IUPAC Nomenclature of Compounds Containing a Functional Group

Many organic molecules carry, in addition to a plain carbon skeleton, one or more functional groups -- specific atoms or groups of atoms, such as $-\text{OH}$, $-\text{CHO}$, $-\text{COOH}$ or $-\text{NH}_2$, that are chiefly responsible for the molecule's characteristic chemical behaviour. When exactly one kind of functional group is present, it is written as a suffix added to the end of the parent chain name (dropping the final $-e$ of the parent name before a suffix that itself begins with a vowel, e.g. $\text{propane} \rightarrow \text{propan-2-ol}$), and the parent chain is numbered so as to give this group the lowest possible locant -- overriding, if necessary, the numbering that a double bond or a substituent would otherwise prefer.

When a molecule contains more than one kind of functional group at once, IUPAC rules do not allow every group to be expressed as a suffix simultaneously. Instead, chemists rank functional groups in a fixed order of seniority, and only the single most senior group present -- called the principal characteristic group -- is expressed as the suffix and given the lowest locant; every other functional group present must then be cited only as a prefix. A short, commonly used part of this seniority order, from most to least senior, is: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine. So, for a molecule containing both an aldehyde and a ketone group, the aldehyde -- being senior to the ketone -- is always chosen as the suffix ($-\text{al}$), while the ketone's carbonyl is cited only as an $\text{oxo}$- prefix. The table below lists the suffix used when a group is the principal characteristic group, together with the prefix used for the same group whenever it is not senior enough to be the suffix. …