# Lactic acid (2-hydroxypropanoic acid) is produced in muscle cells during intense exercise. It exists as two optical isomers. Explain why lactic acid displays optical isomerism and describe the relationship between its two optical isomers, including how they interact with plane-polarised light.

> Edexcel A-Level Chemistry (9CH0) — 17.1 Optical isomerism · Explain · 5 marks

> Lactic acid has the molecular formula C₃H₆O₃. Its central carbon atom is bonded to four different groups: a hydroxyl group (–OH), a carboxyl group (–COOH), a methyl group (–CH₃), and a hydrogen atom.

## Mark scheme (5 marks)

1. The central carbon atom is bonded to four different groups/substituents, making it a chiral centre (or asymmetric carbon).
2. The two optical isomers are non-superimposable mirror images of each other.
3. The two optical isomers are called enantiomers.
4. Each enantiomer rotates plane-polarised light in opposite directions.
5. A racemic mixture (equal amounts of both enantiomers) shows no overall rotation of plane-polarised light / is optically inactive.

## Key terms

- [optical isomerism](https://www.gradenine.co.uk/glossary/optical-isomerism)
- [plane-polarised light](https://www.gradenine.co.uk/glossary/plane-polarised-light)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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/lactic-acid-2-hydroxypropanoic-acid-is-produced-4f891cb0) · Published by Druglandscape Ltd.