Testing for alkenes
The presence of the C=C A covalent bond resulting from the sharing of four electrons (two pairs) between two atoms. allows Unsaturated hydrocarbon with a double bond between the carbon atoms. to react in ways that Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds. 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.
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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 A substance formed by the chemical union of two or more elements. forms. For example:
ethene + bromine â 1,2-dibromoethane
C2H4 + Br2 â C2H4Br2
CH2=CH2 + BrâBr â CH2BrCH2Br
Other addition reactions of alkenes:
- Hydrogen can be added to a C=C double bond. This has the effect of âsaturatingâ the molecule, and will turn an alkene into an alkane. For example: C2H4 + H2 â C2H6.
- If steam (H2O) is added to an alkene, an alcohol is made. For example: C2H4 + H2O â C2H5OH.