# Explain the mechanism by which ethene reacts with hydrogen bromide to form bromoethane, including the role of the π bond and the nature of each step.

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

## Mark scheme (4 marks)

1. The π electrons of ethene act as a nucleophile / electron donor and attack the δ+ hydrogen of the polar H–Br bond.
2. H–Br acts as the electrophile; HBr is described as (acting as) an electrophile in this reaction.
3. A carbocation intermediate forms when H bonds to one carbon, leaving the other carbon with a positive charge.
4. The bromide ion (Br⁻) then attacks the carbocation to form bromoethane; the overall mechanism is electrophilic addition.

## Key terms

- [π bond](https://www.gradenine.co.uk/glossary/bond)

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