Aromatic Stability and Substitution Preference
Benzene undergoes electrophilic substitution rather than addition to preserve its stable, delocalised \(\pi\) aromatic ring system (\(\approx 152\text{ kJ mol}^{-1}\) resonance stabilisation energy).
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.
Why Benzene Does Not Undergo Addition
Substitution (Observed)
One H is replaced by an electrophile. The delocalised aromatic \(\pi\) sextet is restored. Energetically favourable.
Addition (Not Observed)
Would permanently break the delocalised \(\pi\) system, resulting in severe loss of resonance stabilisation energy.
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.
Role of Lewis Acid Catalyst
The Lewis acid catalyst (\(\text{AlCl}_3\) or \(\text{FeBr}_3\)) acts as a halogen carrier, polarising the halogen and generating a powerful electrophile:
\[\text{Cl}_2 + \text{AlCl}_3 \rightarrow \text{Cl}^+ + \text{AlCl}_4^-\]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
Formation of the Nitronium Ion (NO₂⁺)
For nitration, concentrated \(\text{HNO}_3\) and concentrated \(\text{H}_2\text{SO}_4\) (nitrating mixture, 50°C) generate the nitronium ion (\(\text{NO}_2^+\)):
\[\text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightleftharpoons \text{NO}_2^+ + 2\text{HSO}_4^- + \text{H}_3\text{O}^+\]Here \(\text{H}_2\text{SO}_4\) acts as a Brønsted-Lowry acid, protonating \(\text{HNO}_3\) (which acts as a base).
Lewis Acidity in Halogen Carriers
\(\text{AlCl}_3\) has an incomplete octet on aluminium (accepts an electron pair → Lewis acid). By accepting a lone pair from \(\text{Cl}_2\), it forms \(\text{AlCl}_4^-\), liberating the electrophile \(\text{Cl}^+\) (halogen carrier).
Drawing Benzene Electrophilic Substitution
- Curly arrow origin: The arrow in Step 1 must start from the delocalised ring, not from a specific carbon atom.
- Intermediate drawing: Draw a horseshoe with positive charge inside opening towards the \(\text{sp}^3\) carbon.
- Loss of \(\text{H}^+\): Show the \(\text{C}-\text{H}\) bond electrons collapsing back into the ring to regenerate aromaticity.
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