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
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.
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.
Copyright © 2000-2024 Nigel Saunders