Addition reactions
Alkenes are more reactive than alkanes and cycloalkanes because they have a double bond. While alkanes, alkenes and cycloalkanes undergo combustion reactions with oxygen, only alkenes can participate in addition reactions.
In an addition reaction, the double bond of the alkene partially breaks when the reactant molecule attacks and adds on across it.
Example
Addition of bromine water can be used as a test for unsaturation.
Alkenes quickly turn bromine water from red/brown to colourless, while alkanes and cycloalkanes do not.
The experiment shown below is an example of how to distinguish between alkanes and alkenes by adding bromine water.
Image caption, Step one
Two test tubes of bromine water. An alkane is added to one, an alkene to the other.
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The addition of bromine to an alkene is called bromination.
Other small molecules can be added across double bonds in alkenes. If hydrogen is added, then the corresponding alkane molecule is formed.
Example
The addition of hydrogen to an alkene is called hydrogenation.
If water is added to an alkene, then an alkanol (alcohol) is produced.
The addition of water to an alkene is called hydration.