4.4.2Organic Chemistry
Bromine and Alkenes — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 12 of 24
Part 1 of 5Why alkenes are reactive
Saturated and unsaturated
- Saturated and unsaturated Alkanes are saturated: every carbon–carbon bond is a single bond,
- so each carbon atom is already bonded to as many other atoms as it can be.
- Alkenes are unsaturated: they contain at least one carbon–carbon double bond, C=C.
The double bond
- The double bond The C=C bond contains a region of high electron density, which makes it much more reactive than a C–C single bond.
- Because of this, alkenes can take part in reactions that alkanes cannot, such as addition reactions.
General idea of addition
- General idea of addition In an addition reaction two reactants join to form a single product, and nothing else is made.
- A small molecule adds across the double bond, which turns into a single bond.
Part 2 of 5Adding bromine to an alkene
1Alkene meets bromine
- Alkene meets bromine A molecule of bromine, Br2, approaches the C=C double bond of the alkene.
2Double bond opens
- Double bond opens One of the two bonds between the carbon atoms breaks, leaving a single C–C bond.
3Bromine atoms attach
- Bromine atoms attach One bromine atom bonds to each of the two carbon atoms that were joined by the double bond.
4Dibromoalkane forms
- Dibromoalkane forms The product is a saturated dibromoalkane.
- With ethene the product is 1,2-dibromoethane, C2H4Br2.
Part 3 of 5Equation and general pattern
| Reaction | Word equation | Symbol equation |
|---|---|---|
| Ethene + bromine | ethene + bromine → 1,2-dibromoethane | C2H4 + Br2 → C2H4Br2 |
| Propene + bromine | propene + bromine → 1,2-dibromopropane | C3H6 + Br2 → C3H6Br2 |
| Ethene + chlorine | ethene + chlorine → 1,2-dichloroethane | C2H4 + Cl2 → C2H4Cl2 |
| Any alkene + halogen | alkene + halogen → dihaloalkane | Both halogen atoms always end up on the carbons of the old C=C bond |
Part 4 of 5The bromine water test
1
Put a small sample of the unknown hydrocarbon (or a solution of it) into a test tube.
2
Add a few drops of orange bromine water and shake the tube gently so the two mix.
3
Observe the colour. Bromine water is used instead of pure bromine because it is safer and easier to handle.
4
If the mixture stays orange, the hydrocarbon is an alkane (or another saturated compound): there is no C=C bond for the bromine to add to, so free bromine remains.
5
If the orange colour disappears and the mixture becomes colourless, the hydrocarbon is an alkene: the bromine has added across the C=C bond, so no free bromine is left to give colour.
Part 5 of 5Interpreting the result
Correct wording
- Correct wording Say that bromine water is decolourised (orange to colourless).
- Avoid the word 'clear', which means transparent rather than colourless.
- A positive test shows an alkene, or at least a compound with a C=C double bond.
Explaining the change
- Explaining the change The coloured Br2 molecules are used up in the addition reaction, and the dibromoalkane formed is colourless.
- An alkane does not react in this way, so the orange colour stays.
Safety
- Safety Bromine and bromine water are toxic and irritating, so wear eye protection and gloves and work in a well-ventilated space.
Key terms
| Term | Meaning |
|---|---|
| Saturated | Containing only single bonds between carbon atoms, as in alkanes. |
| Unsaturated | Containing at least one carbon–carbon double bond, as in alkenes. |
| Addition reaction | A reaction in which atoms add across a double bond and a single product forms. |
| Dibromoalkane | A saturated compound with one bromine atom on each of two adjacent carbon atoms. |
| Bromine water | An orange solution of bromine used to test for C=C double bonds. |
| Decolourise | To change from a colour (here orange) to colourless. |
Avoid these mistakes
1
Writing that the bromine water turns 'clear' for an alkene. The correct observation is that it turns from orange to colourless.
2
Saying the alkane 'reacts' and stays orange. An alkane does not react with bromine water in the test, so the colour is unchanged.
3
Drawing the product of ethene and bromine with the bromine atoms on the same carbon or leaving a C=C bond in it. The product is saturated, with one Br on each carbon.
4
Describing the product mixture as containing hydrogen or HBr. Addition reactions make only one product.
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