# A student is studying two organic molecules, molecule A and molecule B. Molecule A is butan-2-ol. Molecule B is butan-1-ol. The student claims that butan-2-ol displays optical isomerism but butan-1-ol does not. Explain why the student's claim is correct, including what is meant by optical isomerism and the structural feature responsible for it.

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

> Optical isomerism is a form of stereoisomerism. Two molecules that are optical isomers are non-superimposable mirror images of each other. A molecule displays optical isomerism when it contains a chiral centre.

## Mark scheme (5 marks)

1. Optical isomerism occurs when a molecule has a chiral centre / asymmetric carbon atom
2. A chiral centre is a carbon atom bonded to four different atoms or groups
3. The two optical isomers are non-superimposable mirror images of each other
4. In butan-2-ol, carbon-2 is bonded to four different groups (H, OH, CH3, and C2H5), so it is a chiral centre
5. In butan-1-ol, no carbon is bonded to four different groups, so there is no chiral centre and no optical isomerism

## Key terms

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

## 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/a-student-is-studying-two-organic-515cf4a6) · Published by Druglandscape Ltd.