Even though the halides are reactive, the hydrocarbon portions of the molecules still retain their identity. When the Cl of ethyl chloride (CHCHCl) is replaced by an OH, the molecule becomes ethyl alcohol (CHCHOH), which has properties similar to other alcohols. With COOH the molecule becomes an organic acid, and with NH, a base. Because groups such as Cl, OH, COOH, and NH determine the gross chemical properties of the molecule, they are known as functional groups.

The hydrocarbon chains attached to the functional groups modify the properties of the functional groups while maintaining their essential chemical character. Methyl alcohol, CHOH, is water-soluble, whereas dodecyl alcohol,
CHCHCHCHCHCHCHCHCHCHCHCHOH,
is not; but they both show the chemical properties of an alcohol.