12.3:
IUPAC Nomenclature of Ketones
The IUPAC names of ketones are generated by substituting the final –e of the parent hydrocarbon with –one.
In acyclic ketones, the carbonyl carbon is given the lowest possible number, and it precedes the suffix –one, thereby giving the name pentan-3-one.
Alternatively, the ketone can also be named with the locant preceding the parent name.
In the presence of substituents, the substituent names are prefixed to the parent name along with their locant values.
Cyclic ketones are numbered starting from the carbonyl carbon, and the numbering continues in the direction that gives the lowest value to the next substituent on the ring.
When the ketone functionality is part of the parent chain comprising other higher priority groups like acids and aldehydes, the parent name is prefixed by the word oxo- along with the locant value.
When the ketone functionality is not part of the parent chain, it is named as a substituent, namely, an alkanoyl or an acyl group.
12.3:
IUPAC Nomenclature of Ketones
Like aldehydes, ketones are named using IUPAC rules; in this case, by replacing “e” in the name of the longest hydrocarbon chain with “one.” In acyclic ketones, the ketonic carbon is given the lowest locant value. For instance, as shown below, a simple five-carbon ketone is named pentan-2-one, instead of pentan-4-one. IUPAC rules also allow the placing of the locant value before the parent name to give an alternate name, 2-pentanone.
Cyclic ketones are numbered starting from the carbonyl carbon. The numbering continues in the direction that gives a lower value to the next substituent present on the ring. For example, as shown below, a five-carbon cyclic ketone with a methyl substituent is 3-methylcyclopentanone and not 4-methylcyclopentanone.
The ketone group is treated as a substituent if it is part of a parent chain containing other higher priority functional groups such as acids, esters, and aldehydes. In such cases, the ketone group is indicated by the word “oxo,” which prefixes the parent name along with the locant values—for example, 3-oxobutanoic acid and 3-oxopentanal, which are depicted in the following figures.
However, if the ketone substituent is not part of the parent chain, it is represented as an acyl or alkanoyl group. In 4-acetylbenzaldehyde, which is presented below, the ketone group is not part of the main parent ring. Hence, it is named as a substituent, acetyl, and prefixed to the parent name, benzaldehyde.
