The Hantzsch-Widman System

Heterocyclic compounds with up to ten ring members are generally named by the extended HW System. In order to do this, a hetero symbol (or several of these symbols) from the table of V terms (Table 4) is combined (eliding the final V ) with a stem term of the following Table 5 that indicates both ring size and degree of saturation. (Unsaturation here means maximum number of non-cumulative double bonds - mancude.) 

Ring size Unsaturated Saturated Ring size Unsaturated Saturated 

For rings containing (only) nitrogen the traditional ending irine can be used. 

The terminal e of the above terms is more or less optional (see for example Chem. Abstr.) and could conveniently be dispensed with. 

Naming and numbering of hetereo atoms occurs in accordance with the table of a terms in such a way that the most senior hetero atom is assigned locant 1 only then are all the other hetero atoms as a set given lowest locants possible (but see p. 61). 

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For such purposes as locating substituents, monocyclic heterocycles having only one non-carbon atom are numbered starting with the hetero atom as 1. If there is more than one hetero atom of the same kind, the more saturated hetero atom takes precedence (cf. imidazole in Table 1). In isoxazole and isothiazole, the divalent atom takes precedence, in conformity with the pnaciples of the Hantzsch-Widman system (Section 1.02.2.2). 

There are 19, largely nonmetallic, elements encompassed by the Hantzsch-Widman system the prefixes to indicate them are shown in Table 2. These prefixes, all of which end in a (which may be lost by elision), are the same as those used in replacement nomenclature (Section 1.02.2.3). 

For single rings (i.e. mononuclear systems) up to ten atoms in size, there is no question but that Hantzsch-Widman names are overwhelmingly the most widely used, and specialists can be presumed to recognize them readily. However, when one of the trivial names of Table 1 can be used, it prevails almost exclusively. Thiophene will be immediately understood by specialists and most nonspecialists, whereas thiole will distract the reader s attention from the chemistry while he considers the writer s intention is the Hantzsch-Widman system indeed being used, or is it a typographical error Chemical Abstracts also uses these trivial names in its indexes. 

According to the Hantzsch-Widman system, the seven-membered unsaturated hcterocyclc with one sulfur atom is named thiepin (1). The three different benzothiepins are assigned by the position of sulfur 1-benzothiepin (2), 2-benzothiepin (3) and 3-benzothiepin (4). Of the four possible dibenzothiepins only dibenzo[6,r/]thiepin (5) and dibenzo[A,/]thiepin (6) are of importance for synthesis, while the other two isomers, which contain unfavorable o-quinoid structures, exist mainly as the stable dihydro compounds, i.c. 5,7-dihydrodibenzo[c,t ]thiepin (7) and 6,1 l-dihydrodibenzo[6,c ]thiepin (8). Benzannulation over all double bonds results in tri-benzo[6,(7,/]thiepin (9). 

This method is useful when dealing with simple heterocycles, but it can become clumsy with more complex ones. An alternative is the Hantzsch-Widman system, which uses the same prefixes, but adds a stem name designed to indicate not only the ring size but also the state of unsaturation or saturation (note when the stem name begins with a vowel the last letter, a, of the prefix is dropped). The stem names for rings containing up to 10 atoms are shown in Table 1.1. 

The Hantzsch-Widman system provides names for both fully saturated and fully unsaturated molecules. Partially saturated molecules are named by attaching appropriate hydro prefixes to the names of the fully saturated rings however the special suffixes applicable to four-and five-membered rings may also be used (this is not now recommended... 

Some complications arise with certain heteroatoms when strict application of the Hantzsch-Widman system could lead to confusion in these cases prefixes or suffixes are suitably modified, e.g., 63-66. 

Numbering of the ring starts with the hetero atom and proceeds around the ring so as to give substituents (or other hetero atoms) the lowest numbered positions. When two or more different hetero atoms are present, oxygen takes precedence over sulfur and sulfur over nitrogen for the number one position. Examples follow to illustrate both the heterocycloalkane and the Hantzsch-Widman systems. Trivial names also are included. 

Type of heteroatom, according to the Hantzsch-Widman system (O, S, N, P). 

The Hantzsch Widman system is often used for the nomenclature of organic rings. This indicates the ring size by specific sufSxes the first suffix given is used for saturated rings, the second for unsaturated rings ... 

The names borazole, boroxole and borthiole, respectively, for the three compounds in Examples 4, 5 and 6 have been abandoned long ago as they imply five-membered rings in the Hantzsch-Widman system. The names borazin(e), boroxin and borthiin indicate six-membered rings with unsaturation and only one boron atom and one other heteroatom (although the order of the element name stems is wrong) and are also not acceptable. 

The parent isoxazolidine (1) can be named as 1,2-oxazolidine according to the extension of the Hantzsch-Widman system. However, since the corresponding heterocycle (2) with the greatest number of non-cumulative double bonds is conventionally called isoxazole, 1 is most frequently referred to as isoxazolidine, which is used in Chemical Abstracts (CA). The hydro-isoxazole or -isoxazoline nomenclature is used solely in connection with the condensed ring for instance, compound 3 may be called either octahydro-l,2-benzisoxazole (CA nomenclature) or 4,5-tetramethyleneisoxazolidine (as a derivative of the isoxazolidine), or 7-oxa-8-azabicyclol4.3.0]nonane (3 ) (Stelzner method). A bis-isoxazolidine is called tetrahydro-2//-isoxazolo[2,3-6]-isoxazole (4) (CA method), or conventionally isoxazolizidine (4 ). 

Sequence and numbering of the heteroatoms follow the rules given in 2.1. The two compounds chosen as examples could also be named according to the Hantzsch-Widman system silole, 1,4,2-thiazasilane. 

The Hantzsch - Widman system serves for the nomenclature of heterocyclic compounds. The affixes for the Hantzsch - Widman system are shown in Table 14.2. 

The Hantzsch-Widman system is well suited for the naming of saturated monocyclic compounds. Bridged ring systems, however, present a special challenge, and here the replacement system is preferred. An example is provided by the framework of the bicyclic hydrocarbon norbornane. Heteroatoms may in principle be substituted for any of the carbons. To illustrate, nitrogen substitution at position 7 would give rise to the name 7-azanorbomane. 

In the Hantzsch-Widman system, the suffix epane indicates what stmctural feature ... 

As lactones are cyclic esters, so lactams are cyclic amides. Again, as was the case with lactones, the nomenclature of the lactams is based on the use of Greek letters to show ring size, substitution of nitrogen for carbon in a cyclic array (thus using the prefix azacyclo ), or the Hantzsch-Widman system (vide supra). Some examples follow in Table 5.14. 

Advanced Chemistry Development Labs. Heterocyclic Systems. Rule B-1. Extension of the Hantzsch-Widman System, http //www.acdlabs.com/iupac/ nomenclature/79/r79 702.htm (accessed February 17, 2015). 

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