-ole¹

Also ‑azole and ‑ol.

Organic chemical compounds.

Latin oleum, oil.

This is a systematic ending in chemistry for unsaturated heterocyclic compounds containing a five-membered ring. Examples are indole (from indigo, from which it was first synthesized), a compound with an unpleasant odour, present in coal tar and in faeces, and pyrrole (Greek purrhos, reddish, because a red colour is produced when its vapour comes into contact with concentrated hydrochloric acid), a sweet-smelling liquid present in bone oil and coal tar.

When the ring contains nitrogen, the ending strictly becomes ‑azole (see azo‑). Examples include thiazole (Greek theion, sulphur), a foul-smelling liquid whose molecule contains a ring of one nitrogen, one sulphur, and three carbon atoms; and carbazole (from carbon), a compound obtained from coal tar and used in making dyes, which contains three rings, one with nitrogen in it. The ending ‑azole is sometimes found in the invented generic names of drugs, such as the antibiotics metronidazole and sulphamethoxazole.

The ending ‑ole also appears in the names of esters, such as safrole, anisole, and phenetole, some of which are used in perfumery.

Some names (though not those of the drugs) can appear spelled ‑ol instead, as with eucalyptol, an oil obtained from eucalyptus leaves, also spelled eucalyptole (and which has the alternative name cineole).