Naming and drawing aldehydes and ketones
Where the C=O goes for -al and -one names, why aldehydes never need a locant, ring aldehydes and ketones, and the oxo prefix.
Aldehydes and ketones both contain a carbonyl group, C=O. The difference is what is attached to the carbonyl carbon: in analdehyde it carries at least one hydrogen (–CHO), so it sits at the end of a chain; in a ketone it is bonded to two carbons, so it sits inside a chain or ring.
Aldehydes: -al
Replace the final -e of the alkane with -al. The CHO carbon is part of the chain and is always C1, so no locant is needed for it: propanal, butanal. Substituents are numbered from the CHO carbon.
Worked example: 3-methylbutanal
Butanal has four carbons with the C=O on C1. Count along the chain from the CHO carbon and put a methyl on C3. Total: C5H10O.
When the CHO group is attached to a ring, the CHO carbon is not part of the ring, and the suffix -carbaldehyde is used: cyclohexanecarbaldehyde. Benzaldehyde is the retained name for the benzene compound.
Ketones: -one
Replace -e with -one and give the carbonyl carbon the lowest possible locant: pentan-2-one has C=O on C2, pentan-3-one on the middle carbon. The carbonyl carbon is part of the chain or ring, so propan-2-one (acetone) has three carbons in total.
In a cyclic ketone the carbonyl carbon is C1, so cyclohexanone needs no locant.
Worked example: 4-methylpentan-2-one
Draw five carbons with C=O on C2 and a methyl on C4. Number from the end nearer the carbonyl: from the other end the C=O would be on C4, which is higher. The carbonyl group outranks the methyl group in deciding the numbering.
Several carbonyl groups and the oxo prefix
Two ketone groups give a -dione, with the final "e" of the alkane kept: pentane-2,4-dione. When a higher-ranking group such as a carboxylic acid provides the suffix, a ketone or aldehyde oxygen is named as the prefix oxo-, as in 3-oxobutanoic acid.
Common mistakes
- Aldehyde or ketone? Propanal and propan-2-one have the same formula, C3H6O. "-al" means the C=O is at the end of the chain; "-one" means it is between two carbons.
- Not counting the carbonyl carbon. It belongs to the parent chain: butanal has four carbons including the CHO carbon.
- Drawing an alcohol. -ol (C–OH) and -one (C=O) differ by two hydrogens; the checker reports the hydrogen count and the group.
- Forgetting -carbaldehyde. In cyclohexanecarbaldehyde the CHO carbon is outside the ring, giving seven carbons in total.