Online Chemistry notes

Aniline – Lab preparation, Properties, Reactions and Uses.

Chemist

Aniline (Aromatic amine)

Aniline is also known as aminobenzene or phenylamine.

Laboratory preparation of aniline

Aniline is prepared in laboratory by reducing nitrobenzene with tin (Sn) and conc. HCl.

10 ml nitrobenzene and 20 gm of granulated tin are placed in the 250 ml round bottom flask fitted with a reflux condenser. 50 ml of conc. HCl is added gradually with constant shaking. After each addition, the round bottom flask is cooled so that temperature may not go above 900C. Then the reaction mixture is heated on a boiling water bath for about one hour until the reaction is completed which is indicated by the smell of nitrobenzene, the disappearance of smell indicates the completion of the reaction. The flask is then cooled and a crystalline solid mass of double salt is separated out.

The crystalline solid mass is then treated with conc. NaOH until the solution is cleared and becomes strongly alkaline. Aniline is separated out and is floated on the surface as dark brown oil.

The mixture obtained is then subjected to the process of steam distillation until clear distillate is obtained.

Purification : Aniline is extracted by shaking the distillate several times with ether. The ethereal layer is separated each time with the help of separating funnel. Now, the ethereal aniline is placed for the evaporation where ether evaporates out. Aniline thus obtained is finally purified by redistillation at 182-1840C.

Physical properties of aniline

Chemical properties of aniline

A. Reactions due to amino group :

1. Basic nature of aniline :

2. Alkylation reaction of aniline :

The process of replacement of H-atom of amino group by alkyl group (-R) is called alkylation.

Aniline reacts with alkyl halide to give a mixture of secondary, tertiary and quaternary ammonium compounds. Eg.

3. Acylation reaction of aniline:

The process of replacement of hydrogen atom of amino group by acyl group (RCO -) is called acylation reaction. Eg.

4. Benzoylation reaction of aniline:

The process of replacement of H-atom of amino group by benzoyl group (C6H5CO-) is called benzoylation. Eg.

5. Carbylamine reaction : Test reaction for primary amines :

When chloroform is warmed with a primary amine in the presence of alcoholic KOH, an offensive (unpleasant) smell of carbylamines ( i.e. isocyanide) is obtained. This reaction is known as carbylamine reaction.

6. Diazotization reaction : 

When aniline is treated with NaNO2 and dil.HCl ( i.e. nitrous acid) at temperature below 50C, benzene diazonium chloride is obtained. This reaction is known as diazotization reaction.

Note : Nitrous acid is unstable so it is prepared in situ by the reaction of sodium nitrite and dilute HCl.

Note : If this reaction is carried out at ordinary temperature then phenol is obtained.

B. Reactions due to benzene ring :

NH2 group is electron releasing group. Lone pair of electrons present in N –atom of NH2 group participates in resonance with the benzene ring and following resonating structures are formed:

From the above resonating structures it is clear that the ortho and para positions are relatively rich in electron density. Hence the incoming electrophile attacks on these positions to give ortho and para substituted products.

1. Sulphonation reaction of aniline :

Aniline undergoes sulphonation reaction by heating it with conc. H2SO4 for 4-5 hours to get benzene sulphonic acid, commonly known as sulphanilic acid.

2. Nitration of aniline:

Q) Aniline cannot be nitrated directly, why?

Aniline cannot be nitrated directly because conc. HNO3 besides being a nitrating agent is also an oxidizing agent and may oxidize amino group. Hence amino group is first protected by acetylation before nitration. Then the nitro group can be introduced in the ring at ortho and para position. Hydrolysis of the nitrated-acetylated-aniline gives nitro aniline. The para product is the major product.

3. Halogenation (Bromination) of aniline:

Aniline reacts with aq. bromine ( i.e. bromine water) to give white ppt. of 2,4,6-tribromoaniline.

4. Coupling reaction of aniline:

Aniline couples with diazonium salts at low temperature to form colored azo-compounds called azo-dyes. This reaction is called coupling reaction.

Uses of aniline

Reaction practice chart of aniline

Answer :

References