# 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.

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

## Mark scheme (5 marks)

1. Stereoisomers have the same molecular formula / same structural formula / same bonds
2. Stereoisomers have a different arrangement of atoms in space / different spatial arrangement
3. Cis-trans isomerism arises because there is restricted rotation about the C=C double bond
4. Each carbon of the double bond must have two different groups attached to it (for cis-trans isomerism to exist)
5. 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

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

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-what-is-meant-by-stereoisomers-4cdbee19) · Published by Druglandscape Ltd.