Class 12 Chemistry – Amines: Mechanism, Reactions, and Strengths

1. Mechanism & Theory of Amines

• Amines are derivatives of ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups.

• The basic nature of amines is due to the lone pair of electrons on nitrogen, which can accept a proton (H⁺).

• Mechanism in reactions:

  • Nucleophilic substitution: Nitrogen attacks electrophilic carbon in alkyl halides.

  • Acylation: Amines react with acid derivatives to form amides.

  • Alkylation: Amines react with alkyl halides to form higher amines.

Key Concept: Amines act as nucleophiles because of the lone pair on nitrogen.

2. Important Reactions of Amines (Conceptual)

3. Acidic Strength Comparisons

• Acidity depends on the ease of losing a proton (H⁺).

• Alcohols vs Phenols vs Carboxylic Acids:

  • Carboxylic acids are more acidic due to resonance stabilization of the conjugate base.

  • Phenols are moderately acidic due to resonance of the aromatic ring.

  • Alcohols are least acidic because no resonance stabilization is available.

Key Concept: Resonance stabilizes negative charge, increasing acidity.

4. Basicity of Amines

• Basicity depends on the availability of the lone pair on nitrogen to accept a proton.

• Factors affecting basicity:

  1. Alkyl groups (electron-donating): Increase electron density on nitrogen → more basic.

     • Trend in aliphatic amines: Secondary > Primary > Tertiary (solvation effects can modify).

  2. Aromatic amines (e.g., Aniline): Lone pair delocalized into the aromatic ring → less available → less basic.

• Key Concept:

  • Aliphatic amines: Nitrogen is free → higher basicity.

  • Aromatic amines: Nitrogen lone pair participates in resonance → lower basicity.

5. Summary of Acid-Base Trends

6. Key Points to Remember

• Amines are nucleophilic and basic due to nitrogen’s lone pair.

• Reactions include: salt formation, acylation, alkylation, diazotization (aromatic).

• Acidity: Carboxylic acids > Phenols > Alcohols

• Basicity of amines: Aliphatic > Aromatic

• Resonance in aromatic compounds reduces basicity.