# Cis-trans isomerism is a type of stereoisomerism. Explain what is meant by the term stereoisomers and describe the structural feature required for a molecule to show cis-trans isomerism. Use 2-bromobutene as an example to illustrate 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 atoms bonded in the same order / same structural formula
2. But stereoisomers differ in the arrangement / orientation of atoms in space
3. A C=C double bond is required (restricted rotation around the double bond)
4. Each carbon of the double bond must have two different groups / atoms attached to it
5. Correct use of 2-bromobutene: identifies that the two carbons of the C=C each bear two different groups (e.g. one carbon has H and Br; the other has H and CH3 / ethyl group), allowing cis (same side) and trans (opposite sides) isomers

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/cis-trans-isomerism-is-a-type-of-f31aa7c3) · Published by Druglandscape Ltd.