Classification of organic compounds

Organic compounds are first sorted by the shape of their carbon skeleton:

• Acyclic (open-chain, also called aliphatic) compounds have straight or branched chains, with no ring.
• Cyclic (closed-chain) compounds have rings. They may be alicyclic (rings that behave like open-chain compounds, such as cyclohexane), aromatic (with a special stable ring, such as benzene; they can be benzenoid or non-benzenoid), or heterocyclic (the ring contains an atom other than carbon, such as oxygen in furan or nitrogen in pyridine).

Compounds are then sorted by their functional group: the atom or group of atoms that decides a compound's characteristic chemical reactions. The rest of the molecule changes these reactions only a little. Important groups: –OH (alcohols), –CHO (aldehydes), >C=O (ketones), –COOH (carboxylic acids), –NH₂ (amines), –NO₂ (nitro compounds), and the multiple bonds C=C and C≡C.

A homologous series is a family of compounds with the same functional group, in which each member differs from the next by a –CH₂– unit. The alcohols, for example: CH₃OH (methanol), C₂H₅OH (ethanol), C₃H₇OH (propanol)…

Members of a homologous series show a steady gradation in physical properties (boiling point rises with chain length) and have similar chemical properties, because they share the same functional group.

A molecule can carry several functional groups at once. Lactic acid, the acid in sour milk, CH₃CH(OH)COOH, has both an –OH group and a –COOH group.