Position isomerism
Position isomerism is an example of structural isomerism. It occurs when a functional group can occupy different positions on the same carbon chain.
In GCSE Chemistry, you usually see position isomers of alcohols and alkenes. At A Level, you find out about position isomers involving other functional groups in aliphatic (straight-chain or open-chain) compounds, and in aromatic compounds.
Alcohols
butan-1-ol
butan-2-ol
The functional group in alcohols is the hydroxyl group, —OH.
There are no position isomers of:
- methanol, CH3OH
- ethanol, CH3CH2OH
Propanol has two position isomers:
- propan-1-ol, CH3CH2CH2OH
- propan-2-ol, CH3CH(OH)CH3
The displayed formulae here are modified just to make the position of the functional group easier to see:
- the hydroxyl group is shown in red
- the O—H bond is not shown.
Alkenes
but-1-ene
but-2-ene
The functional group in alkenes is the C=C bond. Methene does not exist. There are no position isomers of:
- ethene, CH2=CH2
- propene, CH3CH=CH2
The models show the two position isomers of butene. Note that but-2-ene exists as two cis–trans isomers:
- the model shown here is trans-but-2-ene
- you can see a model of cis-but-2-ene on the cis-trans isomerism page.
The displayed formulae here are modified just to make the position of the functional group easier to see:
- the carbon-carbon double bond is shown in red.
Aromatic alcohols
2-methylphenol
2-hydroxy-1-methylbenzene
3-methylphenol
3-hydroxy-1-methylbenzene
4-methylphenol
4-hydroxy-1-methylbenzene
The displayed formulae here are modified just to make the position of the functional groups easier to see:
- the hydroxyl group and methyl group are shown in red
- the O—H and C—H bonds are not shown.
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