# But-2-enedioic acid exists as two stereoisomers: maleic acid (cis) and fumaric acid (trans). Explain why but-2-enedioic acid can exist as cis and trans stereoisomers, and explain how the cis and trans forms of but-2-enedioic acid differ from each other in terms of the arrangement of their atoms.

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

> But-2-enedioic acid has the molecular formula C₄H₄O₄ and contains a C=C double bond with a –COOH group attached to each carbon of the double bond.

## Mark scheme (5 marks)

1. The C=C double bond does not allow rotation (restricted/no rotation about the double bond)
2. Each carbon of the double bond has two different groups attached to it
3. In the cis (maleic acid) form, the two –COOH groups are on the same side of the double bond
4. In the trans (fumaric acid) form, the two –COOH groups are on opposite sides of the double bond
5. The two stereoisomers have the same molecular formula and the same bonds/connectivity but different spatial arrangements of atoms

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/but-2-enedioic-acid-exists-as-two-stereoisomers-f2aaca6e) · Published by Druglandscape Ltd.