# 2-bromobutene can exist as cis and trans isomers. Describe the structural features required for cis-trans isomerism to occur in an alkene, and explain why these features prevent rotation that would interconvert the two isomers.

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

> Cis-trans isomers are a form of stereoisomerism found in certain alkenes. The two isomers of 2-bromobutene have identical molecular formulae and the same bonds, yet they are distinct compounds with different properties.

## Mark scheme (5 marks)

1. The molecule must contain a C=C double bond (which restricts rotation)
2. Each carbon of the double bond must have two different groups/atoms attached to it
3. The C=C double bond consists of a sigma bond and a pi bond; it is the pi bond that prevents rotation
4. The pi bond is formed by sideways overlap of p orbitals above and below the plane; rotating would break this overlap
5. Because rotation cannot occur, the groups on either side of the double bond are locked in position, giving rise to two distinct (non-superimposable) arrangements — the cis and trans isomers

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/2-bromobutene-can-exist-as-cis-and-a7f714d9) · Published by Druglandscape Ltd.