3.3.3Organic Chemistry
Electrophilic addition — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 26 of 42
Part 1 of 4Reactions of Ethene and Propene
Hydrogenation
- Alkene + H₂ with a nickel catalyst at about 150 °C gives an alkane: C₂H₄ + H₂ → C₂H₆.
Bromination
- Alkene + Br₂ at room temperature gives a dibromoalkane: C₂H₄ + Br₂ → CH₂BrCH₂Br.
Hydrogen halides
- Alkene + HBr at room temperature gives a bromoalkane: C₂H₄ + HBr → C₂H₅Br.
Hydration
- Alkene + steam with a phosphoric acid catalyst at about 300 °C and high pressure gives an alcohol: C₂H₄ + H₂O → C₂H₅OH.
Pattern
- In every case the C=C π bond opens and two new atoms or groups attach to the carbon atoms.
Part 2 of 4Unsymmetrical Alkenes
1
Propene is unsymmetrical, so its addition products with HBr or H₂O can form in two ways.
2
Hydrogen can add to either carbon of the double bond, giving 1-bromopropane or 2-bromopropane.
3
Both products form, but 2-bromopropane is the major product and 1-bromopropane is the minor product.
4
The major product comes from the more stable carbocation intermediate, which is the secondary carbocation.
5
Tertiary carbocations are more stable than secondary, and secondary more than primary, because alkyl groups donate electron density.
Part 3 of 4Why Alkenes Are Attacked by Electrophiles
Why Alkenes Are Attacked by Electrophiles
Electrophile
- An electrophile is an electron-pair acceptor that is attracted to regions of high electron density.
Attack
- The π bond provides the electron pair that forms the new bond to the electrophile.
Alkenes
- Alkenes therefore react by electrophilic addition, which is the key reaction type of the homologous series.
Industrial use
- Hydration of ethene makes ethanol, and hydrogenation of vegetable oils makes spreads.
Compare alkanes
- Alkanes lack π electrons, so electrophiles do not attack them.
| Reagent | Conditions | Product from propene |
|---|---|---|
| H₂ | Ni catalyst, 150 °C | Propane |
| Br₂ | Room temperature | 1,2-dibromopropane |
| HBr | Room temperature | 2-bromopropane (major) |
| H₂O(g) | H₃PO₄, 300 °C, high pressure | Propan-2-ol (major) |
Part 4 of 4Exam Checklist & Tips
3.3.3 – Key Points for the Exam
- Electrophile A species that accepts a pair of electrons to form a covalent bond.
- Electrophilic addition A reaction in which an electrophile attacks a C=C bond and the two molecules join.
- Hydrogenation Addition of hydrogen to a double bond.
- Hydration Addition of water to a double bond.
- Carbocation An organic ion with a positively charged carbon atom.
Common Mistakes
- Writing that alkenes react by nucleophilic substitution.
- Naming the major product of propene with HBr as 1-bromopropane.
- Forgetting that hydration of alkenes needs a catalyst and high temperature, not just water at room temperature.
- Leaving out the nickel catalyst and heat when describing the hydrogenation of an alkene.
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