Pent-2-ene can exist as cis and trans stereoisomers. Explain why pent-2-ene can form cis and trans isomers, and state one difference between the cis and trans forms of pent-2-ene.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Pent-2-ene has the molecular formula C₅H₁₀ and contains a carbon–carbon double bond between carbon-2 and carbon-3.
Model answer (5 marks)
Pent‑2‑ene has a C=C double bond, so rotation about the bond is restricted, giving two distinct spatial arrangements.
Each double‑bond carbon is bonded to two different groups: a hydrogen and a propyl chain.
In the cis isomer the two larger groups (the propyl chains) lie on the same side of the double bond.
In the trans isomer the two larger groups lie on opposite sides of the double bond.
Because of this different geometry the cis and trans isomers have different physical properties, for example the cis isomer has a higher boiling point than the trans isomer.
Each double‑bond carbon is bonded to two different groups: a hydrogen and a propyl chain.
In the cis isomer the two larger groups (the propyl chains) lie on the same side of the double bond.
In the trans isomer the two larger groups lie on opposite sides of the double bond.
Because of this different geometry the cis and trans isomers have different physical properties, for example the cis isomer has a higher boiling point than the trans isomer.
Examiner tips
- Show the restriction of rotation first, then explain the two different groups on each double‑bond carbon, then describe the cis/trans arrangement, finally give a physical property difference.
- Use the exact terms ‘cis’ and ‘trans’ and mention the larger groups (propyl chains).
Common mistakes
- Forgetting to mention the restriction of rotation or the different groups on each double‑bond carbon.
- Confusing the cis/trans description (e.g. saying the hydrogens are on the same side).
Mark scheme (5 marks)
- There is restricted rotation about the carbon–carbon double bond
- Each carbon of the double bond has two different groups attached to it
- In the cis isomer the two larger/similar groups are on the same side of the double bond
- In the trans isomer the two larger/similar groups are on opposite sides of the double bond
- Cis and trans isomers have different physical properties, e.g. different boiling points / melting points
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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