The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol⁻¹. If benzene behaved like a molecule with three isolated carbon-carbon double bonds, its enthalpy change of hydrogenation would be expected to be −360 kJ mol⁻¹. The experimentally measured value for benzene is only −208 kJ mol⁻¹. Explain what this difference in values tells us about the stability and bonding of benzene.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol⁻¹. A hypothetical molecule with three isolated double bonds would be expected to release −360 kJ mol⁻¹ on complete hydrogenation. The experimentally measured enthalpy change of hydrogenation of benzene is −208 kJ mol⁻¹.
Model answer (5 marks)
The difference between the expected value (−360 kJ mol⁻¹) and the actual value (−208 kJ mol⁻¹) is about 152 kJ mol⁻¹.
This shows that benzene is more stable than a molecule with three isolated double bonds.
The extra stability comes from the delocalisation of the six π electrons over the whole ring, rather than being confined to three fixed C=C bonds.
Because the π electrons are delocalised, all C–C bonds in benzene are equal in length and strength, intermediate between a single and a double bond.
The 152 kJ mol⁻¹ energy difference is called the resonance (or delocalisation) energy, representing the additional stability gained by delocalisation.
This shows that benzene is more stable than a molecule with three isolated double bonds.
The extra stability comes from the delocalisation of the six π electrons over the whole ring, rather than being confined to three fixed C=C bonds.
Because the π electrons are delocalised, all C–C bonds in benzene are equal in length and strength, intermediate between a single and a double bond.
The 152 kJ mol⁻¹ energy difference is called the resonance (or delocalisation) energy, representing the additional stability gained by delocalisation.
Examiner tips
- Use the exact figures (−360, −208, 152) to show calculation of the energy difference. Show that the difference indicates extra stability. Explain delocalisation and equal bond lengths. Mention the term ‘resonance energy’ to match the mark scheme.
Common mistakes
- Mixing up the sign of the enthalpy change (writing +208 instead of −208). Failing to state that the 152 kJ mol⁻¹ is the resonance energy. Not mentioning that all C–C bonds are equal/intermediate in length.
Mark scheme (5 marks)
- The difference between the expected value (−360 kJ mol⁻¹) and the actual value (−208 kJ mol⁻¹) is approximately 152 kJ mol⁻¹ less energy released than expected
- This means benzene is more stable than a hypothetical molecule with three isolated/localised double bonds
- The extra stability arises because the six pi electrons are delocalised over all six carbon atoms in the ring (rather than being in three fixed double bonds)
- Because the electrons are delocalised, all carbon-carbon bonds in benzene are equal / intermediate in length and strength between a single and a double bond
- The energy difference (approximately 152 kJ mol⁻¹) is called the delocalisation / stabilisation / resonance energy, representing the extra stability gained through delocalisation
Key terms in this question
enthalpy change of hydrogenation
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