# Ibuprofen is a common painkiller. It contains a chiral centre in its molecular structure. Only one of its optical isomers is pharmacologically active, meaning only one isomer relieves pain. Explain what is meant by a chiral centre, and explain why the two optical isomers of ibuprofen can have different effects in the human body despite having the same molecular formula and the same structural formula.

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

> Ibuprofen is a common painkiller sold in pharmacies worldwide. Its molecule contains a chiral centre, and it is sold commercially as a racemic mixture, meaning equal amounts of both optical isomers are present.

## Mark scheme (5 marks)

1. A chiral centre is a carbon atom bonded to four different atoms/groups
2. The two optical isomers are non-superimposable mirror images of each other
3. The optical isomers rotate plane-polarised light in opposite directions
4. Biological receptor sites/molecules have a specific three-dimensional shape
5. Only one isomer has the correct three-dimensional arrangement to fit the receptor site, so only one isomer is active

## Key terms

- [chiral centre](https://www.gradenine.co.uk/glossary/chiral-centre)
- [optical isomers](https://www.gradenine.co.uk/glossary/optical-isomers)

## 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/ibuprofen-is-a-common-painkiller-it-8fe7f82b) · Published by Druglandscape Ltd.