Alkenes are unsaturated hydrocarbons that can undergo addition reactions. Describe what happens when ethene reacts with bromine, and explain why alkanes cannot undergo the same type of reaction.

Eduqas A-Level Chemistry — 2.5 Hydrocarbons · Describe and Explain · 5 marks · View as Markdown

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

Ethene (C₂H₄) is the simplest alkene and is used widely in the chemical industry. It reacts readily with bromine to form a new product.

Model answer (5 marks)

Ethene contains a carbon–carbon double bond (C=C). During the reaction the double bond opens; bromine adds across the double bond to give 1,2‑dibromoethane, a single product.
Alkanes are saturated hydrocarbons that only have carbon–carbon single bonds. Without a double bond there is no site for addition, so bromine cannot add across a bond in alkanes.

Examiner tips

  • Use the term ‘double bond opens’ to show understanding of addition. Mention the product name (1,2‑dibromoethane). Explain that alkanes lack a C=C bond, hence no addition site.

Common mistakes

  • Forgetting to state that the double bond opens. Calling the product ‘dibromine’ instead of 1,2‑dibromoethane. Saying alkanes react with bromine without explaining the lack of a double bond.

Mark scheme (5 marks)

  1. Ethene contains a carbon–carbon double bond (C=C)
  2. The double bond opens / breaks during the reaction
  3. Bromine adds across the double bond to form 1,2-dibromoethane / a new single product
  4. Alkanes are saturated / only have carbon–carbon single bonds
  5. Without a double bond, alkanes cannot undergo addition reactions (so bromine cannot add across a bond)

Key terms in this question

addition reaction · saturated · unsaturated

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