Alkenes

Alkenes are a homologous series of unsaturated hydrocarbons. We can identify them by the ‘ene’ at the end of their name, such as ethene. All alkenes have carbon-to-carbon double bonds, which are represented as C=C.

The general formula for alkenes is:

"CnH2n"
  • ‘n’ is the number of carbon atoms in the molecule

Due to the presence of a double bond, alkenes are generally more reactive than alkanes, which only have single bonds between carbon atoms. This double bond is the functional group of alkenes and it determines the chemical properties of alkenes.

Let’s look at the molecular formulae and structures of the first two alkenes:

AlkeneNo. of carbon atomsMolecular formulaStructural formulaBall + stick diagram.
Ethene2C2H4This image has an empty alt attribute; its file name is image-52.pngThis image has an empty alt attribute; its file name is image-50.png
Propene3C3H6This image has an empty alt attribute; its file name is image-53.pngThis image has an empty alt attribute; its file name is image-51.png

As the number of carbon atoms in an alkene molecule increases, its physical properties, such as boiling points, gradually change.

Compared to their corresponding alkanes, alkenes have two fewer hydrogen atoms, making them unsaturated. The double bond between the carbon atoms can open up to form another single bond. Therefore, each carbon atom can form four single bonds instead of one double bond and two single bonds.

This difference in bonding makes alkenes more reactive than alkanes.

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