Benzene is described as an aromatic compound. Explain what is meant by the term 'aromatic' and describe the structure of benzene that makes it aromatic.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Benzene has the molecular formula C₆H₆. It was originally thought to contain alternating single and double carbon–carbon bonds, but experimental evidence showed this was not the case.
Model answer (5 marks)
Benzene is called aromatic because it is unusually stable and has a special electronic structure.
1. The six carbon atoms form a flat, planar ring.
2. Each carbon contributes one p‑orbital electron to a system of six π electrons.
3. These π electrons are delocalised over the whole ring, not confined to individual C=C bonds.
4. Consequently all C–C bonds have the same length, intermediate between a single and a double bond.
5. The delocalised π electrons create a ring of electron density above and below the plane, giving benzene extra resonance stability compared with a structure of alternating single and double bonds.
1. The six carbon atoms form a flat, planar ring.
2. Each carbon contributes one p‑orbital electron to a system of six π electrons.
3. These π electrons are delocalised over the whole ring, not confined to individual C=C bonds.
4. Consequently all C–C bonds have the same length, intermediate between a single and a double bond.
5. The delocalised π electrons create a ring of electron density above and below the plane, giving benzene extra resonance stability compared with a structure of alternating single and double bonds.
Examiner tips
- Use the word ‘delocalised’ and mention the ring of π electrons; this is key for full marks.
- Show that all C–C bonds are equal – this demonstrates the resonance structure.
- Explain that the planarity allows the p‑orbitals to overlap – essential for aromaticity.
Common mistakes
- Confusing aromatic with ‘aroma’ or describing only the smell.
- Failing to state that the ring is planar or that all bonds are equal.
- Omitting the delocalised π‑electron ring or the extra stability it provides.
Mark scheme (5 marks)
- Benzene has a planar (flat) ring of six carbon atoms
- Each carbon atom contributes one electron to a delocalised system / the electrons are delocalised over the whole ring
- All carbon–carbon bond lengths in benzene are equal / the same / intermediate between a single and double bond
- The delocalised pi (π) electrons form a ring of electron density above and below the plane of the ring
- This delocalisation makes benzene more stable than expected / gives extra stability (resonance stabilisation) compared to a structure with alternating double bonds
Key terms in this question
Related
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