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:


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

Here are the names and structures of four alkenes:

Table containing the molecular formula and structural formula of ethene, propene, but-1-ene and but-2-ene.

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.