Practise Edexcel IAL Chemistry 18.1 by comparing hydrogenation enthalpies, using equal X-ray C-C bond lengths and applying dichlorobenzene isomer evidence to support delocalisation.
This question is about benzene, C_6 H_6, a colourless liquid first isolated in 1825, and some related compounds.
Three C_6 H_6 structures proposed in the 1860 s are shown.
The delocalised model for the structure of benzene has been accepted since the 1930s following the study of its X-ray diffraction pattern and the understanding of electron delocalisation in bonding theory.
The Dewar and Ladenburg structures have since been isolated as stable compounds but there is no compound with the Kekulé structure.
Explain how X-ray diffraction shows that benzene has a delocalised structure and not a Kekulé structure.
The Ladenburg and Dewar structures both isomerise to benzene.
The enthalpy changes are -376 kJ mol^-1 and -297 kJ mol^-1 respectively.
Draw a labelled enthalpy level diagram showing the relative thermodynamic stability of the Ladenburg structure, the Dewar structure and benzene.
Include the enthalpy change values in kJ mol^-1.
Your diagram does not need to be to scale.
The enthalpy change of hydrogenation of hex-3-ene is -118 kJ mol^-1.
The table shows the enthalpy changes of hydrogenation of two further alkenes containing six carbon atoms.
Use your knowledge of benzene thermochemistry to suggest explanations for both of these enthalpy changes of hydrogenation in relation to the value for hex-3-ene.