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Alkanes and alkenes
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Alkanes and alkenes are two families of hydrocarbons. One small difference in bonding changes how they react. Separate Chemistry only.
Homologous series
A family of compounds with the same general formula and similar reactions.
Alkanes: C_nH2n+2, only single bonds, saturated. Methane, ethane, propane, butane.
Alkenes: C_nH2n, contain a C=C double bond, unsaturated. Ethene, propene, butene.
Alkanes: C_nH2n+2, only single bonds, saturated. Methane, ethane, propane, butane.
Alkenes: C_nH2n, contain a C=C double bond, unsaturated. Ethene, propene, butene.
Bromine water test
Add orange bromine water.
Alkene: turns colourless (bromine adds across the double bond).
Alkane: stays orange.
Alkene: turns colourless (bromine adds across the double bond).
Alkane: stays orange.
Addition reactions
Alkenes react by addition: the double bond opens.
ethene + hydrogen → ethane (nickel catalyst)
ethene + bromine → dibromoethane
ethene + hydrogen → ethane (nickel catalyst)
ethene + bromine → dibromoethane
Write the formula of the alkene with 3 carbon atoms.
- Alkenes are C_nH2n.
- n = 3, so 2n = 6.
Answer: C3H6 (propene)
Alkanes C_nH2n+2 are saturated; alkenes C_nH2n have a C=C double bond. Alkenes decolourise bromine water and undergo addition reactions.
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