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Amines – Class 12 Chemistry notes, important questions and MCQs

NCERT Class 12 Chemistry, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Amines are derivatives of ammonia where hydrogen atoms are replaced by alkyl or aryl groups. They are basic and form diazonium salts used in dye making.

Classification and basicity

Primary (RNH₂), secondary (R₂NH), tertiary (R₃N).
Aliphatic amines are more basic than ammonia; aniline is less basic (lone pair in resonance with the ring).
In water, basicity order of methylamines: 2° > 1° > 3° > NH₃ (solvation + inductive effect).

Reactions and tests

Hofmann bromamide degradation: RCONH₂ + Br₂ + 4NaOH → RNH₂ (one carbon less).
Carbylamine test (CHCl₃ + KOH) gives foul-smelling isocyanide — only primary amines.
Hinsberg reagent (benzenesulphonyl chloride) distinguishes 1°, 2°, 3° amines; aniline + HNO₂ at 0–5 °C gives benzene diazonium chloride (coupling gives azo dyes).

Key terms

Diazotisation: conversion of a primary aromatic amine into a diazonium salt
Carbylamine test: test for primary amines giving isocyanide smell
Hinsberg's reagent: benzenesulphonyl chloride, used to distinguish primary, secondary and tertiary amines

Formulas

Basicity: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ > C₆H₅NH₂ (in water)

Important questions with answers

How will you distinguish a primary amine from a secondary amine using the carbylamine test?2 marks

On heating with chloroform and alcoholic KOH, only primary amines give an isocyanide with a foul smell; secondary amines do not.

Write the diazotisation of aniline and one coupling reaction.3 marks

C₆H₅NH₂ + NaNO₂ + 2HCl → (273–278 K) → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O. Coupling with phenol in alkaline medium gives p-hydroxyazobenzene, an orange dye: C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅–N=N–C₆H₄–OH + HCl.

Explain how the Hinsberg test distinguishes 1°, 2° and 3° amines, and convert benzene into aniline and aniline into chlorobenzene with equations.5 marks

With benzenesulphonyl chloride: a 1° amine gives a sulphonamide soluble in alkali (acidic N–H); a 2° amine gives a sulphonamide insoluble in alkali (no acidic H); a 3° amine does not react. Benzene → aniline: nitration C₆H₆ + HNO₃ → (H₂SO₄) → C₆H₅NO₂ + H₂O, then reduction C₆H₅NO₂ + 6[H] → (Sn/HCl or Fe/HCl) → C₆H₅NH₂ + 2H₂O. Aniline → chlorobenzene: diazotise to C₆H₅N₂⁺Cl⁻, then Sandmeyer reaction with CuCl/HCl gives C₆H₅Cl + N₂.

Quick revision

Aniline is a weak base.
Carbylamine test only for 1° amines.
Diazonium salts are prepared at 0–5 °C.

Exam tips

Hofmann degradation shortens the chain by one carbon atom.
Basicity in water: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ > C₆H₅NH₂ (solvation and steric effects decide the order).

Practice MCQs

Which is a secondary amine?
1. (CH₃)₂NH
2. CH₃NH₂
3. (CH₃)₃N
4. NH₃
Show answer
Answer: (a) (CH₃)₂NH. Two alkyl groups.
Diazotisation is carried out at
1. 0–5 °C
2. 100 °C
3. room temperature only
4. 500 °C
Show answer
Answer: (a) 0–5 °C. Diazonium salts are unstable.
Aniline is less basic than methylamine because
1. its lone pair is delocalised into the ring
2. it is heavier
3. it is a gas
4. it has no nitrogen
Show answer
Answer: (a) its lone pair is delocalised into the ring. Resonance.
The carbylamine test is given by
1. primary amines only
2. secondary amines
3. tertiary amines
4. all amines
Show answer
Answer: (a) primary amines only. Isocyanide formation.
Aniline is less basic than ammonia because
1. the lone pair on N is delocalised into the ring
2. aniline is heavier
3. aniline has no lone pair
4. it is aromatic gas
Show answer
Answer: (a) the lone pair on N is delocalised into the ring. Resonance.
Hofmann bromamide reaction of propanamide gives
1. propanamine
2. ethanamine
3. butanamine
4. methanamine
Show answer
Answer: (b) ethanamine. One carbon less.
Benzene diazonium chloride coupled with phenol gives
1. an azo dye (p-hydroxyazobenzene)
2. aniline
3. benzene
4. nitrobenzene
Show answer
Answer: (a) an azo dye (p-hydroxyazobenzene). Orange azo dye.
Gabriel phthalimide synthesis is used to prepare
1. primary aliphatic amines
2. tertiary amines
3. secondary amines
4. aromatic amines
Show answer
Answer: (a) primary aliphatic amines. Aryl halides do not react.

All chapters of Class 12 Chemistry

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.