# 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.

> WJEC A-Level Chemistry (Wales) — 4.2 Aromaticity · Explain · 5 marks

> Cyclohexane has the molecular formula C₆H₁₂. Benzene has the molecular formula C₆H₆. Unlike cyclohexene, benzene resists addition reactions and instead undergoes substitution reactions.

## Mark scheme (5 marks)

1. 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.
2. These electrons are delocalised across all six carbon atoms, forming a ring of delocalised electrons / a delocalised pi system above and below the ring.
3. This delocalisation makes benzene more stable than expected (extra stability / delocalisation energy), so the energy required to break the delocalised system is high.
4. Because addition reactions would destroy the delocalised ring system, benzene resists addition and instead undergoes substitution reactions, which preserve the ring.
5. 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

- [addition reaction](https://www.gradenine.co.uk/glossary/addition-reaction)
- [bond length](https://www.gradenine.co.uk/glossary/bond-length)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-studies-two-cyclic-compounds-4dd822da) · Published by Druglandscape Ltd.