Testing for Carbon-Carbon Double Bonds: The Bromine Water Test
Expert reviewed •23 November 2024• 5 minute read
Introduction
The bromine water test is a fundamental analytical method in organic chemistry used to detect the presence of carbon-carbon double bonds (C=C). This test is particularly useful for distinguishing between alkenes and alkanes, making it an essential technique in organic compound analysis.
Principle of the Test
The test relies on the different reactivity patterns of alkenes and alkanes with bromine water. Alkenes readily undergo an addition reaction with bromine (Br₂), while alkanes remain largely unreactive under normal conditions.
Chemical Reaction
When an alkene reacts with bromine water, the following addition reaction occurs:
CH2=CH2+Br2−>CH2Br−CH2Br
Experimental Procedure
Materials Required
Bromine water (orange-brown solution)
Test compound (alkene or alkane)
Test tubes
Dropper
Safety Precautions
Always handle bromine water with care as it is corrosive and irritating
Work in a well-ventilated area or fume hood
Wear appropriate personal protective equipment (PPE)
In school laboratories, cyclohexane and cyclohexene are preferred due to their lower volatility
Method
Place a small sample of the test compound in a clean test tube
Add a few drops of bromine water
Observe any color changes
Record the results immediately
Results Interpretation
Positive Test (Alkene Present): The orange-brown color of bromine water rapidly disappears
Negative Test (Alkene Absent): The orange-brown color persists
Special Considerations
UV Light Effects: Alkanes can react with bromine under UV light through a substitution mechanism:
CH3−CH3+Br2−>[UV light]CH3−CH2Br+HBr
This reaction is much slower than the alkene addition reaction and requires UV light.
Practical Applications
Identifying unknown organic compounds
Quality control in industrial processes
Educational demonstrations in chemistry laboratories
Limitations
The test may give false positives with other reducing agents
Results must be interpreted promptly as the color may fade naturally over time