# Bromoethane can undergo a substitution reaction with potassium cyanide solution to form a new organic compound. Describe what is meant by a substitution reaction and explain why the product formed from bromoethane and potassium cyanide contains one more carbon atom than bromoethane.

> WJEC A-Level Chemistry (Wales) — 2.6 Halogenoalkanes · Describe and Explain · 5 marks

## Mark scheme (5 marks)

1. In a substitution reaction, one atom (or group of atoms) is replaced by another atom (or group of atoms)
2. The bromine atom in bromoethane is replaced during the reaction
3. The cyanide ion / CN⁻ acts as the incoming group that replaces the bromine
4. The cyanide group (CN) contains a carbon atom
5. This carbon from the cyanide group is added to the two-carbon chain of bromoethane, giving a three-carbon product (propanenitrile)

## Key terms

- [substitution reaction](https://www.gradenine.co.uk/glossary/substitution-reaction)
- [cyanide](https://www.gradenine.co.uk/glossary/cyanide)
- [bromoethane](https://www.gradenine.co.uk/glossary/bromoethane)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and 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/bromoethane-can-undergo-a-substitution-reaction-013aaa30) · Published by Druglandscape Ltd.