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 90 0C. 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 182-184 0C.
Physical properties of aniline
Aniline is colourless oily liquid having boiling point 1840C and melting point – 60C.
It has a characteristic unpleasant smell.
It is sparingly soluble (insoluble0 in water but soluble in organic solvents like ethanol, ether and
benzene.
It is steam volatile (and hence can be steam distilled from a mixture containing inorganic
substances).
It is highly toxic in nature.
Chemical properties of aniline
A. Reactions due to amino group :
1. Basic nature of aniline :
It is basic in nature due to presence of loan pair of electron on nitrogen atom.
It is weaker base than aliphatic amines and ammonia.
Being basic in nature it reacts with acid to give salt. Eg.
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. HNO 3 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
It is used in the manufacture of dyes and drugs.
It is used for the preparation of organic reagents like acetanilide, sulphanilic acid, etc.
It is used in the rubber industry for the processing of rubber chemicals and products such as car
tyres, balloons, gloves, etc.
It is used as a pesticide and fungicides in the agricultural industry.
Reaction practice chart of aniline
Answer :
