Nitro- compounds

Nitro-compounds are prepared by the direct action of nitric acid. The reaction is greatly facilitated if a mixture of nitric and sulphuric acid is used. 

Some nitro-compounds are themselves coloured and can be used as dyestuffs, e.g. picric acid. In this case the nitro-group can be considered to be the chromophore. For aliphatic nitro-compounds see nitroparaffins. 

Mix 1 g. of the nitro compound with 4 g, of sodium dichromate and 10 ml. of water in a 50 ml. flask, then attach a reflux condenser to the flask. Add slowly and with shaking 7 ml. of concentrated sulphuric acid. The reaction usually starts at once if it does not, heat the flask gently to initiate the reaction. When the heat of reaction subsides, boil the mixture, cautiously at first, under reflux for 20-30 minutes. Allow to cool, dilute with 30 ml. of water, and filter oflF the precipitated acid. Purify the crude acid by extraction with sodium carbonate solution, precipitation with dUute mineral acid, and recrystaUisation from hot water, benzene, etc. 

Aliphatic nitro compounds. These are isomeric with the alkyl nitrites and may be prepared from the alkyl halide and silver nitrite, for example C,H,aBr + AgNOj — C Hj NO + AgBr 

The alkaline reduction of an aromatic nitro-compound to give the azoxy 

On reduction, the nitro-compounds give rise to a series of products, e.g. 

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

Both aliphatic and aromatic nitro-compounds can be readily reduced in acid solution to the corresponding primary amine. Thus when a mixture of nitrobenzene and tin is treated with hydrochloric acid, the tin dissolves to give stannous chloride, SnCh, which in these circumstances then reacts with more acid to give stannic chloride, SnCl, and the nascent hydrogen produced from 

A number of selected aromatic nitro compounds are collected in Table IV,16A, It will be noted that a few nitro aromatic esters have been included in the Table. These are given here because the nitro group may be the first functional group to be identified aromatic nitro esters should be treated as other esters and hydrolysed for final identification. 

Oxidation of side chains. Aromatic nitro compounds that contain a side chain (e.g., nitro derivatives of alkyl benzenes) may be oxidised to the corresponding acids either by alkahne potassium permanganate (Section IV,9, 6) or, preferably, with a sodium dichromate - sulphuric acid mixture in which medium the nitro compound is more soluble. 

Yellow. Quinones, m- and p-nitroaniline, o-nitrophenol, and many other nitrO"Compounds. [Note that some nitro-compounds often appear yellow (e.g. m-dinitrobenzene and 3, 5 -dinitro-benzoic acid), but are colourless when absolutely pure.] Iodoform. 

Monohydric alcohols, aldehydes (including chloral hydrate), ketones, cinnamic acid, amines (2-naphthylaminc is odourless), nitrophenols (resemble both phenol and nitro-compound), 

The following reactions will assist in the detection of aliphatic nitro compounds. 

For a discussion of the Reactions and Characterisation of Aliphatic Nitro Compounds, see Section IV,16B.) 

Problems still exist with AM 1, freatment of phosphorus-oxygen bonds is inaccurate, nitro compounds are still too positive in energy, and the peroxide bond, for example, is still too short. In many cases, l M3 is an improvement over AM 1, 

C. A typical aromatic amine. Best prepared by the prolonged action of concentrated ammonia solution at a high temperature upon anthraquinone-l-sulphonic acid in the presence of BaClj and by reduction of the corresponding nitro compound or by amination of the chloroanthraquinone. 

NH3) are of great commercial importance practically all modem explosives contain high proportions of ammonium nitrate or organic nitro-compounds. Nitrates are also employed as fertilizers. 

It should be noted that only representative substances are indicated in the above list. Substituted derivatives of the compounds in most classes may be encountered, e.g., nitrobenzoic acid in the aromatic carboxylic acids (p. 347). This acid will contain CH(0)N, but the salient properties are still those of a carboxylic acid, CH(0), Section 14, although the properties of an aromatic nitro-compound (e.g.y reduction to an amino-compound) will also be evident. 

N-phenylhydroxylamine, PhNHOH and further reduction can give azoxybenzene, azobenzene, hydrazobenzene and aniline. The most important outlet commercially for the nitro-compounds is the complete reduction to the amines for conversion to dyestufTs. This is usually done in one stage with iron and a small amount of hydrochloric acid. 

---