# Alkenes are unsaturated hydrocarbons that can undergo addition reactions. Describe what happens when ethene reacts with bromine, and explain why alkanes cannot undergo the same type of reaction.

> Eduqas A-Level Chemistry — 2.5 Hydrocarbons · Describe and Explain · 5 marks

> Ethene (C₂H₄) is the simplest alkene and is used widely in the chemical industry. It reacts readily with bromine to form a new product.

## Mark scheme (5 marks)

1. Ethene contains a carbon–carbon double bond (C=C)
2. The double bond opens / breaks during the reaction
3. Bromine adds across the double bond to form 1,2-dibromoethane / a new single product
4. Alkanes are saturated / only have carbon–carbon single bonds
5. Without a double bond, alkanes cannot undergo addition reactions (so bromine cannot add across a bond)

## Key terms

- [addition reaction](https://www.gradenine.co.uk/glossary/addition-reaction)
- [saturated](https://www.gradenine.co.uk/glossary/saturated)
- [unsaturated](https://www.gradenine.co.uk/glossary/unsaturated)

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/alkenes-are-unsaturated-hydrocarbons-that-can-d80f7c91) · Published by Druglandscape Ltd.