Explain why 1,2-dichloroethene can exist as cis and trans stereoisomers, and describe one difference between the cis and trans forms of this molecule.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
1,2-Dichloroethene has the molecular formula C₂H₂Cl₂ and contains a carbon-carbon double bond with a chlorine atom and a hydrogen atom attached to each carbon.
Model answer (5 marks)
The C=C double bond prevents free rotation, so the relative positions of the groups on the two carbons are fixed.
Each carbon of the double bond is bonded to two different groups (Cl and H), giving each carbon a defined orientation.
Because the two Cl atoms can be on the same side of the double bond, the cis isomer is possible.
Because the two Cl atoms can also be on opposite sides, the trans isomer is possible.
The cis isomer has a higher dipole moment (and higher boiling point) than the trans isomer.
Each carbon of the double bond is bonded to two different groups (Cl and H), giving each carbon a defined orientation.
Because the two Cl atoms can be on the same side of the double bond, the cis isomer is possible.
Because the two Cl atoms can also be on opposite sides, the trans isomer is possible.
The cis isomer has a higher dipole moment (and higher boiling point) than the trans isomer.
Examiner tips
- State the double bond restriction and the two different groups on each carbon. Mention the cis arrangement (Cl on same side) and the trans arrangement (Cl on opposite sides). Give one clear physical property that differs, e.g. dipole moment, boiling point or melting point.
Common mistakes
- Forgetting to mention that the double bond blocks rotation. Confusing the cis/trans definition (e.g. saying Cl on opposite sides is cis). Giving a property that is not actually different between the isomers.
Mark scheme (5 marks)
- The C=C double bond restricts/prevents rotation
- Each carbon in the double bond has two different groups attached to it
- In the cis isomer the two chlorine atoms are on the same side of the double bond
- In the trans isomer the two chlorine atoms are on opposite sides of the double bond
- One valid difference between cis and trans 1,2-dichloroethene, e.g. different physical properties / different melting points or boiling points / different dipole moments
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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