Explain why propene undergoes electrophilic addition with hydrogen bromide but benzene does not react with hydrogen bromide under the same conditions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Propene has a C=C double bond, a region of high electron density that can donate electrons to the electrophile H⁺ in HBr. The H⁺ adds to the double bond forming a carbocation intermediate; Br⁻ then attacks to give 1‑bromopropane. Benzene, however, possesses a delocalised π system; its electrons are spread over all six carbons, lowering the electron density at any one site and making donation to H⁺ much less favourable. Moreover, addition to benzene would break the delocalised π system and destroy aromaticity, an energetically disfavoured step. Thus propene undergoes electrophilic addition, whereas benzene does not react with HBr under the same conditions.
Examiner tips
- Use the term ‘electrophilic addition’ and mention the carbocation intermediate for propene.
- Explain that benzene’s delocalised π system lowers electron density and that addition would destroy aromaticity.
Common mistakes
- Claiming benzene reacts via a free‑radical mechanism; it does not.
- Forgetting to mention the loss of aromaticity as the key reason benzene is unreactive.
Mark scheme (4 marks)
- Propene has a C=C double bond / region of high electron density that can donate electrons to an electrophile such as H⁺ in HBr
- 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
- Addition to benzene would disrupt the delocalised π system / destroy aromaticity, making it energetically unfavourable
- 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 in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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