Section 1 of 5Addition of Bromine
1
Bromine molecules add across the C=C double bond of an alkene, with one bromine atom attaching to each carbon of the double bond.
2
Ethene reacts with bromine to form 1,2-dibromoethane: C₂H₄ + Br₂ → C₂H₄Br₂.
3
The same addition occurs with any alkene and any halogen, giving a dihalogenoalkane.
4
The product has only single bonds, so it is a saturated compound, and no other small molecule is lost in the reaction.
Section 2 of 5The Bromine Water Test
The Bromine Water Test
Reagent
- Bromine water is an orange solution used instead of liquid bromine because it is safer and easier to handle.
Alkane result
- Alkanes have no C=C bond, so the bromine stays in solution and the orange colour remains.
Alkene result
- The bromine adds across the C=C bond, so free bromine is removed and the orange colour disappears.
Conclusion
- Decolourisation of bromine water shows that the compound contains a C=C double bond.
| Substance | Observation with bromine water |
|---|---|
| Ethene (alkene) | Orange to colourless, so it decolourises |
| Propene (alkene) | Orange to colourless, so it decolourises |
| Hexane (alkane) | Stays orange |
Section 3 of 5Addition to Propene
Propene example
- Propene adds bromine to give 1,2-dibromopropane, CH₃CHBrCH₂Br, with a bromine atom on each of the two carbons of the former double bond.
Same pattern
- Each bromine atom bonds to one of the two carbon atoms that were joined by the double bond.
Addition product
- The dibromo compound is saturated, so it no longer decolourises bromine water.
Practical test
- An unknown colourless compound is tested by adding bromine water and observing whether the orange colour disappears.
Control
- An alkane tested alongside it shows the contrast clearly, because its orange colour is retained.
Section 4 of 5Worked Example
Question Describe how to use bromine water to show that an unknown colourless gas is ethene rather than ethane.
- Pass the gas into, or shake it with, a sample of bromine water in a test tube
- Observe the colour of the bromine water
- If the orange colour disappears, the gas is ethene; if the solution stays orange, it is ethane
Answer Ethene decolourises bromine water, but ethane, an alkane, does not change the orange colour.
Section 5 of 5Exam Checklist & Tips
4.4.2 – Key Points for the Exam
- Addition reaction A reaction in which two molecules combine to form one larger product.
- Bromine water An orange aqueous solution of bromine, used to test for C=C double bonds.
- Decolourise To lose colour, as bromine water does when it reacts with an alkene.
- Dihalogenoalkane A compound containing two halogen atoms, formed by adding a halogen across a C=C bond.
- Saturated Containing only single bonds between carbon atoms.
Common Mistakes
- Saying the bromine atoms replace hydrogen atoms; in addition they join across the double bond.
- Stating that bromine water turns green when it reacts with alkenes.
- Reporting that alkanes decolourise bromine water as well as alkenes.
- Naming the product of ethene and bromine as bromoethane; it is 1,2-dibromoethane.