Why does alkyne decolourise bromine water

Carbon chemistry involves many different homologous series of compounds.

A homologous series is a family of hydrocarbons with similar chemical properties who share the same general formula.

SeriesGeneral formulaFunctional group
AlkanesCnH2n+2No functional group. Molecules end with CH3
AlkenesCnH2nCH2
CycloalkanesCnH2nNo specific functional group. Ring of single C-C bonds
AlcoholsCnH2n+1OHOH
Carboxylic acidsCnH2n+1COOHCOOH

Those with only carbon-carbon single bonds are saturated.

If a compound contains at least one carbon-carbon double bond it is unsaturated and can undergo addition reactions.

For example, alkenes can undergo addition reactions with hydrogen to become the corresponding alkanes.

Addition of bromine solution allows saturated and unsaturated compounds to be distinguished. Unsaturated compounds will decolourise bromine solution quickly, whereas it will remain a brown colour with saturated compounds. This is due to the bromine-bromine bond breaking and adding to the double bonds of unsaturated compounds.

Addition reactions

Alkenes are more reactive than alkanes and cycloalkanes because they have a double bond.

In an addition reaction, the double bond of the alkene partially breaks when the reactant molecule attacks and adds on across it.

Example

Addition of bromine water can be used as a test for unsaturation.

Alkenes quickly turn bromine water from red/brown to colourless, while alkanes and cycloalkanes do not.

The experiment shown below is an example of how to distinguish between alkanes and alkenes by adding bromine water.

Step one

Two test tubes of bromine water. An alkane is added to one, an alkene to the other.

Step two

The test tubes are shaken

Step three

The alkane liquid remains orange-brown. The alkene liquid has turned colourless.

The addition of bromine to an alkene is called bromination.