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Alkenes are a homologous series of hydrocarbons that contain a carbon-carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms, so they have the general formula:

C_nH_2n

For example, the molecular formula of ethene is C₂H₄, while for propene it is C₃H₆.

Here are the names and structures of four alkenes:

Butene has two straight-chained isomers, as the double bond can be placed in two different places or locations:

• but-1-ene (double bond with the first carbon)
• but-2-ene (double bond with the second carbon)

Alkenes are unsaturated, meaning they contain a double bond. This bond is why the alkenes are more reactive than the alkanes.