The many names of crowns

What is the best way to name the structure (a)?

(a)

1,4,7-trioxonane (Hantzsch-Widman) 1,4,7-trioxacyclononane (replacement) cyclo[tri(oxyethylene)] (organic macrocycle) 9-crown-3 (Pedersen) 9<O3coronand-3> (Vögtle-Weber)

The general naming method is skeletal replacement applied to the corresponding carbocyclic parent hydride, in our case cyclononane, thus 1,4,7-trioxacyclononane. Or we can use extended Hantzsch-Widman (H-W) system and call it 1,4,7-trioxonane. For rings with up to ten members, the H-W names are preferred [1, p. 96].

What about the structures (b)—(d) then? Since all of these rings have more than ten members, we cannot use H-W system, so we have to give them replacement names: 1,4,7,10-tetraoxacyclododecane (b), 1,4,7,10,13-pentaoxacyclopentadecane (c), 1,4,7,10,13,16-hexaoxacyclooctadecane (d). Rather long, completely unambiguous, and very boring.

Hantzsch-Widman names

Are you tired of carbocycles? Let’s have some ring diversity, I say.

Structures that contain two or more different elements in a ring are called heterocyclic. Perhaps because “heteroatom” is really an organic chemistry concept, the word “heterocycle” is commonly (mis)understood as “organic heterocycle”, that is, a carbocycle where at least one carbon atom is replaced by an heteroatom. I blame organic chemists for that.

For a small number of five- and six-membered organic heterocycles the trival names are retained to be used as parent hydride names. Note that, although “trivial” in chemical parlance means “non-systematic”, there is a system to most of those names. For instance, we can see that imidazolidine (a) is a fully saturated version of 1H-imidazole (b):

(a)	(b)

imidazolidine 1H-imidazole