Aldehydes and ketones
Naming aldehydes and ketones: Practice exercises
How to name aldehydes with -al and ketones with -one, where the C=O goes, diones, ring carbonyls and the oxo prefix, with exercises and worked answers.
Aldehydes and ketones both contain a C=O (carbonyl) group. In an aldehyde it sits at the end of a chain, as –CHO; in a ketone it is between two carbons.
The rules you need
- Aldehydes: replace the final e of the alkane with -al. The CHO carbon is C1 of the chain, so it needs no locant: propanal, 3-methylbutanal.
- Ketones: replace the final e with -one and give the locant of the C=O carbon, as low as possible: pentan-2-one. In propan-2-one and butan-2-one the locant can only be 2, so many books leave it out (propanone, butanone).
- Two C=O groups give -dione, and the e of the alkane stays: pentane-2,4-dione.
- Rings: in a cyclic ketone the C=O carbon is C1 (cyclohexanone). A CHO group on a ring is named with -carbaldehyde, and its carbon is not counted in the ring: cyclohexanecarbaldehyde.
- An aldehyde outranks a ketone. When both are present, the aldehyde gives the suffix and the ketone's C=O becomes the prefix oxo-: 3-oxobutanal. Acids and their derivatives outrank both (see the priority table).
Two names, read step by step
3,3-Dimethylbutanal
The CHO carbon is C1 of a four-carbon chain: butanal. Two methyl groups sit on C3, each with its own locant. The aldehyde itself needs no locant.
Hexan-3-one
Six carbons with the C=O on the third carbon from the nearer end. Numbering from the other end would give it the locant 4, so the name is hexan-3-one.
Watch out for
- An aldehyde is always at the end of a chain. A C=O on C2 is a ketone: butan-2-one, never "butan-2-al".
- Count the C=O carbon in the chain: propanal has three carbons, including the CHO.
- -al and -ol differ by one letter: propanal is an aldehyde, propan-1-ol an alcohol.
Draw from the name: 19 exercises
Each name opens its exercise. Draw the structure and check it: any correct drawing is accepted, and hints and a worked answer help when you are stuck.
- propanal Beginner
- propan-2-one Beginner
- butanal Beginner
- butan-2-one Beginner
- ethanal Beginner
- 3-methylbutanal Intermediate
- cyclohexanone Intermediate
- pentan-3-one Intermediate
- 2-methylpropanal Intermediate
- 3,3-dimethylbutan-2-one Intermediate
- 2-methylcyclohexan-1-one Intermediate
- 4-methylpentan-2-one Advanced
- pentane-2,4-dione Advanced
- cyclohexanecarbaldehyde Advanced
- but-3-en-2-one Advanced
- 4-hydroxybutan-2-one Advanced
- 3-oxobutanal Advanced
- cyclohex-2-en-1-one Advanced
- 1-phenylethan-1-one Advanced
Name the structure
19 structures of this family to name, each with four names to choose from and an explanation of every wrong choice.
Printable worksheet
For practice on paper: names to draw and structures to name, with an answer key to check your work.
The full guide
The guide Aldehydes and ketones explains these names in more detail, with more worked examples and the most common mistakes.