Are halogens substituents or functional groups?

[Chem_Mod]

Are halogens substituents or functional groups?

[Chem_Mod]

For naming, halogens are treated as substituents. However, with respect to chemical reactivity, a carbon atom connected to a halogen is activated and therefore makes that carbon atom more likely to be involved in a reaction. Therefore some books and instructors include halogens along with functional groups such as alcohols, aldehydes, etc., as halogenated carbon atoms have their own characteristic reaction type (or reactivity) like alcohols, aldehydes, etc.