Topic 15 · page 5 of 28

Naming aldehydes and ketones

Aldehydes end in -al with the C=O on the end carbon. Ketones end in -one with the C=O inside the chain. You need to draw each as structural, displayed and skeletal formulae.

14 min read15.6

The carbonyl group 15.6

A carbonyl compound contains a carbon–oxygen double bond, C=O. Where that group sits decides the family.

Family Where the C=O is Group Suffix Example
aldehyde at the end of the carbon chain, bonded to an H –CHO -al propanal, CH₃CH₂CHO
ketone inside the chain, bonded to two carbon atoms –CO– -one propanone, CH₃COCH₃

Write an aldehyde group as CHO, never COH. The COH form looks like an alcohol, and a mark scheme will not give the mark.

Naming rules 15.6

  1. Find the longest carbon chain that includes the C=O carbon. This gives the stem (prop-, but-, pent-).
  2. For an aldehyde, the C=O carbon is always carbon 1, so no number is needed: propanal, butanal.
  3. For a ketone, number the chain from the end that gives the C=O the lowest number, and put that number before "one": pentan-2-one, pentan-3-one. Propanone and butanone need no number, because the C=O can only be in one place.
  4. Name branches as prefixes with their numbers: 3-methylbutanal.
Skeletal formulae with the carbon atoms numbered. Top row, aldehydes: propanal, butanal and 3-methylbutanal, each with the C=O carbon numbered 1. Bottom row, ketones: propanone, pentan-2-one and pentan-3-one, with the C=O shown in orange.
Aldehydes number from the C=O end. Ketones number from the end that gives the C=O the lowest number.
Name Structural formula Family
methanal HCHO aldehyde
ethanal CH₃CHO aldehyde
propanal CH₃CH₂CHO aldehyde
butanal CH₃CH₂CH₂CHO aldehyde
propanone CH₃COCH₃ ketone
butanone CH₃COCH₂CH₃ ketone
pentan-2-one CH₃COCH₂CH₂CH₃ ketone
pentan-3-one CH₃CH₂COCH₂CH₃ ketone

Butanal and butanone both have the molecular formula C₄H₈O. They are functional-group isomers: the same atoms arranged as different families.

Three ways to draw them 15.6

Butanal and butanone drawn three ways. Structural formulae CH3CH2CH2CHO and CH3COCH2CH3. Displayed formulae showing every atom and every bond. Skeletal formulae showing only the carbon chain and the C=O.
The same two molecules as structural, displayed and skeletal formulae.

Check yourself

  1. Name CH₃CH₂CH₂CH₂CHO and CH₃CH₂COCH₂CH₃.
  2. Give the structural formula of 3-methylbutanal and of butanone.
  3. Draw the skeletal formula of pentan-2-one and describe how to tell it from pentan-3-one.
  4. Explain why the name "propan-2-one" is not needed.
  5. State the molecular formula of propanal and propanone, and say what type of isomerism they show.

Answers cover these until you have tried the questions

  1. CH₃CH₂CH₂CH₂CHO is pentanal: an aldehyde with a five-carbon chain. CH₃CH₂COCH₂CH₃ is pentan-3-one: a ketone, five carbons, C=O on carbon 3.
  2. 3-methylbutanal: (CH₃)₂CHCH₂CHO, that is CH₃CH(CH₃)CH₂CHO. Butanone: CH₃COCH₂CH₃.
  3. Pentan-2-one is a five-carbon zig-zag with the C=O on the second carbon from one end, leaving a CH₃ on one side of the carbonyl carbon and a propyl group on the other. Pentan-3-one has the C=O in the middle carbon, with an ethyl group on each side, so its skeleton is symmetrical.
  4. The C=O in a three-carbon chain can only be on the middle carbon, because on an end carbon it would be an aldehyde. There is only one possible ketone, so no number is needed.
  5. Both are C₃H₆O. They are functional-group isomers: propanal is an aldehyde and propanone is a ketone.