Electrophilic substitution of benzeneAQA A-Level Chemistry: Subtopic test

1. 1
   In a laboratory preparation of nitrobenzene, a technician adds benzene slowly to a mixture of concentrated nitric acid and concentrated sulfuric acid. The flask is kept in a water bath at about 50 °C and stirred, and the nitrobenzene is later separated from the acids.

   (a)

   What is the electrophile in the nitration of benzene?

   [1 mark]

   • ANO₂⁻
   • BNO₃⁻
   • CH⁺
   • DNO₂⁺

   (b)

   Why is the temperature kept at about 50 °C and not higher?

   [1 mark]

   • ATo reduce further substitution that would form dinitrobenzene
   • BTo stop the nitric acid reacting with the sulfuric acid
   • CTo prevent the benzene from freezing
   • DBecause the electrophile only forms below 50 °C

   (c)

   Write an equation for the formation of the electrophile in this reaction and state the role of sulfuric acid overall.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist wants to make phenylethanone, C₆H₅COCH₃, from benzene. The chemist adds ethanoyl chloride, CH₃COCl, to benzene in the presence of anhydrous aluminium chloride, AlCl₃, and warms the mixture. Hydrogen chloride gas is given off.

   (a)

   What is the organic product of this reaction?

   [1 mark]

   • AChlorobenzene
   • BEthylbenzene
   • CPhenylethanone
   • DBenzoic acid

   (b)

   What is the role of the aluminium chloride in this reaction?

   [1 mark]

   • AIt acts as an oxidising agent
   • BIt accepts a lone pair from chlorine, forming the electrophile, and is regenerated
   • CIt acts as a nucleophile that attacks the benzene ring
   • DIt acts as a solvent for the reaction

   (c)

   Write an equation for the formation of the electrophile in this reaction and explain why the reagents and apparatus must be anhydrous.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student prepares nitrobenzene, C₆H₅NO₂, from benzene using a mixture of concentrated nitric acid and concentrated sulfuric acid at 50 °C. The student uses 15.6 g of benzene (Mr = 78.0) and obtains 19.7 g of pure nitrobenzene (Mr = 123.0). The reaction is a monosubstitution of the ring.

   (a)

   Calculate the percentage yield of nitrobenzene.

   [3 marks]

   (b)

   Outline the mechanism for the reaction of benzene with the nitronium ion to form nitrobenzene.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two important routes to useful organic compounds start from benzene. Nitration gives nitrobenzene, which is used in the manufacture of amines and explosives. Friedel–Crafts acylation with an acyl chloride and anhydrous aluminium chloride gives aromatic ketones. Both are electrophilic substitution reactions of the benzene ring.

   (a)

   Describe how phenylethanone is made from benzene by Friedel–Crafts acylation, including the reagents, the formation of the electrophile and the mechanism.

   [6 marks]

   (b)

   Compare how the electrophile is generated in nitration and in Friedel–Crafts acylation, and explain why each catalyst is regenerated.

   [6 marks]

   Total for question 4: 12 marks

End of questions