Explain what is meant by stereoisomers and describe how cis and trans isomers of but-2-ene differ from each other.

Eduqas A-Level Chemistry — 4.1 Stereoisomerism · Explain · 5 marks · View as Markdown

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

But-2-ene has the molecular formula C₄H₈ and contains a carbon-carbon double bond. It exists as two stereoisomers.

Model answer (5 marks)

Stereoisomers are compounds that have the same molecular formula and the same connectivity of atoms but differ in the three‑dimensional arrangement of those atoms.

In but‑2‑ene the C=C double bond prevents free rotation, so two distinct spatial arrangements are possible.

• In the cis isomer the two CH₃ groups are on the same side of the double bond.
• In the trans isomer the two CH₃ groups are on opposite sides of the double bond.

Examiner tips

  • Use the definition of stereoisomers first, then explain the role of the double bond. Show the cis/trans difference with a clear statement of the CH₃ positions. Keep it concise and use the exact terms "cis" and "trans".

Common mistakes

  • Confusing stereoisomers with structural isomers; forgetting to mention the double bond restriction.

Mark scheme (5 marks)

  1. Stereoisomers have the same molecular formula
  2. Stereoisomers have the same structural (connectivity) arrangement of atoms but different spatial/3D arrangement
  3. The carbon-carbon double bond restricts rotation, allowing cis and trans isomers to exist
  4. In the cis isomer, the two methyl (CH₃) groups are on the same side of the double bond
  5. In the trans isomer, the two methyl (CH₃) groups are on opposite sides of the double bond

Key terms in this question

stereoisomers · trans isomer

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