References for this section
A general method for naming organic parent compounds containing skeletal atoms that occur in two or more valence states has been developed. This method is consistent with the fundamental principles of substitutive nomenclature and is directly applicable to naming organic derivatives of the inorganic hydrides. Its main principles were introduced briefly in Section D of the IUPAC Organic Nomenclature Rules, as revised for publication in combination with Sections A, B, C, E, F, and H (see Bonding number under Subsection D-0.3 and Rule D-1.62, Ref. 1), and the general convention was described in greater detail in 1982 as provisional recommendations (Ref. 2).
The basic principles of organic nomenclature depend on the classical concepts of constant valence and definite bond order. Therefore, in order to include compounds containing heteroatoms capable of exhibiting two or more valence states within the scope of organic nomenclature, it is necessary to define a standard valence for such heteroatoms. Defined standard valences, related to the Group number in the Periodic Table, for up to thirty heteroatoms are given in Rules B-1.1, D-1.61 (Ref. 1), and RB-1.1 (Ref. 3).
Nonstandard valence states of heteroatoms, i.e., valence states other than those defined in the rules, are often implied by: (a) semisystematic or trivial names of simple parent compounds; (b) class names; (c) names for groups; or (d) names of ring systems, where the nonstandard valence is unambiguous because of specific structure requirements of the system.
Although this method is quite useful for simple compounds, common groups, and ring systems with specific structural constraints, it is limited by the large number of names that must be remembered and the lack of systematization apparent in the creation of names for different compounds and groups.
A number of techniques have been used in organic nomenclature to indicate the presence of heteroatoms in nonstandard valence states. Most of these are useful in one way or another but each has limitations as a general method, or has been considered to be unsuitable for one reason or another.
1. An "indicated hydrogen" assigned to a normally bivalent heteroatom of a ring system clearly indicates a higher valence state. However, this technique is not entirely consistent with the "indicated hydrogen principle" as defined by Rule A-21.6 (Ref. 1).
2H-2-benzothiopyran2. Additive class terms, such as oxide, imide, and bromide, are very useful for indicating nonstandard valence of heteroatoms in parent compounds and can be used for nonstandard valence of heteroatoms in substituents provided unambiguous locants are available, when needed.
|5.||4-(4-methyl-2-pyridyl)-1-naphthoic acid N-sulfide|
However, for some atoms and groups there are no commonly accepted class terms.
3. The additive prefix "hydro" indicates nonstandard valences of heteroatoms in heterocycles by describing the addition of hydrogen atoms, which may be later substituted just as those already implied by the name of the heterocycle.
This method has disadvantages, particularly when hydro prefixes are treated as detachable, and the method can be ambiguous because the practice of omitting locants for hydro prefixes when all double bonds of a ring or ring system are fully saturated is common. For example, the name hexahydro-s-triazatriphosphorine could refer to either of the following structures.
5. A Roman numeral cited as a superscript to an italicized element symbol, and usually associated with a locant, has been used quite extensively for indicating nonstandard valence of heteroatoms in ring systems, but rarely in acyclic structures.
6. A combination of indicated hydrogen (see item 1, above) with the superscript Roman numeral symbolism (see item 5, above) has been used to describe nonstandard valence of heteroatoms in ring systems, especially where unusual structural features, such as cumulative double bonds, are found.
Two of the techniques above (items 2 and 3) are codified briefly, as Rules D-1.54 and D-1.53, respectively, in the IUPAC Organic Nomenclature Rules (Ref. 1) and a third (item 4) can be considered to be an extension of Rule C-41.2 (Ref. 1). However, these techniques were not designed to cover the general case of nonstandard valence states in organic nomenclature and therefore the method described by the following rules should be preferred.
The need for a general method for indicating nonstandard valence of heteroatoms has been recognized and suggestions have been offered to deal with it. For example, the use of different endings (suffixes) added to the name of a parent hydride, indicating the number of nonbonding electron pairs in the valence shell of heteroatoms, has been proposed (Ref. 4). Although apparently sound in principle, this technique did not appear to be consistent with fundamental principles of organic nomenclature, and therefore was not assimilated into general usage.
