IUPAC Nomenclature of Ketones
Introduction
Ketones are organic compounds that contain a carbonyl functional group (C=O) bonded to two carbon atoms. The general formula for ketones is R(C=O)R', where R and R' represent alkyl or aryl groups. In the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature, ketones follow specific naming rules that help identify their structure accurately.
Basic IUPAC Naming Convention
The IUPAC name of a ketone is derived by identifying the longest carbon chain containing the carbonyl group and replacing the -e ending of the corresponding alkane with -one. The position of the carbonyl group is indicated by specifying the number of the carbon atom in the chain to which it is attached.
Example: H3C-CO-CH3 is called propan-2-one or acetone (common name). The carbonyl group is on carbon number 2 of a three-carbon chain.
Detailed IUPAC Naming Rules
1. Select the Longest Carbon Chain
Identify the longest continuous carbon chain that contains the carbonyl group. This chain will form the parent name of the compound.
Example: In the compound CH3-CH2-CH2-CO-CH(CH3)-CH2CH3, the longest chain containing the carbonyl group has six carbons, so the parent name is hexane.
2. Number the Carbon Chain
Number the carbon atoms in the chain so that the carbonyl carbon gets the lowest possible number. This is typically done by numbering from the end nearest the carbonyl group.
Example: In Hexan-2-one: CH3-CO-CH2-CH2-CH3, numbering from left to right gives the carbonyl group position 2 (the lowest possible), so it's named 2-hexanone rather than 5-hexanone.
3. Name the Parent Chain
Change the -e ending of the parent alkane name to -one to indicate the presence of a carbonyl group.
Example: Pentane becomes pentanone, heptane becomes heptanone, etc.
4. Indicate the Carbonyl Position
Specify the position of the carbonyl group by giving the number of the carbon atom to which it is attached. This number precedes the parent name.
Example: A five-carbon chain with a carbonyl group at position 3 would be named pentan-3-one.
5. Name Substituents
If there are other substituents on the carbon chain, name and number them according to standard IUPAC guidelines. The substituent names are prefixed to the main name in alphabetical order.
Example: CH3-CH2-C(CH3)(CO)-CH2-CH3 would be named 3-methylhexan-3-one, indicating a methyl group at carbon 3.
Multiple Carbonyl Groups
When a compound contains multiple carbonyl groups, the suffixes -dione, -trione, etc., are used to indicate two, three, etc., carbonyl groups, respectively. The positions of all carbonyl groups must be specified.
Example: A compound with carbonyl groups at positions 2 and 4 of a six-carbon chain would be named hexane-2,4-dione.
For symmetrical diketones where the carbonyl groups are at the same distance from both ends of the chain, a simplified naming convention may be used.
Example: Pentane-2,4-dione is also known as acetylacetone, a common name for this symmetrical molecule.
Common Names of Ketones
Many ketones have common names that are widely used, especially for simple compounds:
| IUPAC Name | Common Name | Structure |
|---|---|---|
| Propan-2-one | Acetone | H3C-CO-CH3 |
| Butan-2-one | Methyl ethyl ketone | H3C-CO-CH2CH3 |
| 2-Pentanone | Methyl propyl ketone | H3C-CO-CH2CH2CH3 |
| 3-Pentanone | Diethyl ketone | H3CH2C-CO-CH2CH3 |
| Diphenylmethanone | Benzophenone | C6H5-CO-C6H5 |
| 1-Phenylethan-1-one | Acetophenone | C6H5-CO-CH3 |
Special Cases and Exceptions
Ring Ketones
When the carbonyl group is part of a ring structure, the suffix -cycloalkanone is used. The carbonyl carbon is automatically designated as carbon 1.
Example: A five-membered ring with a carbonyl group is named cyclopentanone.
For ring ketones, substituents on the ring are numbered starting from the carbonyl carbon (position 1) and continuing around the ring in the direction that gives the substituents the lowest possible numbers.
Example: A cyclohexanone with a methyl group on the carbon adjacent to the carbonyl would be named 2-methylcyclohexanone.
Complex Substituted Ketones
For ketones with multiple functional groups, priority rules determine the main functional group. The carbonyl group may be named as a substituent (keto or oxo) when a higher-priority functional group is present.
Functional group priority in IUPAC nomenclature follows this general order (from highest to lowest priority):
- Carboxylic acids
- Anhydrides
- Esters
- Acyl halides
- Amides
- Nitriles
- Aldehydes
- Ketones
- Alcohols
- Amines
- Ethers
- Alkenes
- Alkynes
- Haloalkanes
Example: A compound containing both an aldehyde and a ketone group would prioritize the aldehyde in naming. HOOC-CH2-CO-CH3 would be named 4-oxobutanoic acid, with the ketone treated as an "oxo" substituent since the carboxylic acid has higher priority.
Summary of Key Points
- Ketones contain a carbonyl group bonded to two carbon atoms
- The IUPAC name ends with -one, replacing the -e of the parent alkane
- The position of the carbonyl group must be specified in the name
- Multiple carbonyl groups use -dione, -trione, etc.
- Common names exist for many simple ketones
- Ring ketones use the -cycloalkanone naming convention
- When other functional groups are present, priority rules determine the suffix
- Substituents are named and numbered alphabetically
- The carbonyl carbon receives the lowest possible number in the chain
Practice Examples
Test your understanding of ketone nomenclature with these examples:
H3C-CH2-CO-CH2-CH2CH3
Answer: Hexan-3-oneH3C-CO-C6H5
Answer: 1-Phenylethan-1-one or acetophenoneHO-CH2-CH2-CO-CH3
Answer: 4-Hydroxybutan-2-oneCH3-C(CH3)(CO)-CH2-CH2CH3
Answer: 2-Methylhexan-3-oneCO-(CH2)4-CO
Answer: Hexane-1,6-dioneC6H5-CO-CH2-CH3
Answer: 1-Phenylpropan-1-oneCH3-CH2-CO-CH(CH3)-CH3
Answer: 4-Methylpentan-2-one
Hantzsch Widman (IUPAC Or Systematic) Method and Reference File Download Link
