Describe what will be seen when compound A is shaken with bromine water.
How to approach this question
This question asks for the test for an alkene. Compound A has a C=C double bond.
1. What is the standard chemical test for unsaturation (the presence of a C=C double bond)? It involves bromine water.
2. What is the initial colour of bromine water?
3. What is the final colour after a positive reaction with an alkene?
Full Answer
The test for an alkene (a compound containing a C=C double bond) is to shake it with bromine water. Bromine water is an aqueous solution of bromine, which has a distinct orange-brown or yellow colour. Alkenes undergo an addition reaction with bromine, where the double bond breaks and a bromine atom adds to each of the two carbon atoms. The product, a dibromoalkane, is colourless. Because the bromine is used up in the reaction, its colour disappears. Therefore, a positive test for an alkene is that the orange/brown/yellow bromine water is decolourised (turns colourless).
Common mistakes
✗ Just saying "it changes colour" without specifying the start and end colours.
✗ Getting the colours wrong.
✗ Confusing the test with the test for other functional groups.
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