Electrophilic Aromatic Substitution Reactions

Introduction

• Focus on bromination, chlorination, and iodination of benzene.
• General mechanism involves benzene acting as a nucleophile and an electrophile participating in the reaction.

General Mechanism

1. Addition Step

   • Benzene ring attacks the electrophile.
   • Endothermic and slow due to loss of aromaticity.
   • Formation of a carbocation intermediate.

2. Substitution Step

   • Base removes hydrogen, regenerating the aromatic ring.
   • Fast and exothermic.
   • Net result: Hydrogen is replaced with an electrophile.

Bromination of Benzene

• Reagents: Br₂ and FeBr₃ (Lewis acid catalyst).
• Mechanism:
  1. Br₂ reacts with FeBr₃ to form Br⁺ and FeBr₄^-.
  2. Benzene attacks Br, forming a sigma complex (carbocation intermediate).
  3. Base (FeBr₄^-) abstracts a proton, regenerating the aromatic ring.
• Product: Bromobenzene.

Chlorination of Benzene

• Reagents: Cl₂ and AlCl₃ / FeCl₃.
• Mechanism:
  1. Cl₂ reacts with AlCl₃ to form Cl⁺ and AlCl₄^-.
  2. Benzene attacks Cl, leading to a sigma complex.
  3. Base (AlCl₄^-) removes proton, regenerating aromatic ring.
• Product: Chlorobenzene.
• Note: AlCl₃ acts as a catalyst, regenerated in the reaction.

Iodination of Benzene

• Reagents: I₂ with an oxidizing agent (e.g., nitric acid) or I₂ with H₂O₂ and H₂SO₄.
• Mechanism:
  1. Oxidizing agent converts I₂ to I⁺.
  2. Benzene acts as nucleophile, attacking I⁺.
  3. Base abstracts proton, regenerating benzene's aromatic ring.
• Product: Iodobenzene.

Conclusion

• Mechanism for halogenation of benzene involves electrophilic aromatic substitution.
• The general pattern: Benzene ring attacks an electrophile, followed by proton removal to restore aromaticity.
• This knowledge aids in understanding substitution reactions in organic chemistry.