Radical substitution of alkanesEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student places a mixture of methane and chlorine in a flask. Nothing happens in the dark, but the gases react quickly when the flask is exposed to ultraviolet light, forming chloromethane and hydrogen chloride.

   (a)

   What type of bond fission happens when a chlorine molecule absorbs ultraviolet light?

   [1 mark]

   • AHeterolytic fission, forming a Cl⁺ ion and a Cl⁻ ion
   • BHomolytic fission, forming two chloride ions, Cl⁻
   • CNeither: the Cl–Cl bond stays intact and the molecule reacts directly with methane
   • DHomolytic fission, forming two chlorine radicals

   (b)

   Which equation represents a propagation step in this reaction?

   [1 mark]

   • ACl₂ → 2Cl•
   • BCl• + •CH₃ → CH₃Cl
   • C•CH₃ + Cl₂ → CH₃Cl + Cl•
   • DCH₄ + Cl₂ → CH₃Cl + HCl

   (c)

   Explain why the mixture does not react in the dark but reacts when exposed to ultraviolet light.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist exposes a mixture of ethane and bromine to ultraviolet light and obtains bromoethane, C₂H₅Br, and hydrogen bromide, HBr, by a radical chain reaction.

   (a)

   Which species is formed in the initiation step of this reaction?

   [1 mark]

   • ABr•
   • BBr⁺
   • CBr⁻
   • DC₂H₅•

   (b)

   Which species could be produced in a termination step of this reaction?

   [1 mark]

   • AHBr
   • BC₄H₁₀
   • CBr•
   • DC₂H₅•

   (c)

   Write the equations for the two propagation steps in the formation of bromoethane from ethane and bromine.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A mixture of methane and chlorine, with the methane in limited supply, is kept in ultraviolet light. Analysis of the product mixture shows chloromethane, dichloromethane, trichloromethane and a trace of ethane.

   (a)

   Explain how the trace of ethane is formed in this reaction and why its formation stops the chain reaction.

   [3 marks]

   (b)

   Dichloromethane forms as well as chloromethane. Use equations to explain how dichloromethane forms. Suggest how the proportion of chloromethane in the product could be increased.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist plans to make 1-chloropropane for use in a synthesis by reacting propane with chlorine in ultraviolet light.

   (a)

   Describe the mechanism by which propane reacts with chlorine to form a monochloropropane. Give equations for each step and name each stage. Curly half-arrows are not required.

   [6 marks]

   (b)

   Evaluate whether radical substitution of propane with chlorine is a suitable way to make 1-chloropropane.

   [6 marks]

   Total for question 4: 12 marks

End of questions