Halogenoalkanes and nucleophilic substitutionEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student has four halogenoalkanes labelled W to Z: 1-bromobutane (W), 2-bromobutane (X), 2-bromo-2-methylpropane (Y) and 1-iodobutane (Z). She plans to warm each with aqueous silver nitrate in ethanol and observe what happens.

   (a)

   Which of the compounds W to Z is a secondary halogenoalkane?

   [1 mark]

   • AW, 1-bromobutane
   • BX, 2-bromobutane
   • CY, 2-bromo-2-methylpropane
   • DZ, 1-iodobutane

   (b)

   What is observed when compound W is warmed with aqueous silver nitrate in ethanol?

   [1 mark]

   • AA white precipitate
   • BA yellow precipitate
   • CA colourless gas is evolved
   • DA cream precipitate

   (c)

   Explain, with reference to the role of water, why a precipitate forms when a halogenoalkane is warmed with aqueous silver nitrate in ethanol.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist wants to lengthen the carbon chain of bromoethane by one carbon atom. She heats bromoethane under reflux with ethanolic potassium cyanide, KCN, and obtains a nitrile.

   (a)

   What is the name of the organic product?

   [1 mark]

   • AEthanenitrile
   • BButanenitrile
   • CPropanenitrile
   • DEthylamine

   (b)

   Why is this reaction useful in organic synthesis?

   [1 mark]

   • AIt increases the length of the carbon chain by one carbon atom
   • BIt decreases the length of the carbon chain by one carbon atom
   • CIt forms an alkene by elimination
   • DIt forms an amine in a single step

   (c)

   Describe, in words, the movement of electrons when the cyanide ion reacts with bromoethane in this nucleophilic substitution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   2-Bromopropane is warmed separately with aqueous potassium hydroxide and with ethanolic potassium hydroxide, giving different organic products. In a separate experiment, bromoethane is heated with a large excess of ammonia dissolved in ethanol, in a sealed tube.

   (a)

   Name the organic product formed with each potassium hydroxide solution and state the role of the hydroxide ion in each reaction.

   [3 marks]

   (b)

   Write an equation for the reaction of bromoethane with ammonia, describe the mechanism of the reaction in words, and suggest why a large excess of ammonia is used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates how the rate of hydrolysis of halogenoalkanes depends on the halogen and on the structure of the carbon chain. Equal volumes of each halogenoalkane are added to separate test tubes containing ethanol and aqueous silver nitrate in a water bath at 50 °C, and the time for a precipitate to appear is measured. Mean bond enthalpies: C–Cl 346, C–Br 290, C–I 228 kJ mol⁻¹.

   (a)

   The student first compares 1-chlorobutane, 1-bromobutane and 1-iodobutane. Describe how to make this a fair comparison and explain the trend in the times obtained.

   [6 marks]

   (b)

   The student then compares 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane. Predict the order of the rates of hydrolysis, fastest first, and explain it in terms of carbocation stability. Describe the mechanism for the reaction of 1-bromobutane with aqueous potassium hydroxide.

   [6 marks]

   Total for question 4: 12 marks

End of questions