Test for Alkenes (Bromine Water)

Purpose of Bromine Water Test

The bromine water test is used to detect the presence of alkenes, which are hydrocarbons containing a carbon 6carbon double bond (C=C). This test distinguishes alkenes from alkanes, which only have single bonds.

Alkenes react with bromine water, causing a visible colour change, while alkanes do not react and so the bromine water remains the same colour. This colour change is an easy and quick way to identify unsaturation (the presence of double bonds) in a hydrocarbon.

Reaction Mechanism

The reaction between bromine water and an alkene is an addition reaction. Bromine molecules add across the double bond, breaking the C=C bond and forming a colourless dibromoalkane.

This happens because the double bond in the alkene is reactive and can open up to allow bromine atoms to attach to the carbon atoms. The overall effect is that the orange colour of bromine disappears as it reacts.

The general equation for the reaction is:

$$\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$$

Observations

• Bromine water is naturally an orange/red solution.
• If an alkene is present, the bromine water loses its colour and becomes colourless.
• If an alkane is tested, there is no reaction and the bromine water remains orange/red.
• The test is quick and easy to observe with the naked eye.

For instance, when bromine water is added to ethene (an alkene), the orange colour disappears as the bromine reacts with the double bond:

$$\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$$

Example: When ethene reacts with bromine water, the orange bromine water turns colourless because the bromine adds across the double bond, forming 1,2-dibromoethane.

Summary

• The bromine water test detects alkenes by their ability to decolourise bromine water.
• Alkenes undergo an addition reaction where bromine adds across the double bond, forming a colourless dibromoalkane.
• Alkanes do not react, so bromine water remains orange/red.
• This test is a simple, quick, and reliable way to identify unsaturation in hydrocarbons.