1-bromopropane can react with potassium hydroxide dissolved in ethanol to produce an organic compound. Explain what happens during this reaction, naming the type of reaction, the organic product formed, and the role of the potassium hydroxide.

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).

Model answer (5 marks)

The reaction is an elimination (E2) reaction. Potassium hydroxide supplies hydroxide ions (OH⁻) which act as a strong base. The OH⁻ abstracts a hydrogen atom from the β‑carbon of 1‑bromopropane while the bromine atom leaves from the α‑carbon. This concerted process removes HBr and forms a C=C double bond, giving propene as the organic product.

Examiner tips

  • Use the term "elimination (E2)" to show you know the mechanism.
  • Show that OH⁻ is the base and that HBr is eliminated.
  • Mention the formation of the C=C bond and the product propene.
  • Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Common mistakes

  • Calling the reaction a substitution instead of elimination.
  • Failing to identify OH⁻ as the base or not mentioning HBr elimination.
  • Giving the wrong product (e.g. 1‑propene instead of propene).

Mark scheme (5 marks)

  1. The reaction is an elimination reaction
  2. The organic product is propene (an alkene)
  3. A hydrogen atom (and the bromine atom) are removed from 1-bromopropane
  4. The potassium hydroxide acts as a base / provides hydroxide ions (OH⁻)
  5. A C=C double bond is formed in the product

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