Revision of the extended Hantzsch-Widman system of nomenclature for heteromonocycles
(Recommendations 1982)

Rules RB-1.2 to RB-1.5

Contents

RB-1.2 Monocycle with up to ten ring members, one of which is a heteroatom
RB-1.3 Two or more atoms of the same heteroatom
RB-1.4 Two or more different heteroatoms
RB-1.5 Heteromonocycle with less than the maximum number of noncumulative double bonds
References for this section

RB-1.2. Monocycle with up to ten ring members, one of which is a heteroatom.

(Replaces Rules B-1.1, in part, and B-1.51).

A monocycle with no more than ten ring members, one of which is a heteroatom given in Table I, is named by combining the appropriate prefix from Table I with a stem from Table II, except as provided by RB-1.1, with elision of the final "a" of the prefix, when followed by a vowel (see Note).

Note In organic replacement nomenclature the final "a" of the replacement prefix is not elided (ref. 15c).

The presence of at least one hydrogen atom on a ring atom joined to adjacent ring atoms by only single bonds in an unsaturated monocycle containing the maximum number of noncumulative double bonds is denoted by the technique used for indicated hydrogen (see examples 2 and 4 below) (ref. 16).

The numbering of the heterocycle begins with the heteroatom. Thereafter indicated hydrogen takes precedence for low locants.

Examples:

1. Oxirane

2. 1H-Azirene [1H-Azirine (traditional)]

3. Azete

4. 2H-Phosphole

5. Silolane

6. Phosphinine (formerly Phosphorin)

7. Borinane

8. Thiepine

9. Azocane (formerly Octahydroazocine)

RB-1.3. Two or more atoms of the same heteroatom

(Replaces Rules B-1.3 and B-1.52).

Two or more heteroatoms of the same element in a heteromonocycle are indicated by adding an appropriate numerical prefix, such as "di", "tri", etc., to the prefix for the heteroatom (Table 1). A final "a" of a numerical prefix is elided when followed by a vowel.

The positions of the heteroatoms are given by numerical locants cited in front of the name. Numbering begins at a heteroatom and proceeds in the direction that gives the lowest set of locants to the heteroatoms (see Note a).

Note (a). The lowest set of locants is that series which contains the lowest number at the first point of difference when two or more series containing the same number of terms are compared term by term (ref. 15e).

Note (b). Designation of hydrogen atoms bv the technique of indicated hydrogen (see RB-1.2) at ring positions between two bivalent ring atoms may be omitted.

Examples:

1. 1,3-Dithiolane (the locant set 1,3 is lower than 1,4)

2. 1,3,5,7-Tetroxocane

3. 2H,4H-1,3-Dioxine

(may be written as 4H-1,3-Dioxine; indication of the hydrogen atom at position 2 may be omitted as explained in the Note (b) to this rule)

4. 1,2,4-Triazine (the locant set 1,2,4 is lower than 1,2,5; 1,3,4; or 1,4,5)

5. 4H-1,3-Diphosphepine (heteroatoms are preferred to indicated hydrogen for lowest locants)

RB-1.4. Two or more different heteroatoms

(Replaces Rules B-1.4 and B-1.53).

A heteromonocycle with two or more different heteroatoms is named by adding the appropriate prefix for each heteroatom (Table I) together with a numerical prefix as prescribed by RB-1.3 if needed, in the order of appearance of the heteroatoms in Table I (see Note) to an appropre stem (RB-1.1 or RB-1.2). The final "a" of a numerical prefix is elided when followed by a vowel.

Noteiat. This order of citation follows the element sequence now accepted in the rules of both inorganic (ref. 18) and organic (ref. 19) nomenclature.

The positions of the heteroatoms are indicated by numerical locants, all of which are cited before the first heteroatom prefix and in the same order as the heteroatom prefixes to which they refer.

Numbering of the heteromonocycle begins with a heteroatom whose prefix occurs earliest in Table I. If there is any choice the numbering should give the lowest set of locants (see Note (a) to Rule RB-1.3) first to all heteroatoms, then to heteroatoms in the order of their prefixes.

Examples:

1. 1,3-Oxathiolane (the locant set 1,3 is lower than 1,4)

2. 6H-1,2,5-Thiadiazine (preferred)

(the numbering begins with the sulfur atom, and then the locant set 1,2,5 is lower than 1,3,6)

3. 1,2,4,3-Triazasilolidine

Note that numbering beginning with either nitrogen atom adjacent to the single carbon atom results in the same lowest locant set, 1,2,3,4, for the four heteroatoms taken as a complete set without regard to priority. The direction for numbering thus depends on the comparison of these two lccant sets in the order that the heteroatoms to which they refer are cited, namely, 1,3,4,2 and 1,2,4,3; the latter set has the lower second locant and is therefore preferred.

RB-1.5 Heteromonocycle with less than the maximum number of noncumulative double bonds

(Replaces Rule B-1.2) (see Note).

Note. Thus, the special terminations for partially saturated heteromonocycles as given by Rule B-1.2 (ref. 1f) are no longer recommended.

A heteromonocycle containing fewer than the maximum number of noncumulative double bonds may be named by: (a) adding the prefixes "dihydro", "tetrahydro", etc., to a parent name that indicates the presence of the maximum number of noncumulative double bonds; or (b) adding the prefixes "didehydro", "tetradehydro", etc., to a parent name that indicates a fully saturated structure, according to the requirements of Rules C-16.1 and C-41.2 (ref. 15f). Method (a) is usually preferred.

Heteroatoms and indicated hydrogen have preference over hydro and dehydro prefixes for determining the beginning and direction of numbering.

Note: The final "o" of a hydro or dehydro prefix is not elided before a following vowel.

Examples:

1. (a) 1,2-Dihydroazete (b) 2,3-Didehydroazetidine (formerly [Delta]2-Azetine or 2-Azetine)

2. (a) 3,6-Dihydro-2H-phosphepine (b) 1,2,4,5-Tetradehydrophosphepane

3. (a) 2,3,4,7,8,9-Hexahydrooxonine (b) 5,6-Didehydrooxonane

REFERENCES

1. International Union of Pure and Applied Chemistry, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, 1979 edition, Pergamon Press, Oxford, 1979: [a] p. 53-54; [b] Rule B-1.1, Tables I and II, footnotes, p. 53; [c] Rule B-1.1, Table II, footnote (d), p. 53; [d] Rule B-1.1, Table I, footnote, p. 53; [e] Rule B-1.1, Table I, footnote, and Table II, footnote (b), p. 53; [f] Rule B-1.2, exception, p. 54; [g] Rules 1.51, p. 55; [h] pp. 55-63; [i] p.68.

15. Reference 1, Section C: [a] Rule C-41.2, p. 115; [b] Rule C-32.1, p. 114; [c] Preamble, Elision of Vowels, p. 83; [d] Rule C-923.1, p. 289; [e] Rule C-13.11(e), footnote, p. 99; [f] pp. 108, 115-116.

16. Reference 1, Section A, Rule A-21.6, p. 25.

17. 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). Revised version now published as: Treatment of Variable Valency in Organic Nomenclature (Lambda Convention) (Recommendations 1983), Pure Appl. Chem., 56, 769-778 (1984).

18. International Union of Pure and Applied Chemistry, Nomenclature of Inorganic Chemistry (1970), 2nd ed., Butterworths, London, 1971, Table IV, p. 104.

19. Reference 1, Section D, Appendix, Table I, pp. 459-460.

Continued with References

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