Aromatic Chemistry and Electrophilic Substitution
Aromatic Chemistry and Electrophilic Substitution
The Structure of Benzene
Benzene (C₆H₆) is the simplest aromatic compound. Its structure was debated for decades before the modern model was established.
Kekulé's model proposed alternating single and double bonds in a hexagonal ring. However, this model has problems:
- Benzene does not decolourise bromine water (expected if C=C bonds were present)
- All C–C bond lengths are equal (140 pm — intermediate between a single bond at 154 pm and a double bond at 134 pm)
- The enthalpy of hydrogenation is less exothermic than predicted for three isolated C=C bonds (by about 150 kJ mol⁻¹ — the delocalisation energy)
The delocalised model: Each carbon in benzene is sp² hybridised, forming three σ bonds (to two adjacent carbons and one hydrogen) in a planar hexagonal ring. Each carbon has one remaining p orbital perpendicular to the ring. These six p orbitals overlap sideways to form a delocalised π system — a ring of electron density above and below the plane of the molecule.
This delocalisation makes benzene more stable than the hypothetical Kekulé structure. The extra stability explains why benzene undergoes substitution (preserving the aromatic ring) rather than addition (which would destroy it).
Electrophilic Substitution
The electron-rich delocalised π system attracts electrophiles (electron-pair acceptors). Benzene undergoes electrophilic substitution — an electrophile replaces one hydrogen atom on the ring, preserving the aromatic system.
Nitration
Reagents: Concentrated nitric acid + concentrated sulfuric acid (nitrating mixture), heated to ~50 °C.
Product: Nitrobenzene (C₆H₅NO₂)
Generating the electrophile:
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
The nitronium ion (NO₂⁺) is the electrophile.
Mechanism:
1. Curly arrow from the delocalised π system (ring) to NO₂⁺
2. NO₂⁺ bonds to one carbon in the ring, forming a positively charged intermediate (the Wheland intermediate or arenium ion) — the ring temporarily loses its aromaticity at that carbon
3. Curly arrow from the C–H bond on that carbon to the ring, restoring aromaticity
4. H⁺ is lost (picked up by HSO₄⁻ to regenerate H₂SO₄, which acts as a catalyst)
Use: Nitrobenzene can be reduced to phenylamine (C₆H₅NH₂) using tin and concentrated HCl, followed by NaOH. Phenylamine is important in making dyes and pharmaceuticals.
Friedel-Crafts Alkylation
Purpose: To introduce an alkyl group onto the benzene ring.
Reagents: A halogenoalkane (e.g. CH₃Cl) with an AlCl₃ catalyst (a Lewis acid, also called a halogen carrier).
Generating the electrophile:
CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻
(In practice, the electrophile is the highly polarised complex CH₃δ⁺–Clδ⁻···AlCl₃, but it is acceptable to show CH₃⁺ as the electrophile at A-Level.)
Mechanism:
1. Curly arrow from the ring to CH₃⁺
2. Intermediate forms (positive charge on the ring)
3. H⁺ is lost, aromaticity restored
4. H⁺ + AlCl₄⁻ → HCl + AlCl₃ (catalyst regenerated)
Product: Methylbenzene (toluene), C₆H₅CH₃
Friedel-Crafts Acylation
Purpose: To introduce an acyl group (RCO–) onto the ring.
Reagents: An acyl chloride (e.g. CH₃COCl) with AlCl₃ catalyst.
Generating the electrophile:
CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻
The acylium ion (RCO⁺) is the electrophile.
Mechanism: Same pattern — attack by ring, intermediate, loss of H⁺.
Product: A phenyl ketone (e.g. phenylethanone, C₆H₅COCH₃)
Advantage over alkylation: Acylation introduces exactly ONE group (the electron-withdrawing C=O deactivates the ring towards further substitution), whereas alkylation can lead to polysubstitution.
Halogenation
Bromination of benzene:
Reagents: Br₂ with an AlBr₃ or FeBr₃ catalyst (halogen carrier) — NOT bromine water.
Generating the electrophile:
Br₂ + AlBr₃ → Br⁺ + AlBr₄⁻
Mechanism: Ring attacks Br⁺ → intermediate → loss of H⁺
Product: Bromobenzene (C₆H₅Br) + HBr
Without the catalyst, benzene does not react with bromine — the delocalised ring is too stable. This contrasts with alkenes, which decolourise bromine water readily (electrophilic addition).
Directing Effects (Extension)
Substituents already on the ring affect where the next electrophile attacks:
- Electron-donating groups (–OH, –NH₂, –CH₃, alkyl): activate the ring and direct to the 2,4 positions (ortho/para)
- Electron-withdrawing groups (–NO₂, –COOH, –CHO): deactivate the ring and direct to the 3 position (meta)
Comparing Benzene with Alkenes
| Feature | Benzene | Alkenes |
|---|---|---|
| Typical reaction | Electrophilic substitution | Electrophilic addition |
| With Br₂ | No reaction without catalyst | Decolourises Br₂ immediately |
| Reason | Substitution preserves aromatic stability | No special stability to preserve |
| Enthalpy evidence | Less exothermic hydrogenation than expected | Hydrogenation enthalpy as predicted |
Exam Tips
- When drawing the mechanism, the curly arrow must come from the ring (from the π system), not from a specific C=C bond
- Always show the intermediate with a positive charge delocalised over the ring (draw a partial circle with a + sign, or show the charge on one carbon)
- Name the electrophile explicitly (e.g. NO₂⁺ is the nitronium ion)
- State that the catalyst is regenerated in Friedel-Crafts reactions