AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Delocalisation of p electrons makes benzene more stable than the…
Full specification wording
Bonding (A-level only): Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.
Delocalisation lowers energy: spreading six π electrons over six carbon atoms puts them, on average, further from repulsion with each other and closer to more nuclei than three isolated double…
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Delocalisation lowers energy: spreading six π electrons over six carbon atoms puts them, on average, further from repulsion with each other and closer to more nuclei than three isolated double bonds would. Hydrogenation enthalpies measure the difference, because the product is the same in both cases. Adding hydrogen to one C=C in cyclohexene releases 120 kJ mol⁻¹, so the theoretical cyclohexa-1,3,5-triene, with three isolated double bonds, should release three times that, 360 kJ mol⁻¹. Benzene in fact releases only 208 kJ mol⁻¹. Less energy is given out because benzene started at a lower energy: it is about 152 kJ mol⁻¹ more stable than the model predicts, and that gap is the delocalisation energy.
What earns the marks
- Stating that the π electrons are delocalised over all six carbon atoms.
- The predicted value calculated as three times the enthalpy of hydrogenation of cyclohexene.
- The difference between the predicted and measured values identified as the extra stability.
Common mistakes and how to put them right
Arguing that benzene is less stable because 208 is a smaller number than 360.
Multiplying the enthalpy of hydrogenation of benzene by three instead of that of cyclohexene.
Comparing benzene with cyclohexane rather than with the theoretical triene.
Calling the delocalisation energy the enthalpy of hydrogenation of benzene.
Exam tips
- Sketch the energy level diagram: theoretical triene at the top, benzene below it, cyclohexane at the bottom, then label the gap.
- Watch the signs. Enthalpies of hydrogenation are negative, so 'less negative' means 'started lower' means 'more stable'.
Try the lesson
The start of the Bonding (A-level only): Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene. lesson, made from checked teaching notes for this specification point.
Have a go first
In a benzene molecule, each carbon atom contributes one p electron to a system in which the electrons are spread over all six carbon atoms. What name is given to this spreading of p electrons over the ring?
Listen
Delocalisation lowers energy: spreading six π electrons over six carbon atoms puts them, on average, further from repulsion with each other and closer to more nuclei than three isolated double bonds would. Hydrogenation enthalpies measure the difference, because the product is the same in both cases. Adding hydrogen to one C=C in cyclohexene releases 120 kJ mol⁻¹, so the theoretical cyclohexa-1,3,5-triene, with three isolated double bonds, should release three times that, 360 kJ mol⁻¹. Benzene in fact releases only 208 kJ mol⁻¹. Less energy is given out because benzene started at a lower energy: it is about 152 kJ mol⁻¹ more stable than the model predicts, and that gap is the delocalisation energy. A strong answer earns marks for these points. stating that the π electrons are delocalised over all six carbon atoms. the predicted value calculated as three times the enthalpy of hydrogenation of cyclohexene. the difference between the predicted and measured values identified as the extra stability. Exam tip: Sketch the energy level diagram: theoretical triene at the top, benzene below it, cyclohexane at the bottom, then label the gap.
Key words
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delocalisation energy
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enthalpy of hydrogenation
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Kekulé model
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relative stability
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See every definition
- delocalisation energy
- The gap by which benzene is about 152 kJ mol⁻¹ more stable than the model predicts, because it started at a lower energy.
- enthalpy of hydrogenation
- The energy released when hydrogen is added to a C=C, as when cyclohexene releases 120 kJ mol⁻¹.
- Kekulé model
- The model of benzene with three isolated double bonds, which predicts a value three times that of cyclohexene.
- relative stability
- Benzene is more stable than the theoretical cyclohexa-1,3,5-triene because delocalisation of p electrons lowers its energy.
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