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NITROBENZENE, C 6 H 5 NO2, the simplest aromatic nitro compound. It was first isolated in 1834 by E. Mitscherlich (Fogg. Ann., 1834, 31, p. 625), and is prepared commercially by the gradual addition of benzene to a well-cooled mixture of concentrated nitric and sulphuric acids, the oily product being separated, washed with alkali, and then distilled. It also results in the oxidation of aniline by monopersulphuric acid (H. Caro, Zeit. angew. Chem., 1898, p. 845) or by potassium permanganate (E. Bamberger, Ber., 1893. 26, p. 496); by the oxidation of nitrosobenzene (below) with atmospheric oxygen; or by the decomposition of benzene diazonium nitrate mercury nitrite, Hg(NO 2 ) 2 -2C<iH5N 2 -NO3, with copper powder (A. Hantzsch, Ber., 1900, 33, p. 2551). It is a yellowish liquid possessing a strong smell of oil of bitter almonds. It boils at 2O9C., and melts at 3-6 C. (C.E. Linebarger, Amer. Chem. Jour., 1896, 18, p. 437). The products of its electrolytic reduction vary with the conditions: in sulphuric acid solution it yields para-aminophenol (L. Gattermann, Ber., 1893, 26, p. 1844); in alcoholic alkaline solution it yields azoxybenzene; in acid alcoholic solution, benzidine; in ammoniacal alcoholic solution, phenylhydrazine. With chlorine, in the presence of iodine or antimony chloride, it yields meta-chlornitrobenzene. Hydrobromic acid at i85-i9oC. converts it into di- and tri-bromaniline. It occasionally acts as an oxidizing agent, as in the preparation of quinoline and f uchsine. It is used commercially for the preparation of aniline and of benzidine; and in perfumery (oil of mirbane).

Dinitrobenzenes, CeH^NOa^. Ortho-dinitrobenzene is formed in small quantity in the preparation of meta-dinitrobenzene, and also results from the action of nitro-sulphuric acid on bismuth triphenyl (A. Gillmeister, Ber., 1897, 30, p. 2844). It forms colourless crystals which melt at 116-5 C. and boil at 319 C. (773 mm.). On boiling with aqueous caustic soda, it yields ortho-nitrophenol. Meta- dinitrobenzene is formed by the direct nitration of nitrobenzene with fuming nitric acid, the product being poured into water and recrystallized from dilute alcohol. It forms practically colourless needles which melt at 89-7 C., and boil at 302-8 C. It is used for the preparation of meta-phenylene diamine. Para-dinitrobenzene results from the action of nitrogen peroxide on an ethereal solution of quinone dioxime (R. Oliveri-Tortorici, Gazz., 1900, 30, i. p. 533). It crystallizes in colourless needles, which melt at 171 -172 C. It is only slightly soluble in cold water and cold alcohol.

Trtnitrobenzenes, C 6 Hs(NOz)3. Asymmetrical (1.2.4) trinitrobenzene results from the action of fuming nitric and sulphuric acids on para-dinitrobenzene. It forms yellow crystals, which melt at 57-5 C. When boiled with dilute aqueous caustic soda it yields 2-4 dinitrophenol. Symmetrical (1.3.5) trinitrobenzene is formed by the further nitration of meta-dinitrobenzene with fuming sulphuric and nitric acids; by the action of hydrochloric acid on sodium malonyl aldehyde (H. B. Hill and J. Torray, Ber., 1895, 28, p. 2598), or by the action of water on 2.4.6-trinitrobenzoic acid (German patent 77353). It crystallizes in prisms which melt at 121 C. It yields addition compounds, with aniline and naphthalene, and combines directly with potassium methylate, sodio-malonic ester and hydrocyanic ester. Alkaline potassium ferricyanide oxidizes it to picric acid.

Nitrosobenzene, C 6 H 6 NO, was first obtained by the action of nitrosyl bromide or chloride on mercury diphenyl (A. Baeyer, Ber., 1874, 7, p. 1638). It results, with other products, in the oxidation of phenyl diazonium chloride with alkaline potassium ferricyanide; of $- phenylhydroxylamine with chromic acid mixture (E. Bamberger, Ber., 1893, 26, pp. 4_73, 483, 1894, 27, p. 1349), or of aniline by monopersulphunc acid (German patent 110575). I* exists in two crystalline forms. Nitric acid passed into its chloroform solution gives phenyl diazonium nitrate. With aniline and acetic acid it yields azobenzene. It combines with aromatic amines to form azocompounds, with arylhydroxylamines to form azoxy compounds, and with hydroxylamine it gives isodiazobenzene.

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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