Amines

1. Definition of Amines

• Amines are derivatives of ammonia (NH₃) where one, two, or all three H atoms are replaced by alkyl and/or aryl groups.

• Used commercially as intermediates in medicines and fibres.

2. Classification of Amines

• Primary (1°): One alkyl/aryl group (e.g., CH₃NH₂)

• Secondary (2°): Two alkyl/aryl groups (e.g., (CH₃)₂NH)

• Tertiary (3°): Three alkyl/aryl groups (e.g., (CH₃)₃N)

3. Preparation of Amines

(a) By Ammonolysis

• Alkyl halides + Ammonia → 1°, 2°, 3° amines.

• Reactivity: RI > RBr > RCl

(b) By Reduction

• Nitro compounds (aromatic or aliphatic) reduced to 1° amines.

(c) Gabriel Phthalimide Synthesis

• Produces only primary aliphatic amines.

• Aromatic amines NOT formed (aryl halides do not undergo nucleophilic substitution).

(d) Hoffmann Bromamide Degradation

• Amide → Amine with 1 less carbon

• Reagents: Br₂ + KOH

4. Physical Properties

• Solubility in water:

  • 1° & 2° amines: soluble (H-bonding)

  • 3° amines: insoluble (no N-H bond)

• Boiling Point Order: 1° > 2° > 3° (due to H-bonding)

5. Basic Nature of Amines

• Amines act as Lewis bases (lone pair on N).

• Aliphatic > NH₃ > Aromatic (due to +I effect of alkyl; aryl has -I and resonance)

In Gaseous State (basicity order):

• 3° > 2° > 1° > NH₃

In Aqueous Solution:

• Depends on +I effect, solvation, and steric hindrance.

• Example (Ethyl amines):
  (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃
  (Methyl amines): (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

• For substituted anilines:

  • EDG (e.g., –CH₃): increase basicity

  • EWG (e.g., –NO₂): decrease basicity

6. Chemical Reactions of Amines

(a) Acylation

• With acid chlorides or anhydrides in presence of pyridine (removes HCl).

(b) Carbylamine Test (Isocyanide Test)

• 1° amine + CHCl₃ + alc. KOH → foul-smelling isocyanide

(c) Reaction with Nitrous Acid (HONO)

• 1° Aliphatic amines → alcohols

• 1° Aromatic amines → diazonium salts (stable at 273 K)

(d) Hinsberg’s Test

• With C₆H₅SO₂Cl:

  • 1° amines → sulphonamides (soluble in alkali)

  • 2° → insoluble sulphonamides

  • 3° → no reaction

7. Electrophilic Substitution in Aromatic Amines

• –NH₂ group is ortho/para-directing, strongly activating

• Must be protected (acylated) to get monosubstituted products.

Nitration

• Aniline gets protonated in acidic medium → meta-directing anilinium ion

• To get para product → acetyl protection first

Sulphonation

• Forms sulphonic acid derivatives.

Friedel-Crafts Reaction

• Not possible (–NH₂ forms salt with AlCl₃, deactivates ring).

8. Diazonium Salts

• Primary aromatic amines → stable arenediazonium salts at 273–278 K

• Resonance stabilizes diazonium ion.

• Primary aliphatic amines → unstable alkyl diazonium salts

9. Practice Questions

1. Arrange in decreasing pKb:
   C₂H₅NH₂, C₆H₅NHCH₃, (C₂H₅)₂NH, C₆H₅NH₂

2. Basic strength (increasing order):
   C₆H₅NH₂ < C₆H₅N(CH₃)₂ < (C₂H₅)₂NH < CH₃NH₂

3. Arrange:
   Aniline, p-nitroaniline, p-toluidine (basic strength)

4. Chemical tests:

   • Methylamine vs Dimethylamine

   • Secondary vs Tertiary amines

   • Ethylamine vs Aniline