Alkenes
Introduced| English | Meaning |
|---|---|
| double bond/ˈdʌbl bɒnd/ | A covalent bond in which two atoms share two pairs of electrons. |
| alkenes/ˈælkiːnz/ | An unsaturated hydrocarbon containing at least one carbon-carbon double bond. |
| unsaturated/ʌnˈsætʃəreɪtɪd/ | Containing one or more carbon-carbon double or triple bonds rather than only single bonds. |
| addition reactions/əˈdɪʃn rɪˈækʃnz/ | Reactions in which atoms or groups join across a multiple bond to produce a larger molecule. |
| bromine water/ˈbrəʊmaɪn ˈwɔːtə/ | An aqueous solution containing bromine, used in tests such as detecting carbon-carbon double bonds. |
The double-bond 双键 family
- The alkenes 烯烃 look similar to alkanes but have one crucial difference: a double bond.
- That double bond makes them much more reactive.
Alkenes contain a:
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The C=C double bond defines alkenes.
The general formula of the alkenes is:
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Alkenes follow CₙH₂ₙ.
The simplest alkene, with two carbon atoms, is ______.
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Ethene is C₂H₄.
The alkene family
- Alkenes have the general formula CₙH₂ₙ and contain a carbon–carbon double bond.
- Examples: ethene (C₂H₄), propene (C₃H₆).
Alkenes are unsaturated — they contain a C=C double bond, unlike the saturated alkanes
Addition across the C=C double bond
Step through an addition reaction. The C=C double bond is an alkene's reactive spot — molecules add right across it, which also gives the bromine-water test.
Alkenes are unsaturated because of their double bond.
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The double bond can open to add atoms.
Unsaturated 不饱和
- Alkenes are unsaturated — they have a double bond that can open up.
- This lets other atoms add across the double bond (addition reactions 加成反应).
Bromine water added to an alkene turns:
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The bromine adds across the double bond, decolourising.
Testing for a double bond
- Add bromine water 溴水 to an alkene: it turns from orange to colourless.
- This is the test that distinguishes alkenes (unsaturated) from alkanes (saturated).
Worked example. Shake an unknown hydrocarbon with orange bromine water. If it turns colourless, the compound is an alkene (the bromine adds across the double bond). If it stays orange, it's an alkane.
The bromine water test: an alkene decolourises the orange bromine water, while an alkane leaves it orange
You've got it
- alkenes are unsaturated (a C=C double bond), formula CₙH₂ₙ
- the double bond lets atoms add across it (addition reactions)
- the test: bromine water turns from orange to colourless with an alkene