Nomenclature of Aldehydes and Ketones

Aldehydes and ketones are both carbonyl compounds -- organic molecules built around a $\text{C=O}$ group -- but they differ in exactly what else is attached to that carbon. In an aldehyde, the carbonyl carbon carries at least one hydrogen, written generally as $\text{R-CHO}$ (or $\text{H-CHO}$ for the very first member, formaldehyde). In a ketone, the carbonyl carbon is flanked on both sides by carbon-containing groups, written $\text{R-CO-R}'$, with no hydrogen on the carbonyl carbon itself.

IUPAC nomenclature of aldehydes. Select the longest carbon chain that includes the $\text{-CHO}$ carbon as the parent chain, replace the '-e' ending of the corresponding alkane with the suffix '-al', and number the chain so that the carbonyl carbon is C-1 -- since a terminal $\text{-CHO}$ group can only ever be at the end of a chain, it automatically gets the lowest possible locant and its position number ('1') is conventionally dropped from the name. For example, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$ is named by taking the four-carbon parent (butane), dropping the terminal '-e' and adding '-al', giving butanal. If a substituent is present elsewhere on the chain, it is cited with a locant counted from the carbonyl carbon, e.g. $\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_2\text{CHO}$ is 3-methylbutanal.

IUPAC nomenclature of ketones. Select the longest chain that includes both carbons attached to the carbonyl carbon, replace the alkane's '-e' with '-one', and number the chain from whichever end gives the carbonyl carbon the lower locant -- unlike an aldehyde, a ketone's carbonyl group can sit anywhere along an internal chain, so its position must always be stated explicitly. For example, $\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$ is pentan-2-one (numbering from the end nearer the carbonyl gives locant 2, versus 4 from the other end, so 2 is chosen). When the carbon skeleton is symmetric about the carbonyl group, such as $\text{CH}_3\text{COCH}_3$, the compound is named propan-2-one (common name acetone).

Common/trivial names remain widely used alongside the IUPAC system: formaldehyde ($\text{HCHO}$), acetaldehyde ($\text{CH}_3\text{CHO}$), acetone ($\text{CH}_3\text{COCH}_3$), and benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) for the aromatic aldehyde where the $\text{-CHO}$ is attached directly to the benzene ring.

Both families are collectively called carbonyl compounds, and everything that follows in this chapter -- their physical properties, their central reaction (nucleophilic addition), and their characteristic tests -- follows directly from the electronic structure of this one shared functional group.