Bromine water, also called Bromide bromate solution or Bromine solution with the chemical formula Br2. The molecular weight of bromine water is 159.81 and the density is 1.307 g/mL. Bromine water is a yellow mixture solution with high oxidizing property, prepared by dissolving diatomic bromine (Br2) in water (H2O).
Bromine water solution can be prepared in the chemistry lab by direct mixing of fumes of bromine and water. But it is not safe, so the more convenient method of preparation of bromine water solution is breaking of sodium bromide (NaBr) in the presence of bleach and hydrochloric acid (HCl).
Bromine water is used to identify the functional group present in the organic compound by halogenation mechanism.
Bromine water test (saturation test)
The bromine water test is a qualitative test, used to identify the alkene or alkane functional groups present in the compound. Alkene groups react with bromine water in the dark condition and undergo an addition reaction, to give a decolourized solution. On the other hand, alkane doesn’t react with bromine water and the colour of the bromine water remains the same. Enols, alkenes, aniline, glucose, phenols and acetyl groups are the most common compounds to undergo bromine water test. The test also identifies the presence of an aldehyde group in the compound. During the process, the colour of the bromine water changes from yellow to colourless.
The reaction between bromine water and different functional groups are as given below:
Alkane does not react with the bromine water solution and dark yellow colour of bromine solution remains as such.
CH3 + CH2 + CH2 + CH3 → No reaction
Alkene undergoes an addition reaction. For example, ethene reacts with bromine water to give 1,2 dibromo ethanes.
The reaction takes place at room temperature if the reactants are in the gaseous state (ethene). Colour of the bromine water solution is decolourized as it reacts with ethene.
But, liquid alkenes like cyclohexene react with bromine water solution in the presence of tetrachloromethane.
Phenols undergo substitution reactions in the presence of bromine water to give a brominated compound. During the process, bromine water is decolourized and gives white precipitate.
The mechanism of the reaction is given below,
Aniline or phenylamine reacts with bromine water. During the reaction, a white precipitate is formed along with decolouration of brome water.
Enols undergo the bromine water test and brominated ketone is formed.
The bromine water test is a simple test to distinguish between glucose and fructose. The Glucose undergoes oxidation reaction to give glucuronic acid on reaction with the bromine water solution. Since bromine water is a mild oxidizing agent, fructose fails to undergo an oxidation reaction.
A reaction between the ketone and bromine water is an electrophilic alpha substitution reaction, adjacent to the carbonyl group and gives the colourless solution of brominated compounds.
Aldehyde reacts with bromine water and undergoes an oxidation reaction to give a colourless solution.
- The bromine water test is focused on the determination of saturated or unsaturated hydrocarbons.
- Organic compounds like phenols, alkenes, acetyl compounds and anilines will readily undergo bromine water tests.
- During the process, change in the colour of the bromine water indicates the presence of an unsaturated group in the organic compound.
Common questions regarding the bromine water test
1. Does benzene undergo bromine water test?
Even though benzene has an unsaturated double bond, they have stable delocalized pi bonds, and it does not react with a bromine water solution.
2. How does alkane react with bromine water?
Since alkane is a saturated compound, it does not react with a bromine water solution.