Amines are an important class of organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. They play a vital role in biochemistry, medicines, dyes, and industrial chemistry.
👉 Core Idea: Amines are nitrogen-containing organic compounds that behave as bases due to the presence of a lone pair on nitrogen.
1. Classification of Amines
Amines are classified based on the number of hydrogen atoms replaced in ammonia.
(A) Primary Amines (1°)
One hydrogen replaced.
Structure:
R–NH₂
Example:
CH₃–NH₂ (Methylamine)
(B) Secondary Amines (2°)
Two hydrogens replaced.
Structure:
R–NH–R′
Example:
(CH₃)₂NH (Dimethylamine)
(C) Tertiary Amines (3°)
All three hydrogens replaced.
Structure:
R–N(R′)–R″
Example:
(CH₃)₃N (Trimethylamine)
(D) Quaternary Ammonium Salts
Nitrogen bonded to four groups.
Structure:
[R₄N]⁺ X⁻
Concept Clarity
👉 WHY classification matters?
Because properties and reactions differ for 1°, 2°, and 3° amines.
2. Nomenclature of Amines
(A) Common Naming
Name alkyl group + amine
Example:
CH₃NH₂ → Methylamine
(B) IUPAC Naming
- Replace “e” of alkane with “amine”
Example:
CH₃NH₂ → Methanamine
For Secondary/Tertiary Amines
Use prefix N-
Example:
CH₃NHCH₃ → N-methylmethanamine
3. Structure of Amines
Geometry
Nitrogen in amines is sp³ hybridized.
Shape:
Trigonal pyramidal
Diagram (Text Form)
H
|
H — N — R
:
(lone pair)
Bond Angle
≈ 107°
Concept Clarity
👉 WHY pyramidal shape?
Because lone pair repels bonded pairs.
4. Physical Properties
(A) State
- Lower amines → gases
- Higher amines → liquids/solids
(B) Solubility
- Small amines → soluble in water
- Due to hydrogen bonding
(C) Boiling Point
Order:
1° > 2° > 3°
👉 WHY?
Because primary amines form more hydrogen bonds.
5. Basic Nature of Amines
Definition
Amines act as bases because nitrogen has a lone pair.
Reaction
R–NH₂ + H⁺ → R–NH₃⁺
Order of Basicity
In gaseous phase:
3° > 2° > 1°
In aqueous solution:
2° > 1° > 3°
Concept Clarity
👉 WHY tertiary amines less basic in water?
Because steric hindrance reduces solvation.
6. Preparation of Amines
(A) Reduction of Nitro Compounds
R–NO₂ → R–NH₂
(B) Ammonolysis of Alkyl Halides
R–X + NH₃ → R–NH₂
(C) Gabriel Phthalimide Synthesis
Used to prepare primary amines only
(D) Hoffmann Bromamide Reaction
Amide → Amine (one carbon less)
R–CONH₂ → R–NH₂
Concept Clarity
👉 WHY Hoffmann reduces carbon?
Because CO group is removed.
7. Chemical Reactions of Amines
(A) Alkylation
R–NH₂ + R′–X → higher amines
(B) Acylation
R–NH₂ + RCOCl → R–NHCO–R
(C) Carbylamine Reaction (Test for 1° amines)
R–NH₂ + CHCl₃ + KOH → R–NC
(Foul smell)
(D) Reaction with Nitrous Acid (Very Important)
Primary Aliphatic Amine
R–NH₂ + HNO₂ → alcohol
Primary Aromatic Amine
Forms diazonium salt
8. Diazonium Salts
Formation
Ar–NH₂ + HNO₂ → Ar–N₂⁺Cl⁻
Structure
Ar — N≡N⁺ Cl⁻
Importance
Used to make dyes and other compounds.
Reactions of Diazonium Salts
- Sandmeyer reaction
- Replacement reactions
9. Aromatic Amines (Aniline)
Structure
C₆H₅–NH₂
Properties
- Less basic than aliphatic amines
- Lone pair involved in resonance
Diagram (Resonance)
C6H5 — NH2 ↔ Resonance structures
Concept Clarity
👉 WHY aniline is less basic?
Because lone pair is delocalized in benzene ring.
10. Important Differences
Primary vs Secondary vs Tertiary
| Type | Structure | Example |
|---|---|---|
| 1° | R–NH₂ | CH₃NH₂ |
| 2° | R₂NH | (CH₃)₂NH |
| 3° | R₃N | (CH₃)₃N |
11. Important Named Reactions
- Hoffmann bromamide reaction
- Gabriel synthesis
- Sandmeyer reaction
- Carbylamine reaction
12. Important Numericals
(Not calculation-heavy chapter, mostly theory)
13. Concept Clarity (Very Important)
👉 WHY amines are basic?
Because nitrogen has lone pair.
👉 WHY aniline less basic?
Due to resonance.
👉 WHY primary amines have higher boiling point?
Due to more hydrogen bonding.
👉 WHY diazonium salts unstable?
Because N₂ gas easily escapes.
14. Common Mistakes
- Confusing 1°, 2°, 3° amines
- Wrong basicity order
- Forgetting reactions of nitrous acid
Conclusion
Amines are highly important organic compounds with wide applications in pharmaceuticals, dyes, and polymers. Understanding their structure, preparation, and reactions is essential for scoring high in exams.
👉 Focus on reactions + structures + concept clarity.