3.8       describe the addition reaction of alkenes with bromine, including the decolourising of bromine water as a test for alkenes.

Bromine will add to a double bond in a molecule like ethene, as shown above.  This is called an ADDITION reaction because 2 molecules have added together to form only one product.

This works because the alkenes are unsaturated, so there is “room” for Br atoms to add to it. Two can add for each double bond.

Note that the product has no double bonds, so it is saturated. Nothing further can add to it.

This is a test for alkenes:

Bromine water is a solution of bromine in water.  It is orange colour (or yellow if it is more dilute).

A little bromine water (orange) is added to the suspected alkene and shaken. If it is an alkene, the addition reaction above happens.  The product is colourless, so we see a change of orange to colourless in this test.  With an alkane (saturated hydrocarbon) the bromine water stays orange colour.