Benzene is an aromatic compound with the molecular formula C₆H₆. Explain why the structure of benzene is described as a delocalised ring rather than a structure containing alternating single and double carbon–carbon bonds.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Benzene was originally proposed to have a ring structure containing alternating single and double carbon–carbon bonds (a Kekulé structure). Evidence from bond length measurements and reaction behaviour showed this model to be incorrect.
Model answer (5 marks)
All C–C bonds in benzene are equal in length, about 1.39 Å, rather than alternating short double and long single bonds as a Kekulé structure would predict. This equality shows that the six π electrons are delocalised over the whole ring, not confined to specific C=C bonds. Because the π electrons are spread out, benzene does not undergo addition reactions typical of alkenes; instead it undergoes electrophilic substitution, indicating the absence of localized double bonds. The delocalised model also explains benzene’s extra stability – its enthalpy of hydrogenation is less exothermic than that of a hypothetical Kekulé structure, giving it lower energy.
Examiner tips
- Mention equal bond lengths first, then delocalisation, then reaction type, finish with stability/enthalpy
Common mistakes
- Claiming benzene has alternating bonds, or ignoring bond length evidence; confusing addition with substitution reactions; omitting the enthalpy of hydrogenation point
Mark scheme (5 marks)
- All carbon–carbon bond lengths in benzene are the same / equal
- A Kekulé structure would have two different bond lengths (alternating shorter double and longer single bonds)
- The six electrons (one from each carbon's p orbital / π electrons) are delocalised / spread over the whole ring
- Benzene does not readily undergo addition reactions / undergoes electrophilic substitution instead, which is inconsistent with the presence of localised double bonds
- The delocalised model gives benzene greater stability (lower energy) than expected for a Kekulé structure / the enthalpy of hydrogenation is less exothermic than predicted
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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