What is it?
Position isomerism, an example of structural isomerism, occurs when a functional group is in a 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- or open-chain) compounds, and in aromatic compounds.
What is here?
You can see ball-and-stick models of the position isomers of:
In alcohols such as butanol, the functional group 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 hydroxyl group is red in the displayed formulae to make its position easier to see.
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.