Benzene is more stable than expected based on its molecular formula. This extra stability is explained by the delocalised model of bonding in benzene. Explain why the delocalised model of benzene predicts greater stability than a structure containing three isolated carbon–carbon double bonds, and describe the evidence from one reaction of benzene that supports this greater stability.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
In the delocalised model the six π electrons are shared over all six carbon atoms, so each C–C bond contains a fraction of a double bond.
Because the electrons are spread out, the energy of the molecule is lowered compared with a structure that has three fixed C=C bonds; the delocalised π system is therefore more stable.
The delocalised model also predicts that all C–C bonds are equal in length – intermediate between a single and a double bond – which is what is observed experimentally.
Evidence for this stability is that benzene undergoes electrophilic substitution rather than addition. For example, it does not react with bromine water under normal conditions; instead it undergoes substitution, which preserves the delocalised π system and the ring.
Because the electrons are spread out, the energy of the molecule is lowered compared with a structure that has three fixed C=C bonds; the delocalised π system is therefore more stable.
The delocalised model also predicts that all C–C bonds are equal in length – intermediate between a single and a double bond – which is what is observed experimentally.
Evidence for this stability is that benzene undergoes electrophilic substitution rather than addition. For example, it does not react with bromine water under normal conditions; instead it undergoes substitution, which preserves the delocalised π system and the ring.
Examiner tips
- Use the term ‘delocalised π system’ and explain energy lowering. Mention equal C–C bond lengths. Give a specific reaction (e.g. bromination) that shows substitution, not addition.
- common_mistakes
- :
- Confusing the delocalised model with a single Kekulé structure. Failing to mention that all C–C bonds are equal. Giving a reaction that involves addition (e.g. hydrogenation) instead of substitution.
Mark scheme (5 marks)
- In the delocalised model, the six pi electrons are spread over all six carbon atoms (rather than fixed between two carbons)
- Delocalisation lowers the energy of the molecule / makes it more energetically stable than a localised (Kekulé) structure
- All C–C bond lengths in benzene are equal / intermediate between a single and double bond length, consistent with delocalisation rather than alternating single and double bonds
- Benzene preferentially undergoes electrophilic substitution rather than addition (accept: does not readily undergo addition reactions / does not react with bromine water under normal conditions)
- Substitution preserves the delocalised pi system / ring is maintained because the extra stability would be lost if addition occurred
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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