# Explain what is meant by stereoisomers and describe how cis and trans isomers of but-2-ene differ from each other.

> Eduqas A-Level Chemistry — 4.1 Stereoisomerism · Explain · 5 marks

> But-2-ene has the molecular formula C₄H₈ and contains a carbon-carbon double bond. It exists as two stereoisomers.

## Mark scheme (5 marks)

1. Stereoisomers have the same molecular formula
2. Stereoisomers have the same structural (connectivity) arrangement of atoms but different spatial/3D arrangement
3. The carbon-carbon double bond restricts rotation, allowing cis and trans isomers to exist
4. In the cis isomer, the two methyl (CH₃) groups are on the same side of the double bond
5. In the trans isomer, the two methyl (CH₃) groups are on opposite sides of the double bond

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-what-is-meant-by-stereoisomers-33d68eb1) · Published by Druglandscape Ltd.