Testing for alkenes

The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot. This allows us to distinguish alkenes from alkanes using a simple chemical test.

Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene. Alkenes can decolourise bromine water, but alkanes cannot. The slideshow shows this process.

Two test tubes of reddish-brown bromine water. An alkane is added to one, and an alkene is added to the other.

The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction. The bromine is decolourised because a colourless dibromo compound forms. For example:

ethene + bromine → 1,2-dibromoethane

C2H4 + Br2 → C2H4Br2

CH2=CH2 + Br–Br → CH2BrCH2Br

Other addition reactions of alkenes:

  • Hydrogen can be added to a C=C double bond. This has the effect of ‘saturating’ the molecule, and will turn an alkene into an alkane. For example: C2H4 + H2 → C2H6.
  • If steam (H2O) is added to an alkene, an alcohol is made. For example: C2H4 + H2O → C2H5OH.