Nucleophilic properties of aminesAQA A-Level Chemistry: Subtopic test

1. 1
   A chemist heats bromoethane, CH₃CH₂Br, with a large excess of ammonia dissolved in ethanol. The reaction is carried out in a sealed tube and ethylamine is formed.

   (a)

   What type of reaction takes place between bromoethane and ammonia?

   [1 mark]

   • AElectrophilic addition
   • BNucleophilic substitution
   • CElectrophilic substitution
   • DElimination

   (b)

   Why is the mixture heated in a sealed tube?

   [1 mark]

   • ATo keep the mixture free from water
   • BTo stop oxygen entering and oxidising the amine
   • CTo increase the activation energy of the reaction
   • DBecause ammonia and bromoethane are volatile and would escape from an open vessel

   (c)

   Explain why ammonia can act as a nucleophile in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student heats an excess of 1-bromopropane, CH₃CH₂CH₂Br, with a small amount of ethanolic ammonia in a sealed tube. Analysis shows a mixture of amines and an ionic compound.

   (a)

   Which compound is the ionic product when substitution continues as far as possible?

   [1 mark]

   • ATetrapropylammonium bromide, a quaternary ammonium salt
   • BTripropylamine, a tertiary amine
   • CPropylamine, a primary amine
   • DPropanamide, an amide

   (b)

   Which change would increase the proportion of the primary amine in the product?

   [1 mark]

   • AUse an excess of 1-bromopropane
   • BUse aqueous sodium hydroxide in place of ammonia
   • CUse a large excess of ammonia
   • DAdd aluminium chloride as a catalyst

   (c)

   Write an equation for the reaction of propylamine with 1-bromopropane to form a secondary amine, and explain why propylamine can react in this way.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Hexadecyltrimethylammonium bromide, [CH₃(CH₂)₁₅N(CH₃)₃]⁺ Br⁻, is a cationic surfactant used in hair conditioners. It is manufactured from the tertiary amine hexadecyldimethylamine, CH₃(CH₂)₁₅N(CH₃)₂.

   (a)

   Suggest how this surfactant is formed from the tertiary amine, giving the reagent, an equation and the type of mechanism.

   [3 marks]

   (b)

   Explain how this ion acts as a surfactant in a hair conditioner, and why it is described as cationic.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemistry department studies the reactions of bromoethane with ammonia. They want to understand the mechanism of the first substitution, and to learn how the conditions can be varied to form either mainly the primary amine ethylamine or mainly the ionic product tetraethylammonium bromide.

   (a)

   Outline the mechanism for the reaction of bromoethane with ammonia to form ethylamine.

   [6 marks]

   (b)

   Explain how the proportions of products depend on the amounts of bromoethane and ammonia, and why the reaction eventually stops at tetraethylammonium bromide.

   [6 marks]

   Total for question 4: 12 marks

End of questions