10.2.5 Alkenes - Halogenation

• The reaction between alkenes and halogens is known as halogenation
• It is an example of an electrophilic addition where an electrophile ('electron seeker') joins onto to a double bond
• The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms
• The result of this reaction is a dihalogenoalkane
• The reaction occurs readily at room temperature and is the basis for the test for unsaturation in molecules

Halogenation in alkenes

• Halogens can be used to test if a molecule is unsaturated (i.e. contain a double bond)
• Br₂ is an orange or yellow solution, called bromine water
• The unknown compound is shaken with the bromine water
• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The bromine water test is the standard test for unsaturation in alkenes