AlkenesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What are alkenes?
Alkenes are unsaturated hydrocarbons — their molecules contain at least one carbon-carbon double bond (C=C). Because of this double bond, alkenes have the general formula CnH2n, which has fewer hydrogen atoms than the equivalent alkane (CnH2n+2).
The first two alkenes are:
| Name | Formula |
|---|---|
| Ethene | C2H4 |
| Propene | C3H6 |
Section 2
How are alkenes represented?
Alkenes can be shown in several forms:
- Molecular formula: e.g. C3H6 (shows only the numbers of each type of atom)
- Displayed formula: shows every atom and every bond, with = used to represent the double bond
- Structural formula: a shorthand showing how atoms are grouped, e.g. CH2=CHCH3 for propene
Recognising the C=C double bond in any of these forms tells you the molecule is an alkene, not an alkane.
When asked to identify an alkene from a formula, check specifically for a double bond (=) between two carbons — this is the deciding feature, not just the presence of carbon and hydrogen.
Section 3
How can you test for a double bond?
The standard test for unsaturation uses bromine water:
- Bromine water is normally orange
- When shaken with an alkene, it turns colourless, because the C=C double bond reacts with the bromine
- Alkanes do not react with bromine water, so it stays orange — this is how alkenes and alkanes can be told apart
A common error is saying bromine water 'changes colour' without stating the direction — always state orange to colourless for a positive (alkene) result.
Section 4
Where do alkenes come from?
Alkenes are not common in crude oil itself. Instead, they are produced by cracking — breaking down larger, less useful hydrocarbon molecules (often alkanes) from crude oil fractions into smaller, more useful molecules. Cracking produces a mixture of smaller alkanes and alkenes. Alkenes are especially useful as the starting materials (monomers) for making polymers.
Must Know
- Alkenes are unsaturated hydrocarbons containing a C=C double bond; general formula CnH2n
- The first two alkenes are ethene (C2H4) and propene (C3H6)
- In a displayed formula, = represents the double bond
- Bromine water turns from orange to colourless with an alkene (positive test for unsaturation); it stays orange with an alkane
- Alkenes are produced by cracking larger hydrocarbons from crude oil
- Alkenes are used as monomers to make polymers
That's the notes covered.
Carry on to the next subtopic.