Organic chemistry: arenes

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   The enthalpy change of hydrogenation of cyclohexene is $-120\,\text{kJ mol}^{-1}$. The enthalpy change of hydrogenation of benzene to cyclohexane is $-208\,\text{kJ mol}^{-1}$.

   Use these data to show that benzene does not have the Kekulé structure with three separate C=C bonds, and explain its greater stability.

   Mark scheme

   • Expected value for the Kekulé structure $= 3 \times (-120) = -360\,\text{kJ mol}^{-1}$ (1)
   • The actual value is $152\,\text{kJ mol}^{-1}$ less exothermic than expected (1)
2. Question 23 marks

   Phenol decolourises bromine water at room temperature, forming a white precipitate, whereas benzene needs a halogen carrier to react with bromine.

   Explain this difference and name the white precipitate.

   Mark scheme

   • A lone pair of electrons on the oxygen atom of OH overlaps with (is delocalised into) the ring (1)
   • This increases the electron density of the ring, so it polarises bromine molecules and attracts electrophiles more strongly (1)
   • The precipitate is 2,4,6-tribromophenol (1)