Electrophilic substitution of benzeneEdexcel International A Level Chemistry: Subtopic test

1. 1
   Benzene is warmed with a mixture of concentrated nitric acid and concentrated sulfuric acid at 50 °C to make nitrobenzene, C₆H₅NO₂.

   (a)

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

   [1 mark]

   • AAn oxidising agent that oxidises benzene
   • BA strong acid that protonates nitric acid, leading to formation of the electrophile
   • CA nucleophile that attacks the benzene ring
   • DA reducing agent that removes oxygen from nitric acid

   (b)

   Which species is the electrophile that attacks the benzene ring?

   [1 mark]

   • ANO₃⁻
   • BHNO₃
   • CNO₂⁺
   • DNO₂⁻

   (c)

   Write an equation for the formation of the electrophile in this reaction and explain why the temperature is kept at or below 55 °C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Benzene reacts with bromine to form bromobenzene, C₆H₅Br, in the presence of anhydrous iron(III) bromide, FeBr₃, as a catalyst.

   (a)

   Why is a catalyst needed for benzene to react with bromine?

   [1 mark]

   • ABromine is a nucleophile that attacks the ring too slowly
   • BThe catalyst breaks the benzene ring open
   • CThe catalyst reduces bromine to bromide ions
   • DThe delocalised electrons in benzene cannot polarise Br₂ enough, so a stronger electrophile must be generated

   (b)

   Which equation shows the generation of the electrophile?

   [1 mark]

   • ABr₂ + FeBr₃ → Br⁺ + FeBr₄⁻
   • BBr₂ → 2Br•
   • CBr₂ + FeBr₃ → Br⁻ + FeBr₄⁺
   • DBr₂ + FeBr₃ → FeBr₅

   (c)

   Write the overall equation for the reaction, and an equation to show that the catalyst is regenerated.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Benzene is heated under reflux with ethanoyl chloride, CH₃COCl, in the presence of anhydrous aluminium chloride to make phenylethanone, C₆H₅COCH₃.

   (a)

   Give an equation for the formation of the electrophile, the overall equation for the reaction, and one reason why the aluminium chloride must be anhydrous.

   [3 marks]

   (b)

   Describe the mechanism by which the electrophile CH₃CO⁺ reacts with benzene, including the intermediate and how the catalyst is regenerated.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician investigates three reactions of benzene: combustion in air, reaction with fuming sulfuric acid, and reaction with chloromethane in the presence of anhydrous aluminium chloride.

   (a)

   Explain how methylbenzene is made from benzene and chloromethane, including the formation of the electrophile and the mechanism.

   [6 marks]

   (b)

   Benzene burns with a smoky flame in air. Write an equation for its complete combustion and explain the smoky flame. Then name the electrophile in fuming sulfuric acid and write an equation for its reaction with benzene.

   [6 marks]

   Total for question 4: 12 marks

End of questions