Nitro Compounds

organic compounds containing one or more nitro groups, —NO₂, bound to carbon atoms. A distinction is made among mono-, di-, tri-, and poly-nitro compounds.

Nitro compounds may be aliphatic (nitroparaffins and ni-troolefins); aromatic, containing nitro groups in the aromatic ring (for example, nitroanilines, nitrobenzene, nitrotoluenes, and nitronaphthalene); and aliphatic-aromatic compounds, containing nitro groups only in the aliphatic side chain (for example, phenylnitromethane, C₆H₅CH₂NO₂, and nitrostyrene, C₆H₅-CH—CHNO₂), as well as compounds with nitro groups in the side chain and the aromatic ring, such as p-nitrophenylnitro-methane, O₂NC₆H₄CH₂NO₂.

The aromatic nitro compounds are yellowish liquids or crystalline compounds that dissolve readily in organic solvents and poorly in water. They are synthesized in industry and under laboratory conditions by nitration of aromatic compounds by nitrating mixtures. Aromatic polynitro compounds are used as explosives (trinitrotoluene, the ammonium salt of trinitrophenol [picrates], and trinitrophenylmethylnitramine). The reduction of nitro groups leads to aromatic amines, which are intermediates in the production of various dyes.

Aliphatic nitro compounds are produced in industry mainly by the nitration of paraffins by nitric acid or nitrogen oxides. The simplest mononitroparaffins are colorless liquids with a characteristic odor. Nitroparaffins reduce readily, with the formation of hydroxylamines and amines. The action of bases on primary and secondary nitro compounds leads to the formation of salts of the aci form of nitro compounds, which are called isonitro compounds (I) and are readily converted into aldehydes or ketones (II):

where R is an alkyl group and Rʹ is a hydrogen atom or alkyl group.

Treatment of the salts of nitro compounds (III)—and, in some cases, the nitro compounds themselves or their aci forms—with alkylating agents can produce O-alkylation, with the formation of nitronic acid esters (IV):

or C-alkylation:

where X is a halogen atom. The direction of the reaction depends both on the structure of the salt and on the nature of the alkylating agent.

The action of concentrated acids on nitro compounds leads to the formation of carboxylic acids. Aliphatic nitro compounds are readily condensed with aldehydes and ketones, with the formation of nitroalcohols, which may be dehydrated to give ni-troolefins. These reactions are widely used in laboratory practice. Nitroparaffins readily dissolve various organic compounds.

Some nitro compounds have toxic properties and may injure the liver or, sometimes, the eyes (cataracts).