Class 12 Chemistry Amines Notes

Class 12 Chemistry Amines Notes

Class 12 Chemistry Notes

Chapter 9: Amines

1. Amines

  • Amines are derivatives of ammonia (NH₃).
  • They are formed when one or more hydrogen atoms of ammonia are replaced by alkyl or aryl groups.
  • Nitrogen is sp³ hybridised.
  • Shape: Trigonal pyramidal.
  • Nitrogen contains one lone pair of electrons, making amines basic.

2. Classification of Amines

(a) Primary (1°)

One alkyl/aryl group attached to nitrogen.

General Formula: RNH₂

Example:

  • CH₃NH₂ (Methylamine)

(b) Secondary (2°)

Two alkyl/aryl groups attached.

General Formula: R₂NH

Example:

  • (CH₃)₂NH

(c) Tertiary (3°)

Three alkyl/aryl groups attached.

General Formula: R₃N

Example:

  • (CH₃)₃N

3. Nomenclature

Common Name

Alkyl + amine

Example:

  • CH₃NH₂ → Methylamine
  • C₂H₅NH₂ → Ethylamine

IUPAC Name

Replace -e of alkane with -amine

Examples:

  • CH₃NH₂ → Methanamine
  • C₂H₅NH₂ → Ethanamine
  • C₆H₅NH₂ → Benzenamine (Aniline)

4. Preparation of Amines

1. Reduction of Nitro Compounds

Nitro compound → Primary amine

Reagents:

  • H₂/Ni
  • Fe/HCl
  • Sn/HCl

2. Ammonolysis of Alkyl Halides

RX + NH₃ → RNH₂

Reaction carried out in alcoholic ammonia.

3. Reduction of Nitriles

RCN → RCH₂NH₂

Reagent:

  • LiAlH₄

Produces amine with one extra carbon atom.

4. Reduction of Amides

Amide + LiAlH₄ → Amine

5. Gabriel Phthalimide Synthesis

  • Used only for primary aliphatic amines
  • Cannot prepare aromatic amines.

6. Hoffmann Bromamide Reaction

Amide + Br₂ + NaOH → Primary amine

Important Point

  • Product contains one carbon less than the original amide.

5. Physical Properties

Lower Amines

  • Fishy smell
  • Gases

Solubility

  • Lower amines are soluble in water.
  • Solubility decreases as carbon chain increases.

Boiling Point

Primary > Secondary > Tertiary

Reason:

  • Hydrogen bonding decreases from primary to tertiary.

Alcohols have higher boiling points than amines because O–H hydrogen bonding is stronger.

6. Chemical Properties

(A) Basic Nature

Amines behave as Lewis bases due to lone pair on nitrogen.

Reaction:

RNH₂ + HCl → RNH₃Cl

Basic Strength

Aliphatic amines > NH₃ > Aromatic amines

Reason:

  • Alkyl groups donate electrons (+I effect).
  • In aniline, lone pair participates in resonance, reducing basicity.

(B) Alkylation

Amines react with alkyl halides to give higher amines.

(C) Acylation

Primary and secondary amines react with:

  • Acid chlorides
  • Acid anhydrides

Product:

  • Amides

(D) Carbylamine Test

Primary amine + CHCl₃ + alcoholic KOH

↓

Isocyanide (bad smell)

Uses

  • Test for primary amines only.

(E) Reaction with Nitrous Acid

Primary Aliphatic Amines

Produce alcohol + N₂ gas.

Primary Aromatic Amines

Produce diazonium salts (273–278 K).

Secondary and Tertiary Amines

Give different products (not diazonium salts).

(F) Hinsberg Test

Reagent:

  • Benzenesulphonyl chloride

Used to distinguish:

  • Primary
  • Secondary
  • Tertiary amines

(G) Electrophilic Substitution in Aniline

The –NH₂ group is:

  • Activating
  • Ortho-para directing

Important reactions:

  • Bromination
  • Nitration
  • Sulphonation

Aniline does not undergo Friedel–Crafts reaction because it forms a complex with AlCl₃.

7. Diazonium Salts

General Formula:

ArN₂⁺X⁻

Prepared by diazotisation of aromatic primary amines.

Conditions:

  • NaNO₂ + HCl
  • Temperature: 273–278 K

Diazonium salts are stable only at low temperatures.

8. Important Reactions of Diazonium Salts

Diazonium group can be replaced by:

  • Cl → Sandmeyer / Gattermann reaction
  • Br → Sandmeyer / Gattermann reaction
  • I → KI
  • F → Balz–Schiemann reaction
  • OH → Phenol
  • H → Reduction
  • CN → Sandmeyer reaction
  • NO₂ → Sodium nitrite/Cu

Coupling Reaction

Diazonium salt + Phenol/Aniline

↓

Azo compound

Importance: Preparation of coloured azo dyes.

Important Points

  • Amines are derivatives of ammonia.
  • Nitrogen is sp³ hybridised.
  • Shape is trigonal pyramidal.
  • Lower amines are water soluble.
  • Boiling point: 1° > 2° > 3°.
  • Basic strength: Aliphatic > NH₃ > Aromatic.
  • Gabriel synthesis gives only primary aliphatic amines.
  • Hoffmann bromamide reaction decreases the carbon chain by one.
  • Carbylamine test identifies primary amines.
  • Hinsberg test distinguishes 1°, 2°, and 3° amines.
  • Diazonium salts are prepared only at 273–278 K.
  • Diazonium salts are important intermediates for preparing substituted aromatic compounds and azo dyes.