Key Concept
Benzene undergoes electrophilic substitution, not addition. The delocalised π system is so stable that benzene preserves its aromatic ring by replacing a hydrogen atom with the electrophile, rather than breaking the ring's delocalisation.
Why Substitution, Not Addition?
Benzene has a delocalised ring of π electrons above and below the plane. This makes it electron-rich and susceptible to electrophilic attack. However, if addition occurred, the delocalised system would be destroyed. Substitution allows benzene to retain its aromatic stability.
Substitution (what happens)
One H is replaced by the electrophile. The delocalised ring is preserved. Energetically favourable.
Addition (does not happen)
Would break the delocalised π system. Loss of resonance/aromatic stabilisation energy (~150 kJ mol⁻¹). Energetically unfavourable.
Role of Lewis Acid Catalysts
Many electrophiles (e.g. Br₂, Cl₂) are not reactive enough to attack benzene's stable ring on their own. A Lewis acid catalyst (also called a halogen carrier) is needed to generate a stronger electrophile.
How the Catalyst Works
The Lewis acid accepts a lone pair from one of the halogen atoms, polarising the halogen molecule and generating a powerful electrophile.
Br₂ + AlBr₃ → Br⁺ + AlBr₄⁻
AlBr₃ accepts a lone pair from Br (Lewis acid-base reaction)
Common Catalysts
| Reaction | Lewis acid catalyst | Electrophile generated |
|---|---|---|
| Bromination | AlBr₃ or FeBr₃ | Br⁺ (or highly polarised Br₂) |
| Chlorination | AlCl₃ or FeCl₃ | Cl⁺ (or highly polarised Cl₂) |
| Nitration | Conc. H₂SO₄ (catalyst) | NO₂⁺ (nitronium ion) |
Mechanism: Bromination of Benzene
Mechanism Summary
Step 1:
The delocalised π ring electrons attack the electrophile (Br⁺). A C-Br bond forms. The intermediate is a positively charged sigma (σ) complex (arenium ion) with a disrupted aromatic system.
Step 2:
A base (often AlBr₄⁻) removes H⁺ from the carbon bearing the Br. The electron pair from the C-H bond returns to the ring, restoring aromaticity.
Nitration of Benzene
Generating the Electrophile
Nitrating mixture: concentrated HNO₃ + concentrated H₂SO₄
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
The nitronium ion (NO₂⁺) is the electrophile that attacks the ring.
Overall: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (at 50°C)
Think About It
AlCl₃ is described as both a Lewis acid and a "halogen carrier" catalyst. How do both descriptions fit?
AlCl₃ is a Lewis acid because it accepts an electron pair from Cl₂ (Al has an incomplete octet). This generates the Cl⁺ electrophile. It is a catalyst because it is regenerated at the end of the reaction when AlCl₄⁻ reacts with H⁺ to reform AlCl₃ + HCl. The same molecule performs both roles.
Common Exam Mistakes
- Saying benzene undergoes "addition". It undergoes substitution to preserve aromaticity.
- Forgetting the role of the Lewis acid catalyst. Without it, the electrophile is not reactive enough.
- Not drawing the curly arrow from the ring to the electrophile. The delocalised electrons are the nucleophile.
- Forgetting to show H⁺ being lost in step 2 to restore the ring.
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