Ethene reacts with hydrogen bromide gas to form bromoethane. Describe what happens during this reaction and explain why ethene is able to undergo this type of reaction whilst ethane cannot.

Cambridge International IGCSE Chemistry (0620) — 11.5 Alkenes · Describe and Explain · 4 marks · View as Markdown

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

Ethene (C₂H₄) and ethane (C₂H₆) are both small hydrocarbons, but they react very differently with hydrogen bromide gas.

Model answer (4 marks)

Ethene undergoes an addition reaction: HBr adds across the C=C double bond.
The product is bromoethane (CH₃CH₂Br).
Ethene has a C=C double bond which can be broken and opened to give two new C–Br and C–H bonds.
Ethane has only C–C single bonds and is saturated, so it cannot undergo addition reactions.

Examiner tips

  • Use the word ‘addition’ and state the product explicitly.
  • Explain that the double bond is the reactive site and that saturation prevents addition.

Common mistakes

  • Failing to mention that the reaction is an addition across the double bond.
  • Confusing ethane with ethene or saying ethane reacts in the same way.

Mark scheme (4 marks)

  1. Ethene undergoes addition reaction / hydrogen bromide adds across the double bond
  2. The product formed is bromoethane (CH₃CH₂Br)
  3. Ethene has a carbon–carbon double bond (C=C) which can open / break
  4. Ethane has only single bonds (C–C) / is saturated so cannot undergo addition reactions

Related

More Alkenes questions

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