Bromoethane can undergo a substitution reaction with potassium cyanide solution to form a new organic compound. Describe what is meant by a substitution reaction and explain why the product formed from bromoethane and potassium cyanide contains one more carbon atom than bromoethane.

WJEC A-Level Chemistry (Wales) — 2.6 Halogenoalkanes · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

In a substitution reaction one atom or group of atoms is replaced by another.

In the reaction of bromoethane with potassium cyanide the bromine atom is replaced.

The cyanide ion (CN⁻) is the incoming group that takes the place of bromine.

The cyanide group contains a carbon atom.

This carbon is added to the two‑carbon chain of bromoethane, giving a three‑carbon product – propanenitrile.

Examiner tips

  • Use the definition of substitution – replacement of one atom/group by another.
  • Show the cyanide ion as the nucleophile and note it brings a carbon.
  • Explain the chain length increase clearly – 2 + 1 = 3 carbons.

Common mistakes

  • Saying the product has the same number of carbons, ignoring the CN carbon.
  • Not identifying the cyanide ion as the incoming group.
  • Using vague terms like ‘change’ instead of ‘replace’.

Mark scheme (5 marks)

  1. In a substitution reaction, one atom (or group of atoms) is replaced by another atom (or group of atoms)
  2. The bromine atom in bromoethane is replaced during the reaction
  3. The cyanide ion / CN⁻ acts as the incoming group that replaces the bromine
  4. The cyanide group (CN) contains a carbon atom
  5. This carbon from the cyanide group is added to the two-carbon chain of bromoethane, giving a three-carbon product (propanenitrile)

Key terms in this question

substitution reaction · cyanide · bromoethane

Related

More Halogenoalkanes questions

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