# Explain what is meant by optical isomerism and describe the conditions that must be met for a molecule to display optical isomerism. In your answer, state how the two optical isomers of a compound are related to each other.

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

> Thalidomide was a drug used in the 1950s and 1960s. It exists as two optical isomers: one isomer relieved morning sickness, while the other caused serious birth defects. Both isomers have the same molecular formula and the same structural formula.

## Mark scheme (5 marks)

1. Optical isomers are non-superimposable mirror images of each other
2. Optical isomers arise when a molecule contains a chiral centre (chiral carbon)
3. The chiral carbon must have four different atoms or groups attached to it
4. The two optical isomers are called enantiomers
5. The two enantiomers rotate plane-polarised light in opposite directions

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-what-is-meant-by-optical-a4db0d1a) · Published by Druglandscape Ltd.