Nitration of Benzene

Introduction

• Objective: Focus on the mechanism for the nitration of benzene.
• Key Reagents:
  • Benzene
  • Nitric acid (HNO₃)
  • Sulfuric acid (H₂SO₄)
• Reaction Type: Electrophilic aromatic substitution
  • Replaces a hydrogen atom in benzene with an electrophile (NO₂)
• Product: Nitrobenzene

Mechanism Overview

1. Electrophile Generation:

   • Nitric Acid Structure:
     • Nitrogen attached to three oxygens (one with hydrogen, one with three lone pairs and a negative charge, one with two lone pairs).
     • Nitrogen has a positive formal charge.
   • Sulfuric Acid: Assists in generating the electrophile.
   • Protonation:
     • Converts the OH group in nitric acid into a good leaving group by protonating it.
     • Creates an OH₂⁺ group which helps in expelling water, forming the nitronium ion (NO₂⁺).
   • Products: Nitronium ion (NO₂⁺) and water.

2. Reaction with Benzene:

   • Benzene Ring:
     • Contains three double bonds.
     • Attacks the nitronium ion (NO₂⁺), breaking one of the pi bonds.
   • Intermediate Formation:
     • Benzene attached to nitrogen with a positive charge.
     • Nitrogen retains bonds with oxygen (double and single bonds).
     • Positive charge appears on the benzene ring.

3. Regenerating Aromatic Ring:

   • Base Requirement:
     • Use a base to neutralize the positive charge and regenerate the aromatic structure.
     • Possible Bases: Water, bisulfate ion, solvent.
   • Deprotonation:
     • Base removes a proton, restoring the benzene's aromaticity.
   • Final Product: Nitrobenzene

Conclusion

• Summary: The nitration of benzene involves replacing a hydrogen atom with an electrophile (NO₂) through a series of steps to generate and react with the nitronium ion, ultimately producing nitrobenzene.