Reactions of Alkenes with Bromine Water
A concise revision guide to the reaction of alkenes with bromine water, including the orange to colourless observation, why the reaction is a test for the alkene functional group and how the C=C double bond reacts by addition.
The Overall Reaction
Alkenes react with bromine water in an addition reaction. Bromine water is bromine dissolved in water, and the reaction takes place at room temperature.
The C=C double bond opens and atoms add across the double bond. This means the alkene becomes a saturated organic product.
The key observation is that the orange colour of bromine water decolourises to colourless.
Key idea: The colour change happens because bromine reacts with the C=C double bond, so bromine water is used as a chemical test for alkenes.
The C=C double bond reacts with bromine water, causing the orange colour to disappear.
Using Bromine Water to Test for Alkenes
Bromine water is commonly used to test whether an organic compound contains an alkene functional group.
If a C=C double bond is present, bromine water changes from orange to colourless. This positive test shows that the bromine has reacted with the alkene.
If there is no C=C double bond under these conditions, the orange colour is expected to remain.
Positive test
Bromine water decolourises from orange to colourless. This suggests the compound contains a C=C double bond.
Negative test
The orange colour remains. This suggests there is no alkene functional group reacting under the test conditions.
Exam focus: State both the reagent and the observation: bromine water changes from orange to colourless.
Why This Is an Addition Reaction
Alkenes are unsaturated because they contain a carbon-carbon double bond, C=C. The double bond contains an electron-rich π bond, which is the reactive part of the molecule.
During addition, the π bond is broken and new bonds form to the two carbon atoms from the original double bond.
The overall result is that the molecule becomes more saturated because atoms have been added across the C=C bond.
Addition reaction: A reaction in which atoms are added across a double bond, converting an unsaturated molecule into a more saturated product.
Remember: In alkene addition reactions, the C=C double bond is the reactive site. The π bond reacts first because it is electron-rich and more exposed than the σ bond.
The Reaction Mechanism
The reaction of an alkene with bromine water is linked to the same core idea as other alkene addition reactions: the electron-rich π bond reacts first.
The C=C double bond provides electron density, allowing the reaction to begin. The double bond then opens and new bonds form across the two carbon atoms.
This explains why the characteristic orange colour disappears. Bromine is consumed as it reacts with the alkene.
Step 1: The C=C bond reacts
The π bond in the alkene is electron-rich and begins the reaction with bromine water.
Step 2: The double bond opens
The alkene loses the C=C double bond as atoms add across the two carbon atoms.
Step 3: The colour change is observed
The orange colour of bromine water decolourises to colourless as bromine is used up in the reaction.
The mechanism shows how the alkene double bond reacts and why bromine water loses its orange colour.
Common Exam Mistakes
Students often lose marks by saying only that bromine water “goes clear” without giving the full starting colour. The full observation is orange to colourless.
Another common error is to describe the reaction as substitution. This is incorrect for alkenes. The reaction is addition because atoms are added across the C=C double bond.
For a test question, the safest answer should include the reagent, condition and observation.
Exam focus: Link the observation to the alkene functional group. The colour change is evidence that a C=C double bond has reacted.
Check Your Understanding
Use these short activities to check the key ideas before moving on to the next alkene reaction.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.