Question

How do alkenes react with bromine water?

Answer 1

Hint:the carbon- carbon double bond of alkene reacts in such a way that alkanes cannot. When alkenes react with bromine , this reaction is known as electrophilic addition reaction. bromine water is an orange coloured solution.

Complete answer:
The bromine molecule is a polar molecule and therefore bromine induces charges in pi bond also.
To know the reaction, we must know the mechanism of this reaction-
Firstly, bromonium ion is formed, when the bond of bromine-bromine breaks. Bromine induces the charge to double bond of bromine molecule and then attacks the carbon atoms. A triangle shaped intermediate is formed.
After that, the bromonium ion attacks from the back side to the intermediate because a hindrance is created on the same side by bromide ion.
The bromonium ion then attaches to the double bond carbon and forms a dibromo product that is $1,2-$dibromoethane. Both the bromine atoms are attached to both the carbon atoms.
The orange bromine solution is decolourised into colourless dibromo compounds because as the solution is shaken with an alkene, it decolourises.
Alkenes react with bromine solution in presence of an organic solvent tetrachloromethane.

Note:Electrophilic addition reaction is defined as the reaction in which the pi bond is broken into two new sigma bonds. The electrophile attacks on the carbon-carbon double bond and forms an additional product. The electrophile is always attracted to electron rich regions and it carries a positive charge.
The reaction of alkene with bromine solution is also used as a chemical test for carbon- carbon double bonds.