A chemist compares the stability of benzene with that of a theoretical compound called 'cyclohexatriene', which would have three separate C=C double bonds arranged in a six-membered ring. Explain why benzene is more stable than this theoretical cyclohexatriene, and describe one piece of experimental evidence that supports this greater stability.

WJEC A-Level Chemistry (Wales) — 4.2 Aromaticity · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Benzene has the molecular formula C₆H₆ and is described as an aromatic compound. A theoretical structure called cyclohexatriene is sometimes used as a comparison. Unlike benzene, cyclohexatriene would have alternating single and double carbon–carbon bonds with no delocalisation.

Model answer (5 marks)

Benzene is aromatic because the six p‑orbitals on the ring carbons overlap to give a delocalised π system that is spread over all six C atoms. This delocalisation means that the π electrons are not confined to three separate C=C bonds as in cyclohexatriene. Consequently every C–C bond in benzene is identical, with a bond length intermediate between a single and a double bond, giving the ring a lower overall energy. Experimentally, benzene does not undergo addition reactions; for example, bromine water remains orange/brown when benzene is added, showing that the π bonds are not reactive double bonds. This resistance to addition demonstrates the greater stability of benzene compared with a compound that would contain three distinct C=C bonds.

Examiner tips

  • Use the word "delocalised" and mention the π cloud; link this to identical bond lengths.
  • State the experimental evidence (bromine water) and explain how it shows stability.
  • Keep the answer concise – 5 marks, so one sentence per point.

Common mistakes

  • Confusing cyclohexatriene with cyclohexane; forgetting to mention delocalisation.
  • Giving the wrong experimental test (e.g., saying it reacts with hydrogen bromide).
  • Using vague terms like "more stable" without explaining the delocalisation mechanism.

Mark scheme (5 marks)

  1. In benzene, the electrons from the p-orbitals are delocalised / spread across all six carbon atoms (forming a delocalised electron cloud / pi system)
  2. This delocalisation makes benzene more stable than if the electrons were localised in three separate double bonds (as in cyclohexatriene)
  3. All carbon–carbon bonds in benzene are identical / the same length, intermediate between a single and double bond
  4. Experimental evidence: benzene resists addition reactions / does not readily react with bromine (bromine water remains orange/brown / is not decolourised)
  5. This lack of addition reaction shows benzene is more stable than cyclohexatriene would be, since a compound with three real double bonds would readily decolourise bromine water

Key terms in this question

cyclohexatriene

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