11.4 - Test for a Double Bond with Bromine Water
The technique
Aim
To use bromine water to tell an alkane apart from an alkene.
Background
- An alkene has a carbon-carbon double bond. An alkane has only single bonds.
- Bromine water is orange. Bromine adds across the double bond of an alkene in an addition reaction, which uses up the bromine and removes the colour.
- An alkane has no double bond for the bromine to add across, so the colour stays.
- You can't see a double bond, but you can see whether a sample decolourises bromine water.
- So in this practical you add bromine water to four unknown liquids. A liquid that turns it colourless is an alkene, and one that leaves it orange is an alkane.
Method
- Label the tubes - Label four test tubes A, B, C, and D, to match the labels on the four unknown liquid samples.
- Add the samples - Use a fresh dropping pipette for each liquid to put about 1 cm3 into its own tube, and put a stopper in each tube.
- Add the bromine water - Take one tube at a time into a fume cupboard, remove the stopper, and add about 5 drops of bromine water.
- Shake the tube - Replace the stopper at once and shake the tube for about five seconds.
- Record the colour - Hold the tube against a white background, and record whether the orange colour disappears or stays. Repeat steps 3 to 5 for each of the other tubes.
- Identify the samples - Once all four results are recorded, a tube that turned colourless holds an alkene, such as hexene or cyclohexene. A tube that stayed orange holds an alkane, such as hexane or cyclohexane.
Safety
- Fume cupboard - Only remove a stopper and add bromine water inside the fume cupboard.
- Eye protection and gloves - Wear them throughout, and read the hazard labels on the samples first, because all four are flammable and toxic.
- No naked flames - Keep the samples and the bromine water away from any flame.
- Spills - Wash bromine water off your skin at once, and tell your teacher straight away about any spill outside the fume cupboard.
Common exam questions
Question 1
Why must the bromine water be added inside a fume cupboard?
Bromine water gives off bromine vapour, which is toxic and irritating to breathe. The fume cupboard carries the vapour away, so you don't breathe it in.
Question 2
Why is each tube kept stoppered except while the bromine water is added?
The samples are flammable as well as toxic. The stopper stops their vapour escaping into the room, which also reduces the risk of it catching fire.
Question 3
Why is the tube shaken after the bromine water is added?
To mix the two liquids, so the bromine water comes into full contact with the sample and any reaction can happen.
Question 4
Why is the colour judged against a white background?
A faint trace of orange is easy to miss against a dark background. A sample could then be wrongly recorded as having decolourised the bromine water, and be misidentified as an alkene.
Question 5
Cyclohexane and cyclohexene have similar names and similar boiling points. What result tells them apart?
Only cyclohexene decolourises the bromine water, because it's the alkene and has the carbon-carbon double bond. Cyclohexane leaves the bromine water orange.