Electrophilic substitution of benzeneEdexcel A-Level Chemistry: Subtopic test

1. 1
   Nitrobenzene is manufactured by reacting benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid. The reaction is an electrophilic substitution and the reaction mixture is kept at about 50 °C.

   (a)

   Which species is the electrophile that attacks the benzene ring in this reaction?

   [1 mark]

   • AH⁺
   • BNO₂⁺
   • CNO₃⁻
   • DHSO₄⁻

   (b)

   What is the role of the concentrated sulfuric acid in this reaction?

   [1 mark]

   • AIt oxidises benzene to nitrobenzene
   • BIt acts as a nucleophile that attacks the ring
   • CIt acts as a base by accepting a proton from nitric acid
   • DIt protonates nitric acid so that NO₂⁺ forms, and is regenerated

   (c)

   Write an equation for the formation of the electrophile in this reaction, and explain why the temperature is kept at about 50 °C rather than being allowed to rise much higher.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Phenol, C₆H₅OH, decolourises aqueous bromine at room temperature and forms a precipitate, whereas benzene does not react with aqueous bromine. In an experiment, 0.940 g of phenol was added to an excess of bromine water. Relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Br = 79.9.

   (a)

   Which observation is made when excess bromine water is added to phenol?

   [1 mark]

   • AThe bromine water is decolourised and a white precipitate forms
   • BThe bromine water stays orange and nothing happens
   • CA silver mirror forms on the inside of the tube
   • DThe solution turns green

   (b)

   How many moles of hydrogen bromide are formed when one mole of phenol reacts completely with excess bromine water?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (c)

   Calculate the maximum mass of 2,4,6-tribromophenol, C₆H₂Br₃OH, formed in this experiment. Give your answer to three significant figures.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Phenylethanone, C₆H₅COCH₃, is made by reacting benzene with ethanoyl chloride, CH₃COCl, in the presence of anhydrous aluminium chloride, AlCl₃. This is a Friedel-Crafts reaction.

   (a)

   Write an equation for the formation of the electrophile in this reaction. Explain the role of aluminium chloride and why it is classed as a catalyst.

   [3 marks]

   (b)

   Describe the mechanism for the reaction of benzene with the electrophile CH₃CO⁺ to form phenylethanone.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Benzene, C₆H₆, burns in air with a smoky flame. It does not decolourise bromine water, but it reacts with bromine in the presence of iron(III) bromide to give bromobenzene. Phenol reacts with bromine water at room temperature without a catalyst. Cyclohexene decolourises bromine water by addition.

   (a)

   Explain why phenol reacts with bromine at room temperature without a catalyst, whereas benzene needs a catalyst and does not react with bromine water.

   [6 marks]

   (b)

   Explain why benzene burns with a smoky flame. Explain also why benzene reacts with bromine by electrophilic substitution, using a catalyst, rather than by addition like cyclohexene, and include an equation for the formation of the electrophile.

   [6 marks]

   Total for question 4: 12 marks

End of questions