Propene can undergo addition reactions with different reagents. Describe what happens when propene reacts with steam in the presence of an acid catalyst, and explain why this reaction is classified as an addition reaction.

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).

Model answer (4 marks)

Propene reacts with steam in the presence of an acid catalyst to give a single alcohol product – either propan‑1‑ol or propan‑2‑ol. The acid catalyst (e.g. H₂SO₄) protonates the double bond, forming a carbocation. Water then attacks the carbocation, and deprotonation gives the alcohol. The reaction is an addition reaction because the water molecule adds across the C=C double bond, breaking the π bond and forming two new σ bonds, so the double bond is lost and a single product is formed.

Examiner tips

  • Use the term ‘addition reaction’ and state that the double bond is broken and two new bonds are formed.
  • Mention the role of the acid catalyst and the formation of a carbocation intermediate.
  • Show that only one product (an alcohol) is obtained, not a mixture of products.

Common mistakes

  • Saying the reaction gives a mixture of alcohols without noting the single product.
  • Confusing the reaction with a substitution or elimination reaction.
  • Omitting the need for an acid catalyst or high temperature/pressure conditions.

Mark scheme (4 marks)

  1. Propene reacts with steam (water / H₂O) to form propan-1-ol or propan-2-ol (an alcohol)
  2. An acid catalyst is required / the reaction requires high temperature and pressure
  3. The C=C double bond in propene breaks / opens
  4. The atoms of steam / water add across the double bond to form a single bond / one product only is formed with no other products

Key terms in this question

addition reaction · catalyst

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