Benzene was originally proposed to have a Kekulé structure. Explain why the evidence from bond lengths and reactions with bromine water shows that the Kekulé structure is incorrect, and describe the accepted delocalised model of benzene that replaced it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
In a Kekulé structure benzene would have alternating single and double C–C bonds, giving two distinct bond lengths. X‑ray data show all C–C bonds are equal (≈1.39 Å), intermediate between single and double.
A Kekulé benzene would react with bromine water like an alkene, decolourising the solution. Experimentally benzene does not decolourise bromine water, indicating the absence of isolated double bonds.
Thus the Kekulé model is incorrect.
The accepted model shows the six π electrons are delocalised over the whole ring. This gives a planar, symmetrical hexagon with all C–C bonds equal, stabilising the molecule beyond a simple Kekulé structure.
A Kekulé benzene would react with bromine water like an alkene, decolourising the solution. Experimentally benzene does not decolourise bromine water, indicating the absence of isolated double bonds.
Thus the Kekulé model is incorrect.
The accepted model shows the six π electrons are delocalised over the whole ring. This gives a planar, symmetrical hexagon with all C–C bonds equal, stabilising the molecule beyond a simple Kekulé structure.
Examiner tips
- Use the exact wording ‘all C–C bonds are equal’ and ‘delocalised over the ring’ to match the mark scheme.
- Show the two key pieces of evidence (bond lengths and bromine test) before stating the conclusion.
Common mistakes
- Confusing the delocalised model with a single resonance structure; students sometimes write ‘alternating double bonds’ again.
- Failing to mention that benzene does not decolourise bromine water, or mis‑stating the reaction outcome.
Mark scheme (5 marks)
- In the Kekulé structure, alternating single and double bonds would give two different C–C bond lengths, but all C–C bonds in benzene are the same / intermediate length
- Kekulé structure would predict benzene decolourises bromine water (like an alkene), but benzene does not decolourise bromine water
- This shows benzene does not behave as if it contains true/localised double bonds, so the Kekulé structure is incorrect
- In the accepted model, the six electrons (one from each carbon) are delocalised / spread over all six carbon atoms in the ring
- This gives benzene a planar / flat, symmetrical hexagonal ring in which all C–C bonds are equal, making it more stable than a Kekulé structure would be
Key terms in this question
Kekulé structure · delocalised · bromine water · bond length
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