Amines: Structure, Classification, Properties & Uses

What are Amines?

Amines are organic derivatives of ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are classified based on how many hydrogens are replaced:

classDiagram
    class Amine {
        +General formula: R-NH₂, R₂NH, R₃N
        +Basic in nature (due to lone pair on N)
        +Smell: Fishy (low MW), Ammonia-like (high MW)
    }
    class PrimaryAmine {
        +Formula: R-NH₂
        +Example: CH₃NH₂ (Methylamine)
    }
    class SecondaryAmine {
        +Formula: R₂NH
        +Example: (CH₃)₂NH (Dimethylamine)
    }
    class TertiaryAmine {
        +Formula: R₃N
        +Example: (CH₃)₃N (Trimethylamine)
    }
    Amine <|-- PrimaryAmine
    Amine <|-- SecondaryAmine
    Amine <|-- TertiaryAmine

Why are amines important?

• Found in drugs (e.g., adrenaline, caffeine).
• Used in dyes, explosives, and polymers.
• Key intermediates in organic synthesis.

Nomenclature of Amines

Amines are named using IUPAC rules:

1. Primary amines: Add "-amine" to the parent alkane.
   • Example: CH₃NH₂ → Methanamine.
2. Secondary/tertiary amines: Use "N-" prefix for substituents on nitrogen.
   • Example: (CH₃)₂NH → N-Methylmethanamine.
3. Aromatic amines: Use "aniline" (C₆H₅NH₂) as the base name.
   • Example: C₆H₅NHCH₃ → N-Methylaniline.

Common names (still used in exams):

• CH₃NH₂ → Methylamine
• (CH₃)₂NH → Dimethylamine
• C₆H₅NH₂ → Aniline

Preparation of Amines

Amines are prepared by nucleophilic substitution (Sₙ2) or reduction reactions:

1. From Alkyl Halides (Sₙ2 Reaction)

• Reagent: Excess ammonia (NH₃) or primary/secondary amines.
• Mechanism: NH₃ acts as a nucleophile, displacing halide ions.
• Example:

  CH₃Br + NH₃ → CH₃NH₂ + HBr
  

  Problem: Polysubstitution occurs (forms mixtures of primary, secondary, tertiary amines). Solution: Use excess alkyl halide to favor tertiary amines.

2. From Nitro Compounds (Reduction)

• Reagent: LiAlH₄ (strong reducing agent) or Sn/HCl.
• Example:

  C₆H₅NO₂ (Nitrobenzene) + 6[H] → C₆H₅NH₂ (Aniline) + 2H₂O
  

  Why? Nitro group (–NO₂) is reduced to –NH₂.

3. From Amides (Hoffmann Bromamide Reaction)

• Reagent: Br₂ + NaOH (alkaline conditions).
• Example:

  CH₃CONH₂ (Acetamide) → CH₃NH₂ (Methylamine) + CO₂
  

  Key Point: Degradation reaction (amide → amine with one less carbon).

Physical Properties of Amines

Why?

• Primary/secondary amines can form hydrogen bonds (like alcohols).
• Tertiary amines cannot H-bond → lower boiling point.

Chemical Properties of Amines

1. Basic Nature (Due to Lone Pair on Nitrogen)

• Amines react with acids to form ammonium salts.

  CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻ (Methylammonium chloride)
  

• Basicity order: Aromatic < Aliphatic < Ammonia** (due to electron density on N). **Primary > Secondary > Tertiary (steric hindrance in tertiary amines).

2. Reaction with Nitrous Acid (HNO₂)

Why? Primary amines release N₂ gas (used in Liebig’s test for primary amines).

3. Reaction with Carbonyl Compounds (Formation of Imines)

• Amines react with aldehydes/ketones to form imines (Schiff’s bases).

  RNH₂ + R'CHO → R'CH=NR + H₂O
  

• Example: Aniline + Benzaldehyde → Benzalaniline (used in dyes).

4. Reaction with Acyl Chlorides (Formation of Amides)

• Amines react with acid chlorides to form amides.

  CH₃NH₂ + CH₃COCl → CH₃CONHCH₃ + HCl
  

• Used in: Synthesis of pharmaceuticals and polymers.

Uses of Amines

Comparison: Amines vs. Amides vs. Ammonia

Solved Examples

Example 1: Nomenclature

Name the following compound:

(CH₃)₂N-CH₂-CH₃

Solution:

• Longest chain: Propane.
• Nitrogen is on C-1 with two methyl groups.
• N,N-Dimethylpropan-1-amine.

Example 2: Reaction Prediction

What happens when ethylamine (C₂H₅NH₂) reacts with nitrous acid (HNO₂)? Solution:

• Primary aliphatic amine + HNO₂ → Alcohol + N₂ gas.
• Reaction:

  C₂H₅NH₂ + HNO₂ → C₂H₅OH + N₂ + H₂O
  

• Observation: Effervescence (bubbles of N₂ gas).

Example 3: Basicity Comparison

Arrange in order of increasing basicity: NH₃, (CH₃)₂NH, (CH₃)₃N, C₆H₅NH₂ Solution: C₆H₅NH₂ < NH₃ < (CH₃)₃N < (CH₃)₂NH

• Reason:
  • Aromatic amine (aniline) is least basic (electron density delocalized).
  • Tertiary amine is less basic than secondary due to steric hindrance.
  • Aliphatic amines are more basic than ammonia due to +I effect of alkyl groups.

NEB Board-Style Questions

Short Answer (5 marks)

1. Explain the Hoffmann bromamide reaction with a suitable example. Why is it called a degradation reaction?
2. How would you distinguish between primary, secondary, and tertiary amines using nitrous acid?
3. Write the IUPAC names of the following:
   • (CH₃)₃N
   • C₆H₅NHCH₃
   • CH₃CH₂CH₂NH₂

Long Answer (10 marks)

4. Describe the preparation of aniline from nitrobenzene. Explain its reaction with acetyl chloride and the use of the product in industry.
5. Compare the basicity of amines with ammonia and amides. Give reasons for the observed trends.

Exam Tip

✅ Focus on:

• Classification (primary, secondary, tertiary) and nomenclature (IUPAC vs. common names).
• Reactions with HNO₂ (key test for primary amines).
• Basicity trends (aliphatic > aromatic, primary > secondary > tertiary).
• Industrial uses (drugs, dyes, explosives).

❌ Avoid:

• Memorizing long reaction mechanisms without understanding.
• Confusing amines with amides (amides have C=O group).
• Forgetting Liebig’s test (N₂ gas evolution for primary amines).

Pro Tip: Draw structures for amines in reactions—examiners love clear diagrams!

End of Note