Add halogens to alkenes and test for C=C
Use halogen addition and aqueous bromine decolourisation as evidence for an alkene double bond.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to aqueous bromine when it reacts with an alkene?
What you will learn
- Describe X₂ addition across C=C
- Use bromine water as a qualitative test
- State the limit of a positive observation
A halogen adds across the double bond
A halogen molecule X₂ adds to C=C, with one halogen atom bonding to each alkene carbon. Ethene reacts with bromine to form 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br.
The π bond is consumed and the carbon skeleton is retained. The displayed formula can help show that one Br attaches to each carbon.
Aqueous bromine indicates unsaturation
Aqueous bromine is orange-brown. When it reacts with a C=C bond, the bromine colour disappears as the halogen is added.
This is a qualitative test for C=C in the stated context. A colour observation should be interpreted with the sample and controls in mind; it does not identify a unique alkene structure by itself.
Pause and explain
Which bond is indicated by decolourisation of aqueous bromine in this test?
Worked example
Describe and interpret the observation when bromine water is added to ethene.
Show the worked solution
- Ethene contains a C=C bond.
- Br₂ adds across that bond to form 1,2-dibromoethane.
- The orange-brown colour disappears.
Answer The bromine water is decolourised as Br₂ adds across ethene's C=C bond to form CH₂BrCH₂Br.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the product and equation for bromine addition to propene.
Give me a hint
Attach one Br to each carbon that formed C=C.
Compare my reasoning
- Keep the three-carbon chain.
- Convert C=C to C–C.
- Place a Br atom on each former alkene carbon.
CH₃CH=CH₂ + Br₂ → CH₃CHBrCH₂Br; 1,2-dibromopropane.
Look for these in your work
- Carbon skeleton retained
- One Br attached per alkene carbon
- Product named
A colourless liquid decolourises aqueous bromine. What conclusion is supported, and what remains uncertain?
Give me a hint
Separate a qualitative bond test from full structure identification.
Compare my reasoning
- The observation is consistent with C=C reacting with Br₂.
- The sample likely contains an unsaturated compound in this context.
- The observation alone does not identify a unique molecule or prove purity.
The result supports the presence of C=C unsaturation, but it does not identify one unique alkene or establish sample purity.
Look for these in your work
- C=C inference stated
- Context limited
- Identity limit stated
A sample remains orange-brown after bromine water is mixed with it. What does that suggest, and why should the test be repeated carefully?
Give me a hint
Consider whether bromine was consumed and whether the sample actually contacted it.
Compare my reasoning
- No visible decolourisation means the test gives no positive C=C evidence.
- An alkene may be absent, or mixing/contact conditions may be inadequate.
- Use a fresh, correctly mixed sample and suitable comparison before concluding.
There is no positive evidence of C=C from that trial; check mixing and repeat with appropriate controls before concluding the sample is saturated.
Look for these in your work
- Observation interpreted cautiously
- Alternative explanation included
- Repeat/controls noted
Common mistakes
- Calling the test evidence of every possible alkene identity
- Saying bromine substitutes a hydrogen on the alkene
- Claiming a color change without naming the C=C reaction
What can you explain now?
What happens to aqueous bromine when it reacts with an alkene?
Compare with the explanation
Its orange-brown colour disappears
Bromine is consumed by addition across C=C.
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Make it stick
What observation supports C=C, and why does it not identify a unique molecule?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.