# Thalidomide is a drug that was prescribed in the late 1950s. It exists as a pair of optical isomers. One isomer relieved morning sickness in pregnant women, while the other caused severe birth defects. Explain why thalidomide exists as optical isomers and describe the relationship between the two isomers, including how they behave differently when plane-polarised light is passed through their solutions.

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

> Thalidomide has the molecular formula C₁₃H₁₀N₂O₄ and contains a carbon atom bonded to four different groups.

## Mark scheme (5 marks)

1. Thalidomide contains a chiral carbon (chiral centre / 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. An equal mixture (racemic mixture / racemate) of the two enantiomers would show no overall rotation of plane-polarised light

## Key terms

- [optical isomers](https://www.gradenine.co.uk/glossary/optical-isomers)
- [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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/thalidomide-is-a-drug-that-was-98bf8cfb) · Published by Druglandscape Ltd.