# 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.

> Eduqas A-Level Chemistry — 4.2 Aromaticity · Explain · 5 marks

## Mark scheme (5 marks)

1. 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
2. Kekulé structure would predict benzene decolourises bromine water (like an alkene), but benzene does not decolourise bromine water
3. This shows benzene does not behave as if it contains true/localised double bonds, so the Kekulé structure is incorrect
4. In the accepted model, the six electrons (one from each carbon) are delocalised / spread over all six carbon atoms in the ring
5. 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

- [Kekulé structure](https://www.gradenine.co.uk/glossary/kekul-structure)
- [delocalised](https://www.gradenine.co.uk/glossary/delocalised)
- [bromine water](https://www.gradenine.co.uk/glossary/bromine-water)
- [bond length](https://www.gradenine.co.uk/glossary/bond-length)

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- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/benzene-was-originally-proposed-to-have-cf3b1f90) · Published by Druglandscape Ltd.