Reactions and characterisation of aromatic nitro compounds

Nitro compounds, when liquid, have characteristic odours, are insoluble in water, highly refractive and with a density greater than unity. Many are crystalline solids. Most nitro compounds are slightly coloured, generally yellow the intensity of the colour increases with the number of nitro groups. The following reactions will assist in their detection. 

It must be noted, however, that nitroso, azoxy and azo compounds when subjected to the same treatment yield resj ectively hydroxylamines, hydrazo and hydrazine compounds, all of which reduce ammoniacal silver nitrate solution in the cold. 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0- 5 g. of solid ammonium chloride and about 0 5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and test the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not affect the reagent used. 

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

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 off the precipitated acid. Purify the crude acid by extraction with sodium carbonate solution, precipitation with dilute mineral acid, and recrystallisation from hot water, benzene, etc. 

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IV,16A. Reactions and characterisation of aromatic ilitro compounds IV,16B. Reactions and characterisation of aliphatic nitro compounds... 

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