Question

Account for the following:
(i) pKb of aniline is more than that of methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
(iv) Although amino group is o, p− directing in aromatic electrophilic substitution
reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
(v) Aniline does not undergo Friedel-Crafts reaction.
(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic

(vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.

Answer 1

Aniline undergoes resonance and as a result, the electrons on the N-atom are delocalized
over the benzene ring. Therefore, the electrons on the N-atom are less available to donate.

On the other hand, in case of methylamine (due to the +I effect of methyl group), the electron density on the N-atom is increased. As a result, aniline is less basic than methylamine. Thus, pKb of aniline is more than that of methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not: Ethylamine when added to water forms intermolecular H−bonds with water. Hence, it is soluble in water

But aniline does not undergo H−bonding with water to a very large extent due to the presence of a large hydrophobic −C₆H₅ group. Hence, aniline is insoluble in water.

(iv) Although amino group is o,p− directing in aromatic electrophilic substitution
reactions, aniline on nitration gives a substantial amount of mnitroaniline:
Nitration is carried out in an acidic medium. In an acidic medium, aniline is protonated to
give anilinium ion (which is meta-directing).

For this reason, aniline on nitration gives a substantial amount of m-nitroaniline.
(v) Aniline does not undergo Friedel-Crafts reaction:
A Friedel-Crafts reaction is carried out in the presence of AlCl₃. But AlCl₃ is acidic in nature,
while aniline is a strong base. Thus, aniline reacts with AlCl3 to form a salt (as shown in
the following equation).

Due to the positive charge on the N-atom, electrophilic substitution in the benzene ring is
deactivated. Hence, aniline does not undergo the Friedel-Crafts reaction. (vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines: The diazonium ion undergoes resonance as shown below:

This resonance accounts for the stability of the diazonium ion. Hence, diazonium salts of
aromatic amines are more stable than those of aliphatic amines.
(vii) Gabriel phthalimide synthesis is preferred for synthesising primary
amines: Gabriel phthalimide synthesis results in the formation of 1° amine only. 2° or 3° amines
are not formed in this synthesis. Thus, a pure 1° amine can be obtained. Therefore, Gabriel
phthalimide synthesis is preferred for synthesizing primary amines.