An enthalpy level diagram representing the relative stabilities of cyclohexa-1,3,5-triene (Kekulé structure) and benzene (delocalised structure) is shown below.
a.[3]
Use the diagram and the data shown below to calculate ΔH2\Delta H_2ΔH2, in kJ mol−1\text{kJ mol}^{-1}kJ mol−1.
- Enthalpy of atomisation for carbon = +715 kJ mol−1+715 \text{ kJ mol}^{-1}+715 kJ mol−1
- Enthalpy of atomisation for hydrogen = +216 kJ mol−1+216 \text{ kJ mol}^{-1}+216 kJ mol−1
- Bond enthalpy (C−C\text{C}-\text{C}C−C) = +348 kJ mol−1+348 \text{ kJ mol}^{-1}+348 kJ mol−1
- Bond enthalpy (C=C\text{C}=\text{C}C=C) = +612 kJ mol−1+612 \text{ kJ mol}^{-1}+612 kJ mol−1
- Bond enthalpy (C−H\text{C}-\text{H}C−H) = +412 kJ mol−1+412 \text{ kJ mol}^{-1}+412 kJ mol−1
b.[2]
Explain, in terms of structure and bonding, why the delocalised structure of benzene is more thermodynamically stable than the localized Kekulé structure.