What are alkenes?
Alkenes are unsaturated hydrocarbons: they contain at least one carbon–carbon double bond (C=C). They form a homologous series with the general formula CnH2n: ethene C2H4, propene C3H6, butene C4H8. Alkenes are made by cracking larger alkane molecules.
Key idea
The reactive C=C double bond lets alkenes undergo addition reactions. The test for an alkene is bromine water: it changes from orange to colourless when shaken with an alkene (an alkane leaves it orange).
Addition reactions
- With bromine: C2H4 + Br2 → C2H4Br2.
- With hydrogen (catalyst): forms an alkane.
- With steam (catalyst): ethene + water → ethanol.
Worked example
Ethene reacts with bromine. The double bond opens and one Br adds to each carbon: C2H4 + Br2 → C2H4Br2 (1,2-dibromoethane). Because the double bond is used up, the orange bromine water becomes colourless — this is the positive test for unsaturation.
Polymerisation
Many ethene molecules join by addition polymerisation to make poly(ethene), a plastic.
Remember
- Alkenes: CnH2n, unsaturated (C=C).
- Bromine water: orange → colourless = alkene present.
- Alkenes are more reactive than alkanes because of the C=C bond.