Structure and bonding of benzeneEdexcel A-Level Chemistry: Mind map

Exam questions on Structure and bonding of benzene

1. Benzene, C₆H₆, is a planar ring of six carbon atoms. Two models are used to describe its bonding: the Kekulé model and the delocalised model.
   State the shape of a benzene molecule, including the carbon–carbon–carbon bond angle, and the number of electrons in the delocalised π system.2 marks
2. X-ray diffraction shows that all six carbon–carbon bonds in a benzene molecule have the same length, 0.140 nm. In other hydrocarbons a typical C–C single bond is 0.154 nm long and a typical C=C double bond is 0.134 nm long.
   Explain how the bond length data support the delocalised model rather than the Kekulé model.2 marks
3. The standard enthalpy change when cyclohexene, C₆H₁₀, is hydrogenated to cyclohexane is −120 kJ mol⁻¹. The enthalpy change when benzene is hydrogenated to cyclohexane is −208 kJ mol⁻¹. A student also shakes bromine water separately with cyclohexene and with benzene, with no catalyst present.
   Calculate the enthalpy change of hydrogenation predicted for the Kekulé structure of benzene, compare it with the experimental value, and state what the difference shows.3 marks