Bromine test, alkene bromination

The bromine test is able to detect unsaturated compounds:

1. Elemental bromine is dissolved in water, dichloromethane, or tetrachloromethane. The dissolved bromine will make the liquid brown.

2. The dissolved bromine is added to the alkene and shaken. If the alkene is a gas then it can be bubbled through the brown liquid.

3. Discolaration indicates unsaturated bonds. The ratio of bromine and alkenes can also indicate the amount of unsaturated bonds per molecule.

If the mixture contains phenol, you may also get a white precipitate of tribromophenol. Filter and dry this to get white needle-like crystals with an antiseptic smell. It is used as a fungicide and wood preserver.

Tribromophenol

Chlorine is faster at this reaction, iodine is slower. 

If bromine water is used then this complicates the reaction, since the cation created in the mechanism is much more likely to collide with a H₂O molecular than Br₂. For example, if the alkene used is ethene, then the most common product will be bromoethanol and hydrobromic acid, as shown below:

Step 1:

Step 2, without water:

Step 2, with water: