24.2 Naming Aldehydes and Ketones

Learning Objectives

By the end of this section, you will be able to:

  • Use the IUPAC system to name and draw aldehydes and ketones
  • Use common names to name low molecular weight aldehydes and ketones

Naming aldehydes and ketones using IUPAC rules

When following the preferred International Union of Pure and Applied Chemistry (IUPAC) rules for naming either an aldehyde or a ketone, several steps must be followed. The following are the IUPAC rules for naming aldehydes and ketones:

  1. The stem names of aldehydes and ketones are derived from those of the parent alkanes, defined by the longest continuous chain (LCC) of carbon atoms that contains the functional group.
  2. For an aldehyde, drop the –e from the alkane name and add the ending –al. Methanal is the IUPAC name for formaldehyde, and ethanal is the name for acetaldehyde.
  3. For a ketone, drop the –e from the alkane name and add the ending –one. Propanone is the IUPAC name for acetone, and butanone is the name for ethyl methyl ketone.
  4. To indicate the position of a substituent on an aldehyde, the carbonyl carbon atom is always considered to be C1; it is unnecessary to designate this group by number.
  5. To indicate the position of a substituent on a ketone, number the chain in the manner that gives the carbonyl carbon atom the lowest possible number. In cyclic ketones, it is understood that the carbonyl carbon atom is C1.
There are 6 aldehyde structures. Top left to right: methanal, ethanal and pentanal. Bottom left to right: 2-chloropentanal, butanal and 3-methylbutanal.
Figure 24.2a. IUPAC names for some aldehydes. Common names are given in brackets below (credit: Supplemental Modules (Organic Chemistry), CC BY-NC-SA 4.0).

 

6 ketone structures. Top left to right: propanone, acetophenone and benzophenone. Bottom left to right: 2-penatanone, 3-methyl-2-butanone and 3-hexanone.
Figure 24.2b. IUPAC names for some ketones. Common names are given in brackets below (credit: Supplemental Modules (Organic Chemistry), CC BY-NC-SA 4.0).

Exercise 24.2a

Give the IUPAC name for each compound.

There are 6 structures of aldehydes and ketones a) to f). For a) a 5 carbon chain with a methyl group at the 2nd carbon and a carbonyl group at carbon 1; b) a 5 carbon chain with a methyl group at the 2nd and 4th carbon with a carbonyl group at the 3rd carbon; c) a carbonyl group within a cyclohexane; d) a 4 carbon chain with a dimethyl group at the 3rd carbon and a carbonyl group at the 1st carbon; e) a 6 carbon chain with a methyl group at the 2nd carbon and a carbonyl group at the 3rd carbon; and f) a carbonyl group within a cyclobutane.
(credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0).

Naming aldehydes and ketones using common names

Both common and International Union of Pure and Applied Chemistry (IUPAC) names are frequently used for aldehydes and ketones, with common names predominating for the lower molecular weight molecules. The common names of aldehydes are taken from the names of the acids into which the aldehydes can be converted by oxidation (Figure 24.2c.).

 

Formaldehyde and acetaldehyde forms formic acid and acetic acid respectively upon oxidation.
Figure 24.2c. Historical common names of aldehydes stem from the common names of the carboxylic acids that they can be converted into via a process called oxidation (covered in chapter 24.3). (Credits: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

The stems for the common names of the first four aldehydes are as follows:

  • 1 carbon atom: form–
  • 2 carbon atoms: acet–
  • 3 carbon atoms: propion–
  • 4 carbon atoms: butyr–

Because the carbonyl group in a ketone must be attached to two carbon groups, the simplest ketone has three carbon atoms. It is widely known as acetone, a unique name unrelated to other common names for ketones (Figure 24.2d.).

 

Acetone structure
Figure 24.2d. The simplest ketone, commonly known as acetone, contains three carbons, with the carbonyl group located on the central carbon. (Credits: Intro to Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

Generally, the common names of ketones consist of the names of the groups attached to the carbonyl group, followed by the word ketone. (Note the similarity to the naming of ethers.) Another name for acetone, then, is dimethyl ketone. The ketone with four carbon atoms is ethyl methyl ketone (Figure 24.2e.).

 

structure of ethyl methyl ketone
Figure 24.2e. Historical common names for ketones are derived by naming the two side chains attached to the carbonyl group, and adding the word “ketone” at the end. (Credits: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

Exercise 24.2b

Classify each compound as an aldehyde or a ketone and give the common name for each.

There are 6 aldehyde and ketone structures. a) a 4 carbon chain with a carbonyl group at carbon 1; b) a 7 carbon chain with a carbonyl group at the 4th carbon; c) a 4 carbon chain with a methyl group at the 3rd carbon and a carbonyl group at the 2nd carbon; d) a 6 carbon chain with a carbonyl group at the 3rd carbon; e) a 4 carbon chain with a methyl group at the 3rd carbon and a carbonyl group at the 1st carbon; and f) a carbonyl group in-between a cyclobutyl and methyl group.
(credit: Intro Chem: GOB (v. 1.0), CC BY-NC-SA 3.0)

Check Your Answer[2]

Drawing structures for aldehydes and ketones

When it comes to drawing molecular structures for aldehydes and ketones, the best practice is to follow similar rules to what we’ve seen in previous chapters: work your way from right-to-left in the name. Start by drawing a skeleton structure for the parent compound, number your compound, and then add side groups as a final step.

Exercise 24.2c

Draw the structure for each compound.

  1. 7-chlorooctanal
  2. 4-methyl–3-hexanone

Check Your Answer:[3]

Exercise and Image in Solutions Source: Introduction to Chemistry: GOB (v. 1.0), CC BY-NC-SA 3.0.

Carbonyl plus Other Functional Groups in Same Molecule

As with many molecules with two or more functional groups, one is given priority while the other is named as a substituent.  When an aldehyde or ketone is present in a molecule which also contains an alcohol functional group, the carbonyl is given nomenclature priority by the IUPAC system.  This means that the carbonyl is given the lowest possible location number and the appropriate nomenclature suffix is included.  In the case of the alcohols, the OH is named as a hydroxyl substituent (Figure 24.2f.).

4 chemical structures of aldehydes and ketones. Top left to right: 4-hydroxylbutanal and 4-hydroxyl-2-methylbutanal. Bottom left to right: 4-hydroxy-2-butanone and 1,5-dihydroxy-3-pentanone.
Figure 24.2f. Molecules with two or more functional groups (credit: Supplemental Modules (Organic Chemistry), CC BY-NC-SA 4.0).

When and aldehyde or ketone is present in a molecule which also contains an alkene functional group the carbonyl is given nomenclature priority by the IUPAC system. This means that the carbonyl is given the lowest possible location number, and the appropriate nomenclature suffix is included. When carbonyls are included with an alkene the following order is followed:

(Location number of the alkene)-(Prefix name for the longest carbon chain minus the -ane ending)-(an -en ending to indicate the presence of an alkene)-(the location number of the carbonyl if a ketone is present)-(either an –one or and -anal ending).

Remember that the carbonyl has priority so it should get the lowest possible location number. Also, remember that cis/tran or E/Z nomenclature for the alkene needs to be included if necessary.

4 structures of aldehydes and ketones with alkene functional groups. Top left to right: 4-penten-2-one and trans-2-pentenal. Bottom left to right: 3-methyl-2-pentanal and 5-bromo-4-hexen-3-one.
Figure 24.2g. Examples of ketones and aldehydes with alkene functional groups (credit: Supplemental Modules (Organic Chemistry), CC BY-NC-SA 4.0).

For dialdehydes, the location numbers for both carbonyls are omitted because the aldehyde functional groups are expected to occupy the ends of the parent chain (Figure 24.2h.). The ending –dial is added to the end of the parent chain name. For diketones, both carbonyls require a location number (Figure 24.2i.). The ending -dione or -dial is added to the end of the parent chain.

Two structures of dials: pentanedial (left) and butanedial (right).
Figure 24.2h: Examples of dials.
Two structures of diones 2,4-pentanedione (on the left) and 2,3-hexanedione (on the right)
Figure 24.2i. Examples of diones (credit: Supplemental Modules (Organic Chemistry), CC BY-NC-SA 4.0).

Links to Enhanced Learning

For more practice with naming and drawing ketones and aldehydes, see Nomenclature of Aldehydes & Ketones – Chemistry LibreTexts.

Attribution & References

Except where otherwise noted, this page is adapted by Gregory A. Anderson and Samantha Sullivan Sauer from

    1. There are five carbon atoms in the LCC. The methyl group (CH3) is a substituent on the second carbon atom of the chain; the aldehyde carbon atom is always C1. The name is derived from pentane. Dropping the -e and adding the ending -al gives pentanal. The methyl group on the second carbon atom makes the name 2-methylpentanal.
    2. There are five carbon atoms in the LCC. The carbonyl carbon atom is C3, and there are methyl groups on C2 and C4. The IUPAC name is 2,4-dimethyl-3-pentanone.
    3. There are six carbon atoms in the ring. The IUPAC name is cyclohexanone. No number is needed to indicate the position of the carbonyl group because all six carbon atoms are equivalent.
    4. There are four carbon atoms in the LCC. There are two methyl groups on the third carbon atom of the chain; the aldehyde carbon atom is always C1. The IUPAC name is 3,3-dimethylbutanal.
    5. There are six carbon atoms on the LCC. The carbonyl carbon atom is C3, and there is a methyl group on C2. The IUPAC name is 2-methylhexanone.
    6. There are four carbon atoms in the ring. The IUPAC name is cyclobutanone.

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    1. This compound has the carbonyl group on an end carbon atom, so it is an aldehyde. The molecule is 4 carbons long, so its common name is butyraldehyde.
    2. This compound has the carbonyl group on an interior carbon atom, so it is a ketone. Both alkyl groups are propyl groups. The name is therefore dipropyl ketone.
    3. This compound has the carbonyl group between two alkyl groups, so it is a ketone. One alkyl group has three carbon atoms and is attached by the middle carbon atom; it is an isopropyl group. A group with one carbon atom is a methyl group. The name is therefore isopropyl methyl ketone.
    4. This compound has the carbonyl group on an interior carbon atom, so it is a ketone. One alkyl group has three carbon atoms and is attached by a terminal carbon atom; it is a propyl group. A group with two carbon atoms is an ethyl group. The name is therefore propyl ethyl ketone.
    5. This compound has the carbonyl group on an end carbon atom, so it is an aldehyde. The molecule is 4 carbons long, but the carbonyl group is attached to the secondary carbon in the chain; it is a sec-butyl group. The name is therefore sec-butylaldehyde.
    6. This compound has the carbonyl group on an interior carbon atom, so it is a ketone. One alkyl group is 4 carbons long in a ring structure; it is a cyclobutyl group. A group with one carbon atom is a methyl group. The common name is therefore cyclobutyl methyl ketone.

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    1. The octan- part of the name tells us that the LCC has eight carbon atoms. There is a chlorine (Cl) atom on the seventh carbon atom; numbering from the carbonyl group and counting the carbonyl carbon atom as C1, we place the Cl atom on the seventh carbon atom:an 8 carbon chain with a chloro group at the 7th carbon and a carbonyl group at the 1st carbon.
    2. The hexan- part of the name tells us that the LCC has six carbon atoms. The 3 means that the carbonyl carbon atom is C3 in this chain, and the 4 tells us that there is a methyl (CH3) group at C4:a 6 carbon chain with a methyl group at the 4th carbon and a carbonyl group at the 3rd carbon.

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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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