Alkenes and their reactions
Introduced| English | Meaning |
|---|---|
| alkene/ˈælkiːn/ | An unsaturated hydrocarbon containing at least one carbon-carbon double bond. |
| electrophilic addition/ɪˌlektrəʊˈfɪlɪk əˈdɪʃn/ | A reaction in which an electron-pair acceptor initiates addition across a multiple bond. |
| elimination/ɪˌlɪmɪˈneɪʃn/ | A reaction in which atoms or groups are removed from a molecule, usually forming a multiple bond. |
| dehydration/ˌdiːhaɪˈdreɪʃn/ | The removal or loss of water from a substance or organism. |
| bromine water/ˈbrəʊmaɪn ˈwɔːtə/ | An aqueous solution containing bromine, used in tests such as detecting carbon-carbon double bonds. |
The reactive double bond
- Alkenes 烯烃 have a C=C double bond ($\text{C}_n\text{H}_{2n}$).
- That double bond is the functional group, so alkenes are reactive.
- Most reactions are electrophilic addition 亲电加成.
Alkene addition route
Follow how the C=C bond opens and new atoms add.
Alkenes are reactive because they have:
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The electron-rich C=C double bond is the reactive functional group of alkenes (CₙH₂ₙ).
Adding bromine water to an alkene turns it from orange to:
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The bromine adds across the double bond.
Making alkenes
- elimination 消去 of HX from a halogenoalkane (NaOH in ethanol, heat).
- dehydration 脱水 of an alcohol (hot $\text{Al}_2\text{O}_3$ or conc. sulfuric acid).
- cracking of a longer alkane.
Cracking heavy fractions in a refinery produces alkenes
Which makes an alkene?
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Removing water from an alcohol (dehydration), or HX from a halogenoalkane (elimination), makes an alkene.
Match each alkene fact.
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Each item links the term to its correct meaning.
An alkene decolourises bromine water, while an alkane leaves it orange — that is the standard test for a C=C double bond.
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The π electrons of the C=C attack the bromine; alkanes have no double bond, so nothing happens.
Addition reactions
| Reagent | Product |
|---|---|
| $\text{H}_2$ (Pt/Ni, heat) | alkane |
| steam + $\text{H}_3\text{PO}_4$ | alcohol |
| $\text{HX}$ | halogenoalkane |
| $\text{X}_2$ | di-substituted alkane |
- Cold dilute $\text{KMnO}_4$ gives a diol; hot concentrated $\text{KMnO}_4$ breaks the C=C apart.
Adding steam (with an H₃PO₄ catalyst) to an alkene gives:
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Electrophilic addition of water (steam) across the C=C makes an alcohol.
Test for C=C
- Shake with orange bromine water 溴水: an alkene decolourises it (electrophilic addition); an alkane does not.
When an alkene is shaken with orange bromine water, the bromine water:
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Alkenes decolourise bromine water by electrophilic addition; alkanes leave it orange.
You've got it
- alkenes = C=C double bond ($\text{C}_n\text{H}_{2n}$), reactive; made by elimination, dehydration or cracking
- most reactions are electrophilic addition (H₂ → alkane, steam → alcohol, HX → halogenoalkane)
- bromine water test: an alkene decolourises it