imidazole model

We must take into account that many heteromonocycle compounds have been known for more than 100 years, and that they were isolated from natural products. Modelo de la Piridina In the absence of a systematic nomenclature, they were originally given common names related to their origin, with some property, ect. These names are so ingrained that, even if there are systematic alternatives, the IUPAC does not aspire to their use, but rather recommends the former.

The systematic name of the heterocycles is built from a prefix that indicates the type of heteroatom it contains, and a suffix that depends on the size of the cycle.

heterocycle nomenclature 01

[1] Aziridine

[2] Azirine

[3] Oxirane

heterocycle nomenclature 02

[4] Oxirene

[5] Phosphete

[6] Phosphethane

Let us now consider the cases of unsaturated heterocycles (with at least one double bond), but which do not have the maximum number of double bonds. They are named as if they were completely unsaturated, putting the dihydro, tetrahydro, hexahydro, etc. particles before the name. (always indicative of an even number of hydrogens, as corresponds to formal hydrogenations of one, two, three, etc. double bonds). Keep in mind that formal hydrogenations may have occurred on neighboring or remote carbon atoms in the heterocycle.


A pyridine heterocycle missing a double bond at the 2,3-position. (2,3-Dihydropyridine)

Furan heterocycle missing a double bond. (2,5-dihydrofuran).

I Note that dihydros can be used with non-adjacent carbons.

Diarsocine heterocycle missing two double bonds. (1,2,5,8-Tetrahydro-1,4-diarsocine)


Observe the following examples. In all of them there is the maximum number of non-cumulative double bonds, and there is also a ring atom linked to neighboring atoms only by single bonds. If that atom also has one (or two) hydrogen atoms, that hydrogen (or one of the two) receives the name of indicated hydrogen, and must be stated in the name.

indicated hydrogen 01

[1] A pyrrole heterocycle that has the largest possible double bonds, but lacks a double bond on the phosphorous atom. This fact must be indicated in the name through the hydrogen indicated in position 1. (1H-Pyrrole)

[2] Pyrrole heterocycle with the largest double bonds. The lack of double bond in position three should be reflected in the name (3H-Pyrrole)

[3] Pyrane heterocycle with hydrogen indicated in position 4. (4H-Pyrane)

[4] Dithiine heterocycle, with maximum double bonds, but with two saturated carbons that should be reflected in the name (2H,4H-1,3-dithiine)

What might be called the “zero rule” for heteromonocycle numbering calls for locant 1 to be assigned to the highest priority heteroatom according to Table 1. Numbering then proceeds such that the lowest possible locants are given to:
1.- All heteroatoms, together.
2.- All the heteroatoms, taking into account the priorities of table 1.
3.- The indicated hydrogen(s)
4.- The atom(s) of the heterocycle carrier(s) of the main function that is named as a suffix (it is only one, according to the substitutive nomenclature, for it can be repeated).
5.- Substituents not belonging to the Hantzsch-Widman system, as a whole (that is, not oxa, aza, etc., but methyl, dihydro, hydroxy, etc.)
6.- Substituents not belonging to the Hantzsch-Widman system, in alphabetical order.
heterocycle numbering 01

When a heterocycle acts as a substituent, it is named following the rules that we already know, and then the final vowel of the name is deleted, and it is replaced by “n-il”, where n is the locator of the atom of the heterocycle that is attached to the rest of the molecule. If there is a choice, the lowest possible value is given.

Heterocycle numbering follows the rules cited above, no indicated hydrogens or hydro prefixes are used. Unsaturations are indicated by the particle eno or ino.
Regarding the formation of the name, the main novelty is that the locators of the Hantzsch-Widman prefixes precede each one of them, although they continue to appear in the order of priority in Table 1.