Functional derivatives of hydrocarbons are those compounds which are derived from hydrocarbons by replacing one or more hydrogen atoms with the functional groups.
A functional group is an atom or a group of atoms which is responsible for characteristic properties of a compound. The double and triple bonds which respectively give the alkenes and alkynes their characteristic properties are functional groups.
The other examples of functional groups are halogens (-F, -Cl, -Br, -I), -OH (hydroxyl group) and >C=O (carbonyl group).
Since each of these functional groups exhibits the characteristic properties and reactions, all the compounds having the same functional group show the same chemical reactions and constitute one class of compounds. For example, haloalkanes such as chloromethane, chloroethane, chloropropane which have the halo (chloro) functional group show the characteristic reactions of the halo (chloro) group and hence, constitute the class of compounds known as haloalkanes.
Similarly, the alcohols, methanol, ethanol, propanol, which have -OH functional group show characteristic properties and reactions due to the -OH group and they constitute another class called alcohols which is different from that of haloalkanes.