Explain what is meant by stereoisomers and describe how cis-trans isomerism arises in alkenes. Use but-2-ene as an example in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Stereoisomers are compounds that have the same molecular and structural formula but differ in the spatial arrangement of their atoms.
Cis‑trans isomerism in alkenes occurs because rotation about the C=C double bond is restricted. For cis‑trans isomerism to exist, each carbon of the double bond must bear two different groups.
In but‑2‑ene the two carbons of the double bond each have a CH₃ and a H attached. In cis‑but‑2‑ene the two CH₃ groups are on the same side of the double bond, whereas in trans‑but‑2‑ene the CH₃ groups are on opposite sides.
Cis‑trans isomerism in alkenes occurs because rotation about the C=C double bond is restricted. For cis‑trans isomerism to exist, each carbon of the double bond must bear two different groups.
In but‑2‑ene the two carbons of the double bond each have a CH₃ and a H attached. In cis‑but‑2‑ene the two CH₃ groups are on the same side of the double bond, whereas in trans‑but‑2‑ene the CH₃ groups are on opposite sides.
Examiner tips
- Define stereoisomers first, then explain the restriction of rotation and the requirement of two different groups per double‑bond carbon.
- Use the specific example of but‑2‑ene to illustrate cis and trans arrangements.
- Keep the answer concise – 5 marks can be earned with 4–5 short sentences.
- Mention the same formula and different spatial arrangement explicitly.
Common mistakes
- Forgetting that the two carbons must each have two different groups.
- Describing cis‑trans as a type of optical isomerism rather than a form of geometric isomerism.
- Using the wrong terminology (e.g., ‘rotational isomerism’ instead of ‘cis‑trans’).
Mark scheme (5 marks)
- Stereoisomers have the same molecular formula / same structural formula / same bonds
- Stereoisomers have a different arrangement of atoms in space / different spatial arrangement
- Cis-trans isomerism arises because there is restricted rotation about the C=C double bond
- Each carbon of the double bond must have two different groups attached to it (for cis-trans isomerism to exist)
- Correct description of cis and trans but-2-ene: in cis-but-2-ene both CH3 groups are on the same side of the double bond; in trans-but-2-ene the CH3 groups are on opposite sides
Key terms in this question
stereoisomers · cis-trans isomerism
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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