Perhaps the most general technique used for designating nonstandard valence of heteroatoms described above (item 5) was not included as such in Section D of the Organic Rules (Ref. 1) mainly because of its use of the Roman numeral. This symbol is used extensively in inorganic nomenclature to denote oxidation number (Ref. 5a). However, the fundamental principle of the technique was sound, namely, the specification of the bonding number of a heteroatom, i.e., the number of valence bonds connecting a formally neutral nonstandard heteroatom to other skeletal atoms and to the hydrogen atoms associated with it in a parent hydride. Therefore, since it was the Roman numeral as a symbol that was unacceptable, it was necessary only to find a suitable replacement for this symbol; a new principle was not needed.
The use of arabic numerals as superscripts to italicized element symbols was suggested, but not well received because italicized element symbols with numerical superscripts are used quite often as locants in nomenclature and because this superscript position on an element symbol is used to indicate an ionic charge (Ref. 5b). These objections were removed in Section D (Ref. 1) by using the Greek letter "λ" in place of the italicized element symbol (see Note) and a superscript arabic numeral is used in place of the Roman numeral. A locant associated with the resulting symbol specified the position of the nonstandard heteroatom in the structure. This system, which has become known as the "λ" convention, is completely compatible with the principles of organic nomenclature. Substituents are expressed in the usual way by appropriate prefixes or suffixes and the numbering practices established in Sections A, B, and C (Ref. 1) are followed. The "indicated hydrogen" principle, as defined by Rule A-21.6 (Ref. 1), is used, when necessary, to distinguish among isomeric ring systems containing the maximum number of noncumulative double bonds by specifying the position of one or more hydrogen atoms.
Structures containing formal cumulative double bonds at a skeletal atom in a nonstandard valence state have been difficult to describe by principles of organic nomenclature, especially in ring systems where the concept of maximum number of noncumulative double bonds is crucial to the interpretation of the name. As noted above, a combination of the superscript Roman numeral method and the unusual use of "indicated hydrogen" has been used for such structures. The "λ" convention alone also is inadequate in these cases. Therefore, the symbol δc, where c is the number of double bonds in the skeletal structure terminating at the heteroatom, added to the λn symbol, was introduced in the provisional Section D Rules (see Note) (Ref. 1, D-0.3, pp. 324-325, and D-1.62, pp. 334-335, especially example 3). However, structures of this type are part of the general subject of cumulative double bonds, now under study by the Commission, and will be included in a later report.
The formation of a radical or ionic center at a skeletal atom in a nonstandard valence state by the addition or removal of a hydrogen atom or ion is described by a suffix ("yl", "ium", "ylium", "ide") added to the name of the neutral parent hydride in the usual manner as prescribed in Subsection C-0.8 (Ref. 1). Derivation of such names for radical or ionic compounds will be described in a future report.
Note. In the first publications of the provisional Section D rules the symbol σm, where m is the number of a bonds terminating at the heteroatom was suggested either as an alternative to or in conjunction with the λn symbol (Rule 1.62) (Ref. 6). However, the number of hydrogen atoms attached to the heteroatom would have to be included in this symbolism for a parent hydride, which is not consistent with the basic principles of substitutive nomenclature. In organic nomenclature, implied (or expressed) valence requirements of a skeletal atom in a parent hydride are satisfied with hydrogen atoms after other structural requirements are met. Hence, this symbolism was not included in the revision of the Section D rules for the 1979 edition of the IUPAC Organic Rules (Ref. 1).
1. International Union of Pure and Applied Chemistry, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, Pergamon Press, Oxford, 1979.
2. International Union of Pure and Applied Chemistry, The Designation of Nonstandard Classical Valence Bonding in Organic Nomenclature (Provisional), Pure Appl. Chem. 54, 217-227 (1982).
3. International Union of Pure and Applied Chemistry, Revision of the Extended Hantzsch-Widman System of Nomenclature for Heteromonocycles (Recommendations 1982), Pure Appl. Chem. 55, 409-416 (1983).
4. A. F. Clifford, J. Chem. Doc. 5, 91-95 (1965); 10, 180-185 (1970).
5. International Union of Pure and Applied Chemistry, Nomenclature of Inorganic Chemistry, 2nd Edition, Butterworths, London, 1979, (a) Rule 0.1, p. 5; (b) Rule 1.31, p. 11.
6. International Union of Pure and Applied Chemistry, Nomenclature of Organic Chemistry, Section D (Tentative), IUPAC Information Bulletin Appendixes on Tentative Nomenclature, Symbols, Units, and Standards, No. 31, August, 1973.
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