Propan-1-ol and propan-2-ol are both members of the alcohol homologous series. Explain why these two compounds are considered to be structural isomers of each other, and state one chemical property that both compounds share because they belong to the same homologous series.

Cambridge International IGCSE Chemistry (0620) — 11.1 Formulae, functional groups and terminology · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Propan‑1‑ol and propan‑2‑ol have the same molecular formula C₃H₈O but the atoms are arranged differently – the –OH group is on the first carbon in propan‑1‑ol and on the second carbon in propan‑2‑ol. They are therefore structural isomers.
Both are alcohols, so they share the chemical property of reacting with sodium metal to give hydrogen gas and an alkoxide.

Examiner tips

  • Use the word ‘structural isomers’ and explain the different positions of the –OH group. Mention the common functional group (alcohol) and a shared property such as reaction with sodium. Keep the answer concise and use correct IUPAC names.

Common mistakes

  • Saying they are identical because they have the same formula. Forgetting to specify the different position of the –OH group. Choosing a property that is not common to both, e.g. oxidation to a ketone (only propan‑2‑ol).

Mark scheme (4 marks)

  1. Both compounds have the same molecular formula (C₃H₈O)
  2. But the atoms are arranged differently / the hydroxyl (–OH) group is in a different position
  3. Both contain the same functional group, the hydroxyl group (–OH) / both are alcohols
  4. A named chemical property shared by both, e.g. both undergo combustion / both react with sodium to produce hydrogen / both can be oxidised to form a carbonyl compound

Key terms in this question

structural isomers · homologous series

Related

More Formulae, functional groups and terminology questions

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