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.

What is here?

You can see ball-and-stick models of the position isomers of:

  • butanol
  • butene
  • methylphenol

Use your mouse (or finger on touch devices) to move or scale molecules. Double tap to stop and start each animation.

Options
atom labels
ball and stick model
space-filling model
perspective view

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.