AS · Practice questions · Free radical substitution

Start the chain.

Six original Cambridge-style questions on free radical substitution.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Make, pass on, destroy radicals.

Initiation: homolytic fission in UV light.
Propagation: one radical in, one radical out.
Termination: two radicals in, none out.
Mixture of products from further substitution.

Recall

[4 marks]

Describe the mechanism for the reaction of methane with chlorine to form chloromethane, naming each stage.

Initiation: Cl₂ → 2Cl• (UV light, homolytic fission). ✓
Propagation: Cl• + CH₄ → •CH₃ + HCl. ✓
Propagation: •CH₃ + Cl₂ → CH₃Cl + Cl•. ✓
Termination: e.g. •CH₃ + Cl• → CH₃Cl. ✓

Recall

[1 marks]

State the type of bond fission in the initiation step and the condition needed.

Homolytic fission of Cl–Cl, in ultraviolet light. ✓

Explain

[2 marks]

Explain why the product mixture contains a little ethane.

Two methyl radicals can combine in a termination step. ✓
•CH₃ + •CH₃ → C₂H₆. ✓

Analysis

[2 marks]

Write the two propagation steps for the formation of chloroethane from ethane.

Cl• + C₂H₆ → •C₂H₅ + HCl. ✓
•C₂H₅ + Cl₂ → C₂H₅Cl + Cl•. ✓

Explain

[2 marks]

Suggest how the conditions could be changed to make tetrachloromethane the main product.

Use a large excess of chlorine. ✓
Then radicals are much more likely to meet chlorinated molecules and substitute every H atom. ✓

Analysis

[2 marks]

Draw the structures of the two monobromo products formed when propane reacts with bromine in UV light.

1-bromopropane, CH₃CH₂CH₂Br. ✓
2-bromopropane, CH₃CHBrCH₃. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.