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.
- 5.5-a
- 5.5-b
What these spec points say
- 5.5-a know the qualitative test for a C=C double bond using bromine
- 5.5-b know the qualitative test for a C=C double bond using bromine water
Before You Start
This page builds on the GCSE bromine water test for alkenes.
- Bromine water is orange. Alkenes decolourise it; alkanes do not.
- Saturated hydrocarbons contain only C-C single bonds; unsaturated hydrocarbons contain at least one C=C double bond.
- Alkenes have the general formula CnH2n.
- In an addition reaction, atoms add across the C=C bond to form one product.
GCSE Recap: Saturated or Unsaturated?
Four quick GCSE questions on alkenes, alkanes and cracking before you look at the test in detail.
Watch: the test for a C=C double bond
The standard test for a C=C double bond, step by step. What colour change would you see?
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.
Quick Check: Drop by Drop
Decide what it means when the orange colour stops disappearing.
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.
Shaken with an alkene, bromine water is decolourised from orange to colourless; with an alkane the orange colour remains.
Quick Check: Which Ones React?
Click every substance in the list that turns bromine water 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.
Quick Check: Pick the Accurate Statement
In each round, choose the one statement that is accurate.
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 π bond attacks Brδ+
As Br2 approaches, the π bond induces a dipole in it. The π electrons form a bond to Brδ+, the Br-Br bond breaks heterolytically, and a carbocation and Br– form.
Step 2: Water attacks the carbocation
Water is in large excess, so a lone pair on the oxygen atom of H2O usually bonds to the carbocation before Br– can. Some Br– does attack, so a little 1,2-dibromoethane also forms.
Step 3: H+ is lost
The oxygen now carries a positive charge. An O-H bond breaks and H+ is lost, giving 2-bromoethanol from ethene. Bromine has been used up, so the orange colour disappears.
Quick Check: How Far Does the Result Go?
Write a short explanation, then compare it with the mark points and the model answer.
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.
A positive bromine water test shows that a C=C bond is present, but not how many there are or where they are.
Quick Check: Bromine Water from Start to Finish
Each round covers one part of the page, from the observation to the mechanism. Choose the one accurate statement.
FAQs
These questions secure the observation, product and limits of the bromine water test.
Is the product with bromine water 1,2-dibromoethane?
Mainly not. Water is in large excess, so it attacks the carbocation more often than Br– does. The main product from ethene is 2-bromoethanol, with some 1,2-dibromoethane. For the test itself, the observation earns the mark.
Why must I write “colourless” and not “goes clear”?
Clear means transparent, and orange bromine water is already clear. Colourless is the correct word. Write the full change: orange to colourless.
Can bromine water tell me which alkene I have?
No. Every compound with a C=C bond decolourises bromine water, so the test only shows that a C=C bond is present. It cannot tell ethene from propene or show how many double bonds a molecule has.
Why does an alkane not decolourise bromine water?
An alkane has only σ bonds and no π electrons to attack bromine. At room temperature in the dark no reaction happens, so the orange colour stays. In UV light, alkanes react slowly by radical substitution instead.
Do I need to heat the sample with bromine water?
No. Shake the sample with a few drops of bromine water at room temperature. An alkene decolourises it straight away, so heating is not needed.
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.
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