Answer:
Due
to the strong activating effect of group,
aromatic amines readily undergo electrophilic substitution reactions and it is
difficult to stop the reaction at the monosubstitution stage. Usually the
reaction occurs to give 2, 4, 6-trisubstituted amines. For- example, aniline on
treatment with bromine water gives 2, 4, 6-
tribromoaniline.
If, however, we wish to stop the reaction at the monosubstitution stage,
the activating effect of the - group is
reduced by acetylation. The acetyl group being electron-withdrawing
attracts the lone pair of electrons on the N-atom towards itself. As a result,
the lone pair of electrons on the N-atom is not exclusively available for
donation to the benzene ring and hence the activating electrons of the ? group is reduced. This method
is known as protection of the amino group by acetylation and can be used to
stop the reaction at the monosubstitution stage by preventing the formation of
di- and
trisubstitution products. For example, acetylation of
aniline gives acetanilide. Bromination of acetanilide with followed by acid hydrolysis
gives p-bromoaniline as the major product. Similarly, nitration of acetanilide
with a mixture of cone. + cone. followed by acid hydrolysis
give p-nitroaniline as the major product. For equations, refer to page 13/35.
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