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Position isomerism

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:

  • butanol
  • butene
  • methylphenol
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chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

butan-1-ol

CH3CH2CH2CH2OH

Displayed formula of butan-1-ol

butan-2-ol

CH3CH2CH(OH)CH3

Displayed formula of butan-2-ol

 

Alcohols

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.

but-1-ene

CH3CH2CH=CH2

Displayed formula of but-1-ene

but-2-ene

CH3CH=CHCH3

Displayed formula of but-2-ene

 

Alkenes

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.

2-methylphenol
2-hydroxy-1-methylbenzene

Displayed formula of 2-methylphenol

3-methylphenol
3-hydroxy-1-methylbenzene

Displayed formula of 3-methylphenol

4-methylphenol
4-hydroxy-1-methylbenzene

Displayed formula of 4-methylphenol