Classification of Organic Compounds

Organic compounds are first classified by the shape of their carbon skeleton. Open-chain (acyclic or aliphatic) compounds have carbon atoms joined in a continuous chain, straight or branched, with no ring anywhere in the molecule -- hexane, $\text{CH}_3(\text{CH}_2)_4\text{CH}_3$, is a straight-chain example. Cyclic compounds contain at least one ring of atoms, and are further divided into two kinds. Homocyclic (carbocyclic) compounds have a ring made entirely of carbon atoms; these split again into alicyclic compounds, whose ring behaves chemically much like an open-chain compound (cyclohexane and cyclopropane, for instance, readily undergo addition reactions just as alkanes and alkenes do), and aromatic compounds, based on the benzene ring, whose delocalised electron system gives it an unusual extra stability and a strong preference for substitution reactions that preserve the ring rather than addition reactions that would destroy it. Heterocyclic compounds contain a ring in which one or more atoms other than carbon -- a 'heteroatom' such as oxygen, nitrogen or sulphur -- take the place of a ring carbon; furan (one ring oxygen), pyridine (one ring nitrogen) and thiophene (one ring sulphur) are common examples. …