Key ideas
A functional group is a small arrangement of atoms that gives a molecule its characteristic reactions. Learning to recognize the groups tells you what family a compound belongs to and how it will behave.
- Hydrocarbons: alkanes have only single bonds, alkenes contain C=C, alkynes contain C≡C, and arenes contain a benzene ring.
- Oxygen groups: alcohol R–OH, ether R–O–R′, aldehyde R–CHO, ketone R–CO–R′, carboxylic acid R–COOH and ester R–COO–R′.
- Nitrogen groups: amine R–NH₂, amide R–CONH₂ and nitrile R–C≡N.
- Halogen groups: haloalkanes R–X, with X = F, Cl, Br or I.
Key equations
C=C alkeneC≡C alkyne–OH alcoholC=O carbonyl–COOH acid–NH₂ amine
Worked example
Classify CH₃COOCH₂CH₃. It has a C=O joined to an oxygen that carries an alkyl group, so it is an ester, ethyl ethanoate. CH₃CH₂CHO has a C=O at the end of the chain with a hydrogen on it, so it is an aldehyde (propanal), while CH₃COCH₃ has the C=O between two carbons, so it is a ketone (propanone).
Common mistakes
- Calling an ester a carboxylic acid because both contain C=O and an oxygen. An acid has –OH on the carbonyl, an ester has –OR.
- Confusing aldehyde and ketone. In an aldehyde the carbonyl carbon has at least one hydrogen and sits at the end of the chain.
- Treating an ether as an alcohol. An ether has no O–H bond.
Practice
Test yourself with practice questions on this topic: Functional groups