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 Hueckel's "2n+4" 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 (ie, those compounds containing alkene or alkyne functional groups. The alkane series of hydrocarbons are aliphatic compounds.\n