Principal functional group. If the organic compound contains two or more functional groups, one of the functional groups is selected as the principal functional group while all the remaining functional groups (also called the secondary functional groups) are treated as substituents. The following order of preference is used while selecting the principal functional group.
Amine salts > carboxylic acids > sulphonic acids > anhydrides > esters > acid chlorides > acid amides > nitriles > isonitriles > aldehydes > ketones > alcohols > phenols > thiols > amines.
All the remaining functional groups such as halo (fluoro, chloro, bromo, iodo), nitroso (–NO), nitro (–NO2), alkoxy (–OR), alkyl (R), phenyl (–C6H5), etc, are always treated as substituent groups.