📘 IB Understanding
Benzene undergoes electrophilic substitution rather than addition. This preserves the thermodynamic stability of the delocalised π-system. A Lewis acid catalyst generates the electrophile.
Nitration of Benzene
Conditions
Concentrated HNO₃ + concentrated H₂SO₄ at 50 °C
Electrophile Generation
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
The nitronium ion (NO₂⁺) is the electrophile.
Halogenation of Benzene
Conditions
Cl₂ or Br₂ with a Lewis acid catalyst (AlCl₃, AlBr₃, or FeBr₃)
Electrophile Generation
Br₂ + AlBr₃ → Br⁺ + AlBr₄⁻
The Lewis acid polarises the halogen molecule, generating the Br⁺ electrophile.
The General Mechanism
- The electrophile (E⁺) attacks the delocalised π-system of benzene
- A C–E bond forms, creating a positively charged intermediate (arenium ion)
- A proton (H⁺) is lost, restoring the aromatic ring
📋 Exam Tip
Always show the curly arrow from the benzene ring (π-electrons) to the electrophile in step 1. The ring acts as the nucleophile. Show the loss of H⁺ in the final step to restore aromaticity.