Testing for Alkenes

Bromine is a diatomic molecule, which means it consists of two bromine atoms. When dissolved in a solvent, it forms an orange-brown solution. We can use bromine water to test for alkenes.

When we add bromine water to a compound containing a double bond, an addition reaction takes place. In this reaction, the double bond breaks and the bromine molecule splits, forming a dibromo alkane.

This product is colourless, unlike the orange-brown colour of bromine water. For example, when we add bromine water to ethene, it forms a dibromoethane, and the solution becomes colourless.

Diagram showing an ethene molecule and a bromine molecule reacting to form a dibromo alkane.

On the other hand, when bromine water is added to alkanes, which are already saturated, no reaction takes place. This is because the bromine atoms cannot add to or bond with the alkane molecule. Therefore, no reaction takes place and the solution remains orange-brown.

A diagram shows two solutions: one orange, labelled "alkane", and one colourless, labelled "alkene".