Question

How does bromine water show if something is an alkane or alkene?

Answer 1

Because an olefin, an alkene, will undergo facile bromination with bromine.

Explanation:

You know that the bromine molecule is polarizable, i.e. it can form `Br-Br^(delta+)` (the atom on the LHS is `delta^-`!). This electrophilic centre is capable of reaction with the electron-rich double bond to form a cationic intermediate, i.e. with ethylene:

`H_2C=CH_2 + Br_2 rarr H_2^+C-CH_2Br + Br^-`

The orange colour of bromine should therefore dissipate, and of course this is a positive test for olefinic bonds. Now that you have a carbocation, this will react with any nucleophile present: this could be water to give `HOCH_2-CH_2Br` (and of course water is a potent nucleophile). In the absence of water in an inert solvent it could be the bromide ion that was delivered in the first part of the bromination, to give 1,2-dibromoethane.