The structure of benzene
Benzene, C₆H₆, is the simplest aromatic compound. Its six carbon atoms form a flat hexagon. Each carbon is sp² hybridised and uses one p orbital to join a ring of delocalised π electrons above and below the plane. All six C–C bonds are the same length (0.139 nm), between a single (0.154 nm) and a double bond (0.134 nm).
The evidence for delocalisation: benzene does not decolourise bromine water as an alkene does, all its bonds are equal in length, and it is more stable than expected. The enthalpy of hydrogenation of cyclohexene is −120 kJ/mol, so three isolated double bonds would give −360 kJ/mol. For benzene it is only −208 kJ/mol, so benzene is 152 kJ/mol more stable: this is the delocalisation (resonance) energy.
Enter the measured enthalpy of hydrogenation of benzene. Compare it with three times that of cyclohexene (−120 kJ/mol).
Electrophilic substitution
Because the ring is rich in π electrons, benzene is attacked by electrophiles. It then substitutes (keeps its ring) instead of adding, so the stable delocalised system is kept.
| Reaction | Reagents and conditions | Product |
|---|---|---|
| Nitration | Concentrated HNO₃ + concentrated H₂SO₄, 50 °C | Nitrobenzene |
| Halogenation | Cl₂ or Br₂ with AlCl₃ or FeBr₃ catalyst, room temperature | Chlorobenzene or bromobenzene |
| Friedel–Crafts alkylation | Haloalkane with AlCl₃ | Alkylbenzene, e.g. methylbenzene |
| Friedel–Crafts acylation | Acyl chloride with AlCl₃ | Phenyl ketone |
| Sulphonation | Fuming H₂SO₄, warm | Benzenesulphonic acid |
Side chains and directing effects
In methylbenzene (toluene) the CH₃ group makes the ring more reactive and directs a second substituent to the 2- and 4-positions. Oxidation of the side chain with hot alkaline potassium manganate(VII) gives benzoic acid. In nitrobenzene the NO₂ group makes the ring less reactive and directs the next group to the 3-position.
Key terms
- Aromatic
- Containing a benzene ring or similar delocalised ring.
- Delocalisation
- Sharing of electrons over several atoms instead of two.
- Nitronium ion
- NO₂⁺, the electrophile in nitration.
- Friedel–Crafts reaction
- Attaching an alkyl or acyl group to a benzene ring using AlCl₃.
Summary
- Benzene has a planar ring with delocalised π electrons and equal bond lengths.
- The extra stability is shown by a delocalisation energy of about 152 kJ/mol.
- It undergoes electrophilic substitution: nitration, halogenation, Friedel–Crafts.
- Side chains can be oxidised; substituents direct further reactions.