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Alkenes and the bromine water test
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Alkenes contain a C=C double bond. This makes them more reactive than alkanes.
Alkenes
Functional group: C=C (a carbon-carbon double bond).
General formula: C_nH2n
Ethene C2H4, propene C3H6, butene C4H8.
Alkenes are unsaturated: atoms can be added across the double bond.
General formula: C_nH2n
Ethene C2H4, propene C3H6, butene C4H8.
Alkenes are unsaturated: atoms can be added across the double bond.
Isomers of butene
Butene has isomers that differ in the position of the double bond:
but-1-ene: CH2=CHCH2CH3
but-2-ene: CH3CH=CHCH3
The number tells you the first carbon of the double bond, counting from the nearer end.
but-1-ene: CH2=CHCH2CH3
but-2-ene: CH3CH=CHCH3
The number tells you the first carbon of the double bond, counting from the nearer end.
Reaction with bromine
Alkenes react with bromine by addition, forming a dibromoalkane.
ethene + bromine → 1,2-dibromoethane: C2H4 + Br2 → C2H4Br2
Test: shake with bromine water. An alkene turns it from orange to colourless. An alkane leaves it orange.
ethene + bromine → 1,2-dibromoethane: C2H4 + Br2 → C2H4Br2
Test: shake with bromine water. An alkene turns it from orange to colourless. An alkane leaves it orange.
How could you tell propane from propene?
- Add bromine water to each and shake.
- Propene (alkene) decolourises it; propane (alkane) does not.
Answer: Propene turns bromine water from orange to colourless; propane leaves it orange.
Alkenes contain C=C, have the general formula CnH2n and are unsaturated. But-1-ene and but-2-ene are isomers. Alkenes add bromine to form dibromoalkanes, so they decolourise bromine water; alkanes do not.
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