Bonding and stability of benzeneAQA A-Level Chemistry: Mind map

Exam questions on Bonding and stability of benzene

1. A research chemist uses X-ray diffraction to measure the carbon–carbon bond lengths in solid benzene, C₆H₆. For comparison, the C–C bond length in alkanes is 0.154 nm and the C=C bond length in alkenes is 0.134 nm. All six carbon atoms of the benzene ring and all six hydrogen atoms are found to lie in one plane.
   Explain why all the carbon–carbon bonds in benzene are the same length.2 marks
2. A student compares the enthalpy changes when cyclohexene and benzene are each hydrogenated to cyclohexane using hydrogen gas and a nickel catalyst. The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol⁻¹ and that of benzene is −208 kJ mol⁻¹. Kekulé proposed that benzene is cyclohexa-1,3,5-triene, with three localised C=C double bonds.
   Calculate the difference between the predicted and measured enthalpy changes of hydrogenation of benzene, and state what this difference shows about benzene.2 marks
3. A technician shakes cyclohexene with bromine water and the orange colour disappears at once. When benzene is shaken with bromine water under the same conditions, the orange colour remains. Benzene does react with bromine in the presence of anhydrous aluminium bromide, AlBr₃, forming bromobenzene, C₆H₅Br, and hydrogen bromide. The enthalpy change of hydrogenation of benzene is −208 kJ mol⁻¹, whereas that predicted for cyclohexa-1,3,5-triene is −360 kJ mol⁻¹.
   Explain why cyclohexene decolourises bromine water but benzene does not.3 marks