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.