Final answer:
Bromine water reacts with alkenes in a process called halogenation to form a disubstituted alkyl halide, which results in the decolorization of the bromine water.
This reaction is a test for the presence of alkenes and differs from halogenation of alkanes which normally requires UV light or high temperatures.
Step-by-step explanation:
When bromine water is combined with an alkene, a chemical reaction occurs known as halogenation, where the bromine adds across the double bond of the alkene, resulting in the formation of a disubstituted alkyl halide. Bromine water, which is brownish red in color, will decolorize, signaling that the reaction has taken place.
For example, when bromine is added to ethene (an alkene), a disubstituted alkyl halide called 1,2-dibromoethane is formed, highlighting that the alkene has reacted with the bromine.
The disappearance of the brownish red color of bromine is a classic test to confirm the presence of alkenes. This process is an example of halogenation, specifically the addition of a halogen to an unsaturated hydrocarbon.
It is important to note that alkanes can also undergo a halogenation reaction with bromine, but this usually requires ultraviolet light or high temperatures and results in a substitution reaction rather than an addition reaction like with alkenes.