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Functional group isomerism

What is it?

Functional isomerism occurs when substances have the same molecular formula but different functional groups. This means that functional isomers belong to different homologous series. You won’t meet these in GCSE Chemistry, but you do come across them at A Level.

There are three functional group isomers of which you need to be aware:

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

What is here?

You can see models of:

  • ethanol and methoxymethane - functional group isomers of C2H6O
  • propanal and propanone - functional group isomers of C3H6O
  • propanoic acid and methyl ethanoate - functional group isomers of C3H6O2

These are typical of the sorts of molecules you meet at A Level. You should be prepared to identify functional group isomers for simple organic compounds like these, and you also need to be able to name them.

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ethanol

CH3CH2OH

Displayed formula of ethanol

methoxymethane

CH3OCH3

Displayed formula of methoxymethane

 

Alcohols and ethers

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.

R1 and R2 are methyl groups, —CH3, in methoxymethane.

propanal

CH3CH2CHO

Displayed formula of propanal

propanone

CH3COCH3

Displayed formula of propanone

 

Aldehydes and ketones

Aldehydes and ketones both have the carbonyl group C=O. This functional group can be in different positions:

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

propanoic acid

CH3CH2COOH

Displayed formula of propanoic acid

methyl ethanoate

CH3COOCH3

Displayed formula of methyl ethanoate

Carboxylic acids and esters

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.

R1 and R2 are methyl groups, —CH3, in methyl ethanoate.