A student studies two cyclic compounds: cyclohexane and benzene. The student notices that benzene does not undergo addition reactions in the way that alkenes do, and that all the carbon–carbon bond lengths in benzene are identical and intermediate in length between a single bond and a double bond. Explain why benzene behaves in this way, referring to its structure and bonding.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cyclohexane has the molecular formula C₆H₁₂. Benzene has the molecular formula C₆H₆. Unlike cyclohexene, benzene resists addition reactions and instead undergoes substitution reactions.
Model answer (5 marks)
Each carbon atom in benzene is bonded to two neighbouring carbons and one hydrogen, leaving one p‑orbital electron per carbon available for delocalisation.
The six p‑orbital electrons form a continuous ring of delocalised π‑electrons above and below the ring.
Delocalisation gives benzene extra stability (delocalisation energy), so breaking the π‑system requires a large amount of energy.
Because addition reactions would disrupt the delocalised ring, benzene resists addition and instead undergoes substitution reactions that preserve the ring.
The delocalised electrons are shared equally over all six C–C bonds, giving identical, intermediate bond lengths between a single and a double bond.
The six p‑orbital electrons form a continuous ring of delocalised π‑electrons above and below the ring.
Delocalisation gives benzene extra stability (delocalisation energy), so breaking the π‑system requires a large amount of energy.
Because addition reactions would disrupt the delocalised ring, benzene resists addition and instead undergoes substitution reactions that preserve the ring.
The delocalised electrons are shared equally over all six C–C bonds, giving identical, intermediate bond lengths between a single and a double bond.
Examiner tips
- Use the phrase ‘delocalised π‑system’ to show understanding of aromaticity.
- Explain that delocalisation gives extra stability, which is why addition is unfavourable.
- Mention that substitution preserves the ring and the delocalised electrons.
Common mistakes
- Confusing the number of bonds per carbon (e.g., saying four bonds instead of three).
- Failing to link delocalisation to both bond length equality and resistance to addition.
Mark scheme (5 marks)
- Each carbon atom in benzene forms three covalent bonds (to two neighbouring carbon atoms and one hydrogen atom), leaving one electron per carbon available for delocalisation.
- These electrons are delocalised across all six carbon atoms, forming a ring of delocalised electrons / a delocalised pi system above and below the ring.
- This delocalisation makes benzene more stable than expected (extra stability / delocalisation energy), so the energy required to break the delocalised system is high.
- Because addition reactions would destroy the delocalised ring system, benzene resists addition and instead undergoes substitution reactions, which preserve the ring.
- The identical, intermediate carbon–carbon bond lengths result from delocalisation: the electrons are shared equally between all six bonds rather than alternating between single and double bonds.
Key terms in this question
addition reaction · bond length
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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