Bonding and stability of benzeneAQA A-Level Chemistry: Subtopic test

1. 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.

   (a)

   Which statement about the carbon–carbon bonds in benzene is correct?

   [1 mark]

   • AAll six bonds are 0.154 nm long, the same as in alkanes
   • BAll six bonds are the same length, 0.140 nm, which is between a single and a double bond
   • CThree bonds are 0.154 nm and three are 0.134 nm, alternating around the ring
   • DAll six bonds are 0.134 nm long, the same as in alkenes

   (b)

   What is the shape of the benzene ring and the C–C–C bond angle?

   [1 mark]

   • APuckered ring with bond angles of 109.5°
   • BPlanar ring with bond angles of 90°
   • CPlanar hexagon with bond angles of 120°
   • DPuckered ring with bond angles of 120°

   (c)

   Explain why all the carbon–carbon bonds in benzene are the same length.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Using the data for cyclohexene, what enthalpy change of hydrogenation is predicted for cyclohexa-1,3,5-triene?

   [1 mark]

   • A−360 kJ mol⁻¹
   • B−240 kJ mol⁻¹
   • C−120 kJ mol⁻¹
   • D−208 kJ mol⁻¹

   (b)

   Which statement best explains why the measured value for benzene is less exothermic than the predicted value?

   [1 mark]

   • ABenzene has fewer C–H bonds than cyclohexa-1,3,5-triene
   • BThe C=C bonds in benzene are stronger than those in an alkene
   • CBenzene reacts with fewer moles of hydrogen than cyclohexa-1,3,5-triene
   • DThe π electrons in benzene are delocalised, so benzene is more stable than the Kekulé structure

   (c)

   Calculate the difference between the predicted and measured enthalpy changes of hydrogenation of benzene, and state what this difference shows about benzene.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Explain why cyclohexene decolourises bromine water but benzene does not.

   [3 marks]

   (b)

   Use the enthalpy data to explain why benzene undergoes substitution reactions in preference to addition reactions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Kekulé suggested that benzene has a ring of six carbon atoms with alternating single and double bonds. A chemistry class is asked to test this model against experimental evidence. The following data are available. Bond lengths: C–C 0.154 nm, C=C 0.134 nm, and every C–C bond in benzene 0.140 nm. Enthalpy changes of hydrogenation: cyclohexene −120 kJ mol⁻¹, benzene −208 kJ mol⁻¹.

   (a)

   Evaluate the Kekulé structure of benzene using the bond length and enthalpy data.

   [6 marks]

   (b)

   Describe the bonding in a benzene molecule, including its shape and why the carbon–carbon bonds have the same length.

   [6 marks]

   Total for question 4: 12 marks

End of questions