Structure and stability of benzeneEdexcel International A Level Chemistry: Subtopic test

1. 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.

   (a)

   Which statement describes the shape of the benzene molecule revealed by X-ray diffraction?

   [1 mark]

   • APlanar, with all C–C–C bond angles 120°
   • BPuckered, with all C–C–C bond angles 109.5°
   • CPlanar, with alternating bond angles of 109.5° and 120°
   • DPyramidal, with all C–C–C bond angles 90°

   (b)

   What carbon–carbon bond lengths would X-ray diffraction show if benzene were cyclohexa-1,3,5-triene with three localised double bonds?

   [1 mark]

   • ASix bonds of 0.140 nm
   • BSix bonds of 0.134 nm
   • CSix bonds of 0.154 nm
   • DAlternating bonds of 0.154 nm and 0.134 nm

   (c)

   Explain why the carbon–carbon bonds in benzene all have a length of 0.140 nm.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   What enthalpy change of hydrogenation would be predicted for the hypothetical cyclohexa-1,3,5-triene with three localised C=C bonds?

   [1 mark]

   • A−120 kJ mol⁻¹
   • B−360 kJ mol⁻¹
   • C−208 kJ mol⁻¹
   • D−480 kJ mol⁻¹

   (b)

   What do the data show about the stability of benzene?

   [1 mark]

   • ABenzene is 152 kJ mol⁻¹ less stable than the hypothetical triene
   • BBenzene is 208 kJ mol⁻¹ more stable than the hypothetical triene
   • CBenzene is 152 kJ mol⁻¹ more stable than the hypothetical triene
   • DBenzene and the hypothetical triene have the same stability

   (c)

   Explain why the hydrogenation of benzene is less exothermic than predicted for cyclohexa-1,3,5-triene.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Describe the bonding in the benzene ring using the delocalised model.

   [3 marks]

   (b)

   Explain why cyclohexene reacts with bromine water but benzene does not.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The enthalpy changes of hydrogenation to form cyclohexane are: cyclohexene −120 kJ mol⁻¹, cyclohexa-1,3-diene −232 kJ mol⁻¹ and benzene −208 kJ mol⁻¹.

   (a)

   Use the data to compare the stability of cyclohexa-1,3-diene and of benzene relative to the stability predicted for localised C=C bonds, and explain what the comparison shows about benzene.

   [6 marks]

   (b)

   A student claims that benzene is cyclohexa-1,3,5-triene with three localised double bonds. X-ray diffraction shows that all six carbon–carbon bonds in benzene are 0.140 nm long (0.154 nm in cyclohexane and 0.134 nm in cyclohexene). The infrared spectrum of benzene has no absorption in the 1620–1680 cm⁻¹ range, where the C=C bond in alkenes absorbs. Use these data and the enthalpy changes of hydrogenation to evaluate the student's claim.

   [6 marks]

   Total for question 4: 12 marks

End of questions