3.4.5—Electrophilic addition to alkenes
- Syllabus
- First assessment 2025
- Objective
- 3.4.5
- Level
- SL
Electrophilic Addition to Alkenes
The electron-rich C=C attacks an electrophile. Deduce addition products with water, halogens or hydrogen halides within the SL mechanism boundary.
Treat the C=C as the reactive site and place the two added groups on its two carbon atoms. Bromine addition removes the double bond and forms a dibromoalkane; hydration forms an alcohol. At SL, deducing these products does not require a mechanism.
| Reagent | Groups added across C=C | Product check |
|---|---|---|
| X₂ (for example Br₂) | X and X | vicinal dihalogenoalkane; C=C becomes C–C |
| HX | H and X | halogenoalkane; conserve the H and halogen from HX |
| H₂O/steam under acid-catalysed hydration conditions | H and OH | alcohol; conserve the carbon skeleton |
At SL, use reagent and atom conservation to deduce products; curly-arrow mechanisms are not assessed in this card.
Deducing Alkene-Addition Products
Assessment in practice
Representative question
Question 1
[Maximum number: 1]
Predict the product of the reaction between ethene and bromine.
1,2-dibromoethane
Marking guidance:
Accept name or structure.
Electron-Pair Sharing Summary
Retrieve the route: classify nucleophiles and electrophiles, show heterolysis, write substitution and addition mechanisms, map Lewis coordination, compare SN1/SN2, and restore aromaticity in benzene substitution.
Check electron-pair arrow origin and destination, leaving-group departure, intermediate identity, carbocation stability and the assessed mechanism boundary.