Aniline is the most important aromatic amine and is the best representative of this class of compounds.
Laboratory Preparation. Aniline
is prepared in the laboratory by reducing nitrobenzene with tin and concentrated hydrochloric acid.
The acid salt of aniline is then treated with sodium hydroxide solution to liberate aniline.
Aniline is separated from the mixture by steam distillation.
The apparatus used is shown in fig. 38.1. Take 10 ml of nitrobenzene and 25 g of granulated tin in the flask. Add 50 ml of conc HCl in portions from the dropping funnel. Shake the mixture, immersing the flask in cold water if the reaction becomes vigorous. When the whole of the acid has been added, heat the flask for about 20 minutes in a boiling water-bath. Cool the flask and add excess of conc NaOH solution which liberates aniline as an ioly liquid. The contents of the flask are then steam-distilled to remove aniline. To the distillate, add 10 g of solid NaCl (salting out) till it has all dissolved. Transfer the oily layer into a test tube and add to it some potassium carbonate to remove water. Redistil using an air-condenser and collect the fraction passing between 180°
C and 185°
C which is pure aniline.
Manufacture of Aniline by Vapour-phase Reduction of Nitrobenzene. Vaporised nitrobenzene mixed with H2 is Passed over a copper catalyst on silica at 250°C.
This is the modern method of making aniline and has superseded the two older methods. The obsolete methods are:
(a) Reduction of Nitrobenzene with iron and dil HCl.
(b) Reaction of Chlorobenzene with Ammonia.
Physical Properties Of Aniline. Aniline when pure is a colourless oil, bp 184.4 °C, density 1.002 at 20°C. It becomes pale yellow, and then rapidly darkens on exposure to air owing to oxidation. It has a faint characteristic odour, It is sparingly soluble in water but dissolves readity in ethanol, diethyl ether and chloroform. It is steam-volatile Aniline is toxic.
Chemical Properties Of Aniline. Aniline molecule is made of one NH2 group bonded to benzene ring, It gives all the replacement reactions of the aliphatic primary amines, and differs only in its behaviour towards nitrous acid. The benzene ring gives the usual electrophilic substitution reactions in ortho and para position.
Uses Of Aniline.
Aniline is used:
- For preparing dyes and dye intermediates.
- For manufacture of antioxidants and vulcanization accelerators in rubber industry.
- For synthesis of drugs, notably sulpha drugs.
- For making isocyanates required for polyurethane plastics used for insulation.