Key Concept
Alkenes are susceptible to electrophilic attack because of the high electron density of the carbon-carbon double bond (C=C). The pi (π) bond acts as a region of electron density that attracts electrophiles. These reactions lead to electrophilic addition.
Why Do Alkenes React with Electrophiles?
The C=C double bond consists of a sigma (σ) bond and a pi (π) bond. The electron density in the π bond sits above and below the plane of the molecule, making it exposed and available for attack by electrophiles.
What is Addition?
In an addition reaction, two molecules combine to form a single product. The C=C double bond "opens up" (the π bond breaks) and two new bonds form, one to each carbon. There is only one product, with no leaving group.
Reactions of Alkenes
At SL you need to be able to deduce equations for the reactions of alkenes with the following reagents.
1. Reaction with Hydrogen Halides (HBr, HCl, HI)
Ethene + Hydrogen bromide → Bromoethane
CH₂=CH₂ + HBr → CH₃CH₂Br
The H and Br atoms add across the double bond. The π bond breaks and two new single bonds form.
2. Reaction with Halogens (Br₂, Cl₂)
Ethene + Bromine → 1,2-dibromoethane
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
This is the classic bromine water test for unsaturation. The orange bromine is decolourised as it adds across the double bond.
3. Reaction with Water (H₂O)
Ethene + Water → Ethanol
CH₂=CH₂ + H₂O → CH₃CH₂OH
This is the hydration of an alkene. In practice, requires an acid catalyst (e.g. concentrated H₂SO₄) or high temperature and pressure with steam.
Summary Table
| Reagent | Product from ethene | Type of product |
|---|---|---|
| HBr | CH₃CH₂Br | Halogenoalkane |
| HCl | CH₃CH₂Cl | Halogenoalkane |
| Br₂ | CH₂BrCH₂Br | Dihalogenoalkane |
| Cl₂ | CH₂ClCH₂Cl | Dihalogenoalkane |
| H₂O | CH₃CH₂OH | Alcohol |
Note for HL Students
The actual mechanisms of electrophilic addition (curly arrows, carbocation intermediates, Markovnikov's rule) are covered at HL in R3.4.11 and R3.4.12. At SL you only need to deduce the equations and understand why electrophiles are attracted to the double bond.
Think About It
Why do alkenes undergo addition reactions but alkanes do not?
Alkanes contain only strong C-C and C-H sigma bonds with no exposed electron density. Alkenes have a pi bond with electron density above and below the molecular plane, making them susceptible to electrophilic attack. The π bond can break to form two new σ bonds, which is energetically favourable.
Common Exam Mistakes
- Writing "substitution" instead of "addition". Alkenes undergo addition (one product), not substitution (product + leaving group).
- Forgetting that the double bond breaks. Both carbons in the C=C bond gain a new substituent.
- Not balancing the equation. Check that atoms on both sides match.
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