# Explain why propene undergoes electrophilic addition with hydrogen bromide but benzene does not react with hydrogen bromide under the same conditions.

> IB DP Chemistry Standard Level (2023 syllabus) — R3.3 Electron sharing reactions · Explain · 4 marks

## Mark scheme (4 marks)

1. Propene has a C=C double bond / region of high electron density that can donate electrons to an electrophile such as H⁺ in HBr
2. Benzene has a delocalised π electron system / electrons are spread over all six carbon atoms, lowering electron density at any one point and making it less available for donation
3. Addition to benzene would disrupt the delocalised π system / destroy aromaticity, making it energetically unfavourable
4. In propene, addition of H⁺ to the double bond forms a carbocation intermediate, and Br⁻ then attacks; this mechanism is not available to benzene because the carbocation intermediate formed would not be stabilised / would destroy the ring

## Key terms

- [electrophilic addition](https://www.gradenine.co.uk/glossary/electrophilic-addition)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](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/explain-why-propene-undergoes-electrophilic-addition-964f84f1) · Published by Druglandscape Ltd.