In organic chemistry, aliphatic compounds are saturated or unsaturated chains of carbon. Aliphatic molecules consist, generally, of a backbone of carbon atoms and other atoms bound to this carbon chain—most frequently hydrogen, oxygen, nitrogen, sulfur, and various halides.
The term aliphatic is used to distinguish such molecules from those deemed aromatic because of an aromatic ring structure (e.g., the benzene ring)—in accordance with Hückel's "4n+2" rule for ring valence electrons—and to distinguish them from other "unsaturated" compounds which contain carbon-carbon chemical bonds of an order higher than 1 (i.e., those compounds containing alkene or alkyne functional groups). The alkane series of hydrocarbons are aliphatic compounds.