# Cis-trans isomerism is observed in some alkenes but not others. Explain why 1,2-dichloroethene can exist as cis and trans isomers, and predict whether 1,1-dichloroethene can exist as cis and trans isomers, giving a reason for your answer.

> WJEC A-Level Chemistry (Wales) — 4.1 Stereoisomerism · Explain · 5 marks

## Mark scheme (5 marks)

1. The C=C double bond cannot rotate (freely) / rotation is restricted about the C=C double bond
2. Each carbon of the double bond in 1,2-dichloroethene has two different groups attached to it
3. This gives rise to two different spatial arrangements / the groups can be on the same side (cis) or opposite sides (trans)
4. 1,1-dichloroethene cannot exist as cis and trans isomers
5. Because one carbon of the double bond in 1,1-dichloroethene has two identical groups (both Cl) attached to it

## Key terms

- [cis-trans isomerism](https://www.gradenine.co.uk/glossary/cis-trans-isomerism)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/cis-trans-isomerism-is-observed-in-some-cdc40d5a) · Published by Druglandscape Ltd.