Structure and stability of benzeneEdexcel International A Level Chemistry: Mind map

Exam questions on Structure and stability of benzene

1. X-ray diffraction shows that benzene, C₆H₆, is a flat molecule in which all six carbon–carbon bonds have the same length, 0.140 nm. The carbon–carbon bond length is 0.154 nm in cyclohexane and 0.134 nm in cyclohexene.
   Explain why the carbon–carbon bonds in benzene all have a length of 0.140 nm.2 marks
2. The enthalpy change of hydrogenation of cyclohexene to cyclohexane, C₆H₁₀ + H₂ → C₆H₁₂, is −120 kJ mol⁻¹. The enthalpy change of hydrogenation of benzene to cyclohexane, C₆H₆ + 3H₂ → C₆H₁₂, is −208 kJ mol⁻¹.
   Explain why the hydrogenation of benzene is less exothermic than predicted for cyclohexa-1,3,5-triene.2 marks
3. A teacher compares the reaction of bromine water with cyclohexene and with benzene at room temperature. The cyclohexene decolourises the bromine water immediately but the benzene does not. Benzene is a planar ring of six carbon atoms, each bonded to one hydrogen atom.
   Describe the bonding in the benzene ring using the delocalised model.3 marks