Nucleophilic substitution in halogenoalkanesAQA A-Level Chemistry: Subtopic test

1. 1
   1-Bromobutane, CH₃CH₂CH₂CH₂Br, is warmed with aqueous sodium hydroxide, NaOH. Butan-1-ol is formed by a nucleophilic substitution reaction.

   (a)

   Why is the carbon atom attached to bromine in 1-bromobutane attacked by nucleophiles?

   [1 mark]

   • AIt carries a partial negative charge because carbon is more electronegative than bromine
   • BIt has a lone pair of electrons
   • CIt is part of a C=C double bond
   • DIt carries a partial positive charge because bromine is more electronegative than carbon

   (b)

   Which term describes the hydroxide ion in this reaction?

   [1 mark]

   • ANucleophile
   • BElectrophile
   • CFree radical
   • DOxidising agent

   (c)

   Write an equation for the reaction of 1-bromobutane with hydroxide ions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist converts 1-bromopropane, CH₃CH₂CH₂Br, into butanenitrile, CH₃CH₂CH₂CN, by heating it under reflux with a solution of potassium cyanide, KCN, in ethanol.

   (a)

   How does the length of the carbon chain change in this reaction?

   [1 mark]

   • AIt decreases by one carbon atom
   • BIt increases by one carbon atom
   • CIt does not change
   • DIt doubles

   (b)

   Which statement about the mechanism is correct?

   [1 mark]

   • AThe cyanide ion accepts a pair of electrons from the carbon atom
   • BThe C–Br bond breaks homolytically to form two radicals
   • CThe cyanide ion donates a lone pair to the δ+ carbon atom, and the C–Br bond breaks with both electrons going to bromine
   • DA bromine radical attacks the carbon atom of the cyanide ion

   (c)

   Suggest why this reaction is useful in organic synthesis.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student compares the rates of reaction of three halogenoalkanes with water. Each of 1-chlorobutane, 1-bromobutane and 1-iodobutane is warmed with aqueous silver nitrate in ethanol and the time taken for a precipitate of the silver halide to form is measured. The bond enthalpies are: C–Cl 338 kJ mol⁻¹, C–Br 276 kJ mol⁻¹ and C–I 238 kJ mol⁻¹.

   (a)

   Predict which halogenoalkane forms a precipitate first and explain your answer.

   [3 marks]

   (b)

   Chlorine is more electronegative than iodine. Explain why 1-chlorobutane nevertheless reacts more slowly than 1-iodobutane.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Bromoethane, CH₃CH₂Br, reacts with ammonia to form ethylamine, CH₃CH₂NH₂. The reaction is carried out by heating bromoethane with a large excess of ammonia dissolved in ethanol in a sealed tube.

   (a)

   Outline the mechanism of this reaction, explaining the movement of electrons in each step and giving the overall equation.

   [6 marks]

   (b)

   Explain why a large excess of ammonia is used, and what would be formed if a small amount of ammonia were used instead.

   [6 marks]

   Total for question 4: 12 marks

End of questions