can be converted into phenol, CeHsOH. As aromatic amines are prepared from nitro compounds, which are formed by the action of nitric acid on hydrocarbons, the reaction furnishes what is at times the most convenient method of introducing a hydroxyl group into a hydrocarbon. 

Nitro Compounds as Explosives.—Nitro compounds prepared from aromatic hydrocarbons and certain of their derivatives were very important explosives used in the recent war. The compounds differ markedly in the properties which are characteristic of explosives namely, (1) sensitiveness to shock, (2) explosive force, and (3) the velocity of the explosion through the substance. If (1) is very high the explosive can not be transported very safely (2) determines the amount of the explosive to be used if (3) is very high the pressure is developed to its maximum so suddenly that rupture of the gun in which it is used may take place. Substances which are very sensitive to shock are used as detonators or boosters a small amount of the material is exploded by the trigger and the explosive wave set up causes the explosion of the less sensitive material. Mercury fulminate, lead azide, Pb(Ns)2, and several nitro derivatives of aniline (see below) are used for this purpose. 

Aliphatic nitro compounds cannot as a rule be prepared in the same way as the aromatic nitro compounds. The more rapid oxidation of aliphatic hydrocarbons by nitric acid is the main interfering factor, so that conditions must be chosen which minimize oxidation and promote nitration. The oxidation reactions are of such complexity in these cases that no attempt will be made to formulate them. Only a summary of the conditions favoring nitration will be given. The use of a solvent such as ether for carrying out the reaction is often successful. Also dilute nitric acid has been used, and alkyl (generally ethyl) nitrate. In the Friedel-Crafts reaction with ethyl nitrate, aluminium chloride is used as catalyst. In aliphatic nitrations with ethyl nitrate, alkalis such as metal alkoxides (NaOC Hs) are found to be best. The use of alkalis brings out the similarity of this reaction to aldol condensations which are also favored by alkalis. An example of aliphatic nitration, in comparison with an aromatic one may be given ... 

Advances continue to be made in the synthesis and use of polymer-supported homogeneous catalysts. A rhodium(i) hydrogenation catalyst, prepared as outlined in Scheme 1, is much more versatile than most homogeneous rhodium catalysts. This catalyst will not only reduce a variety of olefinic and aromatic hydrocarbons under mild conditions but also carbonyl, nitrile, and nitro-groups. The catalyst is air stable, but its lifetime appears to be less than those of other polymer-supported catalysts. 

Sometimes it is advantageous to prepare a trinitro derivative [m, p-xylene (36)]. The preparation of mononitroderivatives, which are usually liquid, is generally combined with reduction and the acetylation of the amine formed (37, 38). Nitration is often utilized for the detection and the determination of small amounts of aromatic hydrocarbons, for example, in air. After nitration the nitro derivatives fimmed can be detected by a sensitive color reaction or polarographically. 

Aromatic nitro compounds can also be identified by the introduction of an additional nitro group or by the oxidation of the alkyl group—if present on the nucleus —and, finally, in the case of di- and polynitro compounds by the preparation of addition compounds (1), preferably with aromatic hydrocarbons. 

---