# 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.

> Edexcel A-Level Chemistry (9CH0) — 18.1 Aromatic chemistry · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. All carbon–carbon bond lengths in benzene are the same / equal
2. A Kekulé structure would have two different bond lengths (alternating shorter double and longer single bonds)
3. The six electrons (one from each carbon's p orbital / π electrons) are delocalised / spread over the whole ring
4. Benzene does not readily undergo addition reactions / undergoes electrophilic substitution instead, which is inconsistent with the presence of localised double bonds
5. 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

- [aromatic](https://www.gradenine.co.uk/glossary/aromatic)
- [delocalised](https://www.gradenine.co.uk/glossary/delocalised)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
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