Chemistry Class 12 NCERT Solutions: Chapter 13 Amines Part 5

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Q: 3. Account for the following:

(i) 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 amines

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

Answer:

(i) of aniline is more than that of methylamine.

Q 3 i Aniline and Methylamine

Q 3 I Aniline and Methylamine

Q 3 i Aniline and Methylamine

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.

Q 3 i 1 The Electrons on the N-atom are Delocalized over the Benzene ring

Q 3 I 1 Electrons on the N-Atom Delocalized Over Benzene

Q 3 i 1 The Electrons on the N-atom are Delocalized over the Benzene ring

On the other hand, in case of methylamine (due to the effect of methyl group), the electron density on the N-atom is increased. As a result, aniline is less basic than methylamine. Thus, 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 bonds with water. Hence, it is soluble in water.

Q 3 ii Ethylamine when added to water forms Intermolecular H-Bonds with Wate

Q 3 Ii Ethylamine Forms Intermolecular H-Bonds With Wate

Q 3 ii Ethylamine when added to water forms Intermolecular H-Bonds with Wate

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

Q 3 ii 1 Structure of Aniline

Q 3 Ii 1 Structure of Aniline

Q 3 ii 1 Structure of Aniline

(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide:

Q 3 iii Structure of Methylamine and Water

Q 3 Iii Structure of Methylamine and Water

Q 3 iii Structure of Methylamine and Water

Due to the effect of group, methylamine is more basic than water. Therefore, in water, methylamine produces ions by accepting ions from water.

Ferric chloride dissociates in water to form and ions.

Then, reacts with to form a precipitate of 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.

Nitration is carried out in an acidic medium. In an acidic medium, aniline is protonated to give anilinium ion (which is meta-directing).

Q 3 iv Aniline on Nitration gives a Substantial Amount of m-Nitroaniline

Q 3 Iv Aniline on Nitration to M-Nitroaniline

Q 3 iv Aniline on Nitration gives a Substantial Amount of m-Nitroaniline

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 . But is acidic in nature, while aniline is a strong base. Thus, aniline reacts with to form a salt (as shown in the following equation).

Q 3 v Aniline and Salt

Q 3 v Aniline and Salt

Q 3 v Aniline and Salt

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:

Q 3 vi The Diazonium ion Undergoes Resonance

Q 3 Vi the Diazonium Ion Undergoes Resonance

Q 3 vi The Diazonium ion Undergoes Resonance

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 amine only. or amines are not formed in this synthesis. Thus, a pure amine can be obtained. Therefore, Gabriel phthalimide synthesis is preferred for synthesizing primary amines.