1-iodopropane can react with potassium cyanide solution to form a new organic compound. State what type of reaction this is, identify the functional group that replaces the iodine atom, and explain why 1-iodopropane reacts more readily with potassium cyanide than 1-chloropропane does under the same conditions.

Eduqas A-Level Chemistry — 2.6 Halogenoalkanes · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Halogenoalkanes can undergo reactions in which the halogen atom is replaced by another atom or group of atoms. The ease with which this happens depends on the strength of the carbon–halogen bond.

Model answer (5 marks)

The reaction is a nucleophilic substitution.
The iodine is replaced by a cyano group (–CN).
The C–I bond in 1‑iodopropane is weaker than the C–Cl bond in 1‑chloropropane, so less energy is required to break it. Consequently the substitution occurs more readily with 1‑iodopropane.

Examiner tips

  • Use the word ‘nucleophilic substitution’ to name the reaction type. Mention the –CN functional group explicitly. Explain the bond‑strength difference and link it to reaction rate.

Common mistakes

  • Calling the reaction a radical substitution instead of nucleophilic. Forgetting to state the –CN group. Saying the C–Cl bond is weaker instead of the C–I bond.

Mark scheme (5 marks)

  1. Names the type of reaction as substitution (nucleophilic substitution / nucleophilic substitution reaction)
  2. Identifies the functional group that replaces iodine as the nitrile / cyano group (–CN)
  3. States that the C–I bond is weaker than the C–Cl bond
  4. States that less energy is needed to break the C–I bond
  5. Concludes that the reaction of 1-iodopropane is therefore faster / more readily because the bond breaks more easily

Related

More Halogenoalkanes questions

▶ Try answering this question with AI marking (free) →