Replacement names

It is apparent that replacement names can be formed for rings of alt sizes, and the rules for forming them are simple. On the other hand, the names are usually quite long, and as a consequence, chemists more often than not avoid such names in favor of recognized trivial names or Hantzsch-Widman names for ring sizes for which they are available. Examples are given in (17)-(20). 

As described in Section 1.02.2.3, any heterocycle can be named by indicating (with appropriate prefixes) the positions of heteroatoms in the corresponding carbocycle. The carbocycle can be named systematically, as described in later Sections however, the replacement procedure can also be applied to naming heterocyclic analogues of trivially named carbocycles. A list of polycarbocycles with their trivial names is given in Table 5 (taken from Rule A-21.2 in the lUPAC Blue Book (B-79MI10200)) replacement names may be derived as shown in examples (27) and (28). As demonstrated by example (28), it is unnecessary to add hydro prefixes if the heterocycle cannot accommodate as many double bonds as the original carbocycle. 

When applying this principle to replacement names generated from fusion nomenclature, it is essential to keep in mind that fusion names for hydrocarbons ending in -cycloalkene are for fully unsaturated skeletons the -ene ending implies whatever number of double bonds may be necessary, without a multiplier. Thus (117) has six double bonds in the twelve-membered ring, and one must add ten hydrogens to saturate it to the stage of a simple benzene derivative, compound (118). 

Replacement names are much less popular, and are not commonly used for the smaller monocycles, with the exception of analogs of pyridine having a heavier Group V element in place of nitrogen. Phosphabenzene occurs more frequently in primary publications than phosphinine , for example, probably because of the ambi uous state of Hantzsch-Widman... 

The fusion name l//-triazolo[4,5-d]pyrimidine for (140) is preferred by practitioners in the field and appears in CA indexes. On the other hand, the nonspecialist, who may well be uncertain about his command of fusion nomenclature, might more easily grasp the replacement name l//-l,2,3,4,6-pentaazaindene. (This is an appropriate place to emphasize that, by lUPAC rules, replacement names are to be based only on a completely carbocyclic parent. Notwithstanding the attractive simplicity of 8-azapurine for the example at hand, it has no sanction see, however. Section 1.02.3.2. The purpose of this avoidance is to forestall a multiplicity of names for systems containing several hetero atoms.)... 

Replacement nomenclature is used to name heteromonocycles that contain more than ten atoms. In developing a replacement name, carbon atoms are regarded as exchanged for heteroatoms. The non-detachable prefixes (Table 4.8) are used to indicate the exchange. Cycloalkane or annulene names are the bases for transformation into the name of a heterocycle. 

A heterocycle can be derived formally from a carbocycle by replacement of one or more carbon atoms with heteroatoms. In a replacement name, this is indicated by use of prefixes terminating in a, cited in the... 

Stelzner (S) and Chemical Abstracts (CA) types of replacement name for the same molecules will make the difference clear. 

Thus in many cases Stelzner replacement names require hydro- prefixes where the heterocyclic skeleton is in fact maximally unsaturated. This is a notable disadvantage, which often causes confusion, and the method is not recommended. 

Heterocyclic components can be given replacement names e.g., 49 would then be named spiro-[3//-l-azaindene-3,r-cyclopenta[2,4]diene]. Both types of method a spiro nomenclature are readily extensible to molecules containing more than one spiro linkage, e.g., 51 and 52. 

For replacement names the parent carbocycles, apart from benzene, are all named as cycloalkanes (modified to -ene, -diene, etc., as necessary) with the heteroatom replacing /CH2, CH, or C— as appropriate to its valency. Replacement names derived from benzene are not used unless the three formal double bonds are retained otherwise names of the type oxacyclohexa-2,4-diene are appropriate [see 1-3 and 57-60 replacement names for 7-11 are thiacyclopenta-2,4-diene (7), 1,3-diazabenzene (8), l-oxa-2,5-diazacyclopenta-2,4-diene (9), l-oxa-4-azacyclohexane (10), and l,2-diazacyclopent-3-ene (11)]. 

The application of replacement nomenclature has already been discussed (Section II, B, 1, b) and will not be dealt with further here (although replacement names will be given where appropriate). Also it is unnecessary to discuss the use of the trivial names allowed by IUPAC they may be applied whenever it appears appropriate. Those cases in which ring assembly nomenclature should be used are generally obvious and require no comment. Problems do arise, however, in deciding whether to use fusion nomenclature or von Baeyer nomenclature. 

In replacement names, the position of indicated hydrogen is determined after performing the replacement operation(s) and inserting the maximum number of double bonds its location does not necessarily bear any relation to that of indicated hydrogen in the parent hydrocarbon, cf. 80 and 81.14b... 

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