Chemists once proposed that benzene had a ring structure containing alternating single and double carbon-carbon bonds, known as the Kekulé structure. Explain why experimental evidence does not support the Kekulé structure for benzene, and describe the accepted model of bonding in benzene that accounts for this evidence.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The Kekulé structure predicts that benzene should behave like an alkene and undergo addition reactions, yet experimentally benzene only undergoes substitution.
It also predicts two distinct C–C bond lengths (single and double), but X‑ray diffraction shows all six C–C bonds are equal.
In the accepted model each carbon contributes one p‑electron to a delocalised π‑system.
These π‑electrons are spread above and below the ring, over all six atoms.
The delocalisation makes all C–C bonds identical and gives benzene extra stability (delocalisation energy).
It also predicts two distinct C–C bond lengths (single and double), but X‑ray diffraction shows all six C–C bonds are equal.
In the accepted model each carbon contributes one p‑electron to a delocalised π‑system.
These π‑electrons are spread above and below the ring, over all six atoms.
The delocalisation makes all C–C bonds identical and gives benzene extra stability (delocalisation energy).
Examiner tips
- Show the failure of the Kekulé structure to predict reaction type and bond lengths. Explain the delocalised π‑system and its consequences for bond equality and stability. Use the exact terminology: substitution, delocalised electrons, π‑system, stabilisation energy.
Common mistakes
- Assuming benzene undergoes addition reactions. Claiming the ring has alternating single/double bonds without mentioning equal bond lengths. Using vague terms like "resonance" without describing delocalisation.
Mark scheme (5 marks)
- The Kekulé structure predicts that benzene should readily undergo addition reactions (like an alkene), but benzene undergoes substitution reactions instead
- The Kekulé structure predicts two different C–C bond lengths (single and double), but all six carbon-carbon bonds in benzene are the same / equal length
- Each carbon atom in the benzene ring contributes one electron to form a delocalised system / pi system
- The delocalised electrons are spread above and below the plane of the ring / over all six carbon atoms
- This delocalisation makes all six C–C bonds identical / gives extra stability (delocalisation energy / stabilisation) to benzene
Key terms in this question
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