Page No. 278

9.1. Write IUPAC names of the following compounds and classify them into primary, secondary, and tertiary amines.

(i) (CH₃)₂ CHNH₂

(ii) CH₃(CH₂)2NH₂

(iii) CH₃NHCH(CH₃)₂

(iv) (CH₃)3CNH₂

(v) C₆H₅NHCH₃

(vi) (CH₃CH₂)2NCH₃

(vii) m-BrC₆H₄NH₂

Solution

(i) Propan-2-amine (1° amine)

(ii) Propan-1-amine (1° amine)

(iii) N-methylpropan-2-amine (2° amine)

(iv) 2-Methylpropan-2-amine (1° amine)

(v) N-Methylbenzamine or N-methylaniline (2° amine)

(vi) N-Ethyl-N-methylethanamine (3° amine)

(vii) 3-Bromobenzenamine or 3-bromoaniline (1° amine)

(viii) N, N-dimethylethanamine (3° amine)

9.2. Give one chemical test to distinguish between the following pairs of compounds:

(i) Methylamine and dimethylamine

(ii) Secondary and tertiary amines

(iii) Ethylamine and aniline

(iv)  Aniline and benzylamine

(v) Aniline and N-Methylaniline.

Solution

(i) Methylamine being a 1° amine can be distinguished by the carbylamine test.

Carbylamine test: Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form foul-smelling isocyanides or carbylamines. Methylamine (being an aliphatic primary amine) gives a positive carbylamine test, but dimethylamine does not.

(ii) Secondary and tertiary amines can be distinguished by allowing them to react with Hinsberg’s reagent (benzenesulphonyl chloride, C₆H₅SO₂Cl).

Secondary amines react with Hinsberg’s reagent to form a product that is insoluble in an alkali. For example, N, N−diethylamine reacts with Hinsberg’s reagent to form N, N−diethylbenzenesulphonamide, which is insoluble in an alkali. Tertiary amines, however, do not react with Hinsberg’s reagent.

(iii) Ethylamine and aniline can be distinguished using the azo-dye test. A dye is obtained when aromatic amines react with HNO₂ (NaNO₂ + dil.HCl) at 0-5°C, followed by a reaction with the alkaline solution of 2-naphthol. The dye is usually yellow, red, or orange in colour. Aliphatic amines give a brisk effervescence due (to the evolution of N₂ gas) under similar conditions.

Aliphatic amines such as ethylamine give a brisk effervescence due to the evolution of N₂ gas under similar conditions.

(iv) Aniline and benzylamine can be distinguished by their reactions with the help of nitrous acid, which is prepared in situ from a mineral acid and sodium nitrite. Benzylamine reacts with nitrous acid to form unstable diazonium salt, which in turn gives alcohol with the evolution of nitrogen gas.

On the other hand, aniline reacts with HNO₂ at a low temperature to form stable diazonium salt. Thus, nitrogen gas is not evolved.

(v) Primary amines, on heating with chloroform and ethanolic potassium hydroxide, form foul smelling isocyanides or carbylamines. Aniline, C₆H₅NH₂ is a primary aromatic amine and N-methylaniline, C₆H₅NH(CH₃) is a secondary amine. Hence, aniline will give positive carbylamine test while N-methylaniline will not.

Or,

Aniline and N-methylaniline can be distinguished using the Carbylamine test. Primary amines, on heating with chloroform and ethanolic potassium hydroxide, form foul-smelling isocyanides or carbylamines. Aniline, being an aromatic primary amine, gives positive carbylamine test. However, N-methylaniline, being a secondary amine does not.

9.3. Account for the following

(i) pK_b 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 and 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.

Solution

(i) pK_b of aniline is more than that of methylamine: Higher the pKb value lower is the basic strength.

In aniline due to resonance, the electrons on the N-atom are delocalised 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, pK_b 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 bulky hydrophobic –C₆H₅ group. Hence, aniline is insoluble in water.

(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide: Methylamine is more basic than water due to the +I effect of the CH₃ group. Therefore, in water, methylamine produces OH^– ions by accepting H⁺ ions from water.

CH₃​−NH₂​+ H – OH → CH₃​−NH₃⁺​ + OH⁻

Then, OH^– ion reacts with Fe³⁺ ion, from dissociated FeCl₃ to form a precipitate of hydrated ferric oxide.

2Fe³⁺ + 6OH⁻→ Fe₂O₃.3H₂O

Fe₂O₃.3H₂O: 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).

For this reason, aniline on nitration gives a substantial amount of m-nitroaniline.

(v) Aniline does not undergo Friedel-Craft’s reaction : A Friedel-Craft’s reaction is carried out in the presence of AlCl₃ but AlCl₃ is a Lewis acid while aniline is a strong base. Thus, aniline reacts with AlCl₃ to form a 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-Craft’s 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 phthamide 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.