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.