Structure and bonding of benzeneEdexcel A-Level Chemistry: Subtopic test

1. 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.

   (a)

   Which statement describes the carbon–carbon bonding in the Kekulé model?

   [1 mark]

   • AThree C–C single bonds alternate with three C=C double bonds
   • BAll six carbon–carbon bonds are identical and intermediate in length
   • CAll six carbon–carbon bonds are C=C double bonds
   • DAll six carbon–carbon bonds are C–C single bonds

   (b)

   How is the delocalised π system in benzene formed?

   [1 mark]

   • ABy end-on overlap of p orbitals along each C–C axis
   • BBy sideways overlap of p orbitals on the six carbon atoms, above and below the plane of the ring
   • CBy overlap of the hydrogen 1s orbitals with each other
   • DBy transfer of electrons from each carbon atom to its neighbour

   (c)

   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]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which carbon–carbon bond lengths would the Kekulé model predict for benzene?

   [1 mark]

   • ASix bonds, each 0.140 nm
   • BSix bonds, each 0.134 nm
   • CThree bonds of 0.154 nm alternating with three bonds of 0.134 nm
   • DSix bonds, each 0.154 nm

   (b)

   Which statement is the best interpretation of the measured bond length of 0.140 nm?

   [1 mark]

   • ABenzene contains only C–C single bonds
   • BBenzene contains three C–C and three C=C bonds
   • CBond lengths cannot be used to compare the two models
   • DThe bonds are intermediate between single and double, consistent with delocalised electrons shared equally round the ring

   (c)

   Explain how the bond length data support the delocalised model rather than the Kekulé model.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   State what the student sees with each compound, and explain the difference in terms of electron density.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Kekulé proposed in 1865 that benzene is a ring of six carbon atoms with alternating single and double bonds. Modern chemists describe benzene with a ring of delocalised π electrons. Experimental evidence has been used to decide between the two models.

   (a)

   Experimental data: the enthalpy change of hydrogenation of benzene is −208 kJ mol⁻¹, whereas that of cyclohexene is −120 kJ mol⁻¹. All the carbon–carbon bonds in benzene are 0.140 nm long, compared with 0.154 nm for C–C and 0.134 nm for C=C. Evaluate the evidence for the Kekulé and delocalised models.

   [6 marks]

   (b)

   Cyclohexene decolourises bromine water readily but benzene does not. Explain what the Kekulé model would predict, and use the delocalised model to explain the observation.

   [6 marks]

   Total for question 4: 12 marks

End of questions