Functional group isomerism

Functional group isomerism occurs when substances have the same molecular formula but different functional groups:

  • functional group isomers belong to different homologous series.

You won’t meet these in GCSE studies, but you do at A Level.

There are three pairs of these isomers you need to be aware of:

  • alcohols and ethers
  • aldehydes and ketones
  • carboxylic acids and esters

You should be prepared to identify and name functional group isomers for simple organic compounds like the ones on this page.

What is here?

You can see models of:

  • ethanol and methoxymethane – C2H6O isomers
  • propanal and propanone – C3H6O isomers
  • propanoic acid and methyl ethanoate – C3H6O2 isomers

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 and ethers

ethanol

CH3CH2OH

methoxymethane

CH3OCH3

The functional group in alcohols is the hydroxyl group, —OH.

The functional group in ethers is R1–O–R2. The symbols R1 and R2 represent the structures of the rest of the molecule:

  • they can be the same or different
  • if one is a hydrogen atom, the compound contains a hydroxyl group and is not an ether.
In methoxymethane, R1 and R2 are methyl groups, —CH3.

Aldehydes and ketones

propanal

CH3CH2CHO

propanone

CH3COCH3

The functional group in aldehydes and ketones is the carbonyl group, C=O. This can be in different positions:

  • attached to one or two hydrogen atoms in aldehydes
  • attached to two carbon atoms in ketones.

Carboxylic acids and esters

propanoic acid

CH3CH2COOH

methyl ethanoate

CH3COOCH3

These have different functional groups:

  • carboxylic acids have the carboxyl group —COOH
  • esters have the functional group R1COOR2

The symbols R1 and R2 represent the structures of the rest of the molecule:

  • they can be the same or different
  • if one is a hydrogen atom, the compound contains a carboxyl group and is not an ester.
In methyl ethanoate, R1 and R2 are methyl groups, —CH3.