Functional groups: alcohol, aldehyde, ketone, carboxylic acid

What is a characteristic group?

A characteristic group (or functional group) is a cluster of atoms that gives a molecule specific chemical properties, regardless of the length of the carbon chain to which it is attached.

The four groups you need to know

Group	Formula	Example	Name of example
Alcohol	-OH	CH3-CH2-OH	Ethanol
Aldehyde	-CHO	CH₃-CHO	Ethanal
Ketone	-CO-	CH₃-CO-CH₃	Propanone (acetone)
Carboxylic acid	-COOH	CH₃-COOH	Ethanoic acid

Identifying a group in a formula

• An alcohol has an oxygen atom bonded to a hydrogen atom (-OH) on a carbon atom which otherwise has only single bonds.
• An aldehyde must have the -CHO group at the end of the chain.
• A ketone has a carbonyl group C=O located within the chain, never at the end.
• A carboxylic acid has the group -COOH, which combines a carbonyl group and an -OH group on the same carbon atom.

Why it matters

The characteristic group determines much of the chemical behaviour: carboxylic acids are acidic (they release an H+ ion in solution), whilst alcohols can be oxidised to aldehydes and then to carboxylic acids.

Common mistake

Don’t confuse aldehydes and ketones: both contain a carbonyl group C=O, but their position on the chain makes all the difference. An aldehyde is always at one end of the chain (the carbon in the C=O group also has an H attached to it), whilst a ketone is always in the middle (the carbon in the C=O group is bonded to two other carbons).