Mancude rings and annulenes

What do the structures (a), (b) and (c) have in common?

(a)	(b)	(c)

[18]annulene cyclooctadeca-1,3,5,7,9,11,13,15,17-nonaene (PIN) 1,3,5,2,4,6-triazatriphosphinine thiophene

Well, it is obvious that they all are rings. Also, apart from hydrogens in (a) and (c), they have no side chains. Otherwise, they are quite different. The structure (a) is a hydrocarbon. The ring (b) is purely inorganic while (c) is an organic heterocycle. What else?

You can see that in all these structures single bonds alternate with double bonds. Ring systems like this are referred to as mancude, which is an abbreviation of the “maximum number of non-cumulative double bonds”.

Alicyclic monocycles

Now let us have a look at monocyclic hydrocarbons, starting with cycloalkanes. By the way, I think this term is a bit misleading: cycloalkanes indeed contain cycles but are not alkanes because these latter, by definition, are acyclic. Gold Book defines cycloalkanes as “saturated monocyclic hydrocarbons (with or without side chains)”, where “side chains” are alkyl groups. The general molecular formula of cycloalkanes, with or without side chains, is C_nH_2n. I wish there was an elegant collective term for cycloalkanes-with-no-side-chains, or unsubstituted cycloalkanes, because only this subset of cycloalkanes can be used as parent hydrides in systematic organic nomenclature; I am not aware of any. Here, I will refer to unsubstituted cycloalkanes as ‘cycloalkane parents’^*.