Bromine Water Test

This is part of the HSC Chemistry course under Module 8 Section 2: Analysis of Organic Substances. 

HSC Chemistry Syllabus

Conduct qualitative investigations to test for the presence in organic molecules of the following functional groups:

  • carbon–carbon double bonds

  • hydroxyl groups

  • carboxylic acids (ACSCH130)

Identifying C=C Bonds: Bromine Water Test

This video explores how the bromine water test detects the presence of carbon-carbon double bonds. 

How Does the Bromine Water Test Work?

  • Reaction with bromine water distinguishes an alkene from an alkane. Alkanes and alkenes are non-polar molecules which can dissolve in bromine water since it is also a non-polar substance.

 

  • Safety Considerations
    • Cyclohexane and cyclohexene are typically used in schools as they are less volatile than other smaller alkanes and alkenes due to their stronger dispersion forces.
    • Bromine water is also safer to use than bromine gas.

 

  • Method and experimental condition:
    • A few drops of orange/brown coloured bromine water are added to a solution of cyclohexane and cyclohexene in the absence of UV light
    • Any changes in the solutions appearance are then recorded.

 

  • Observation: the reactive C=C bond in alkene undergoes addition reaction with bromine (Br2) to form an haloalkane. This decreases [Br2] and hence decolorises the solution. For example, bromination of ethene:

$$C_2H_4(aq) + Br_2(aq) \rightarrow C_2H_5Br(aq) + HBr(aq) $$

 

  • If the bromine water containing alkane is exposed to UV light, the alkane will undergo substitution reaction to produce haloalkane. This will also decolorise the solution, but at a much slower rate.

 

$$C_2H_6 + Br_2 \rightarrow C_2H_5Br + HBr$$

 

$$C_2H_5Br + HBr \rightarrow C_2H_4Br_2 + H_2$$