Why are there so many organic compounds?

Because of the special properties of the carbon atom:

1. Carbon can join to other carbon atoms and to atoms of other elements (H, O, N, halogens, S …) by covalent bonds.
2. It can form single, double and triple bonds (C–C, C=C, C≡C; also C=O, C≡N).
3. Carbon atoms can link together in many different arrangements: straight chains, branched chains, and rings (homocyclic rings contain only carbon; heterocyclic rings contain carbon plus another atom such as N, O or S).
4. Isomerism: the same molecular formula can correspond to several different structures, multiplying the number of compounds.

Because there are so many compounds, chemists classify them into families (groups), each with its own functional group and naming rules; we study the important ones in this chapter.