A Comparison of Nondetachable Hydro Prefixes (IUPAC), Added Hydrogen (CAS), and Indicated Hydrogen (Beilstein), in Expressing Substitutive Suffixes

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Contents

Introduction
Discussion
A. Nondetachable-hydro-prefix method.
B. The CAS "added hydrogen" method.
C. The Beilstein "extended indicated hydrogen method".
D. A comparison of the above three methods.
E. Comparing the methods
References

Introduction

The most important objective of the Commission on Nomenclature of Organic Chemistry of the International Union of Pure and Applied Chemistry is to codify and systematise sound practices for naming organic compounds while at the same time providing guidance in reducing the number of methods in use. The eventual goal is to produce a single set of rules for naming the vast majority of organic compounds. In many instances, this is a very difficult task. Before it is able to recommend extensions or improvements to existing practices, it is highly desirable, and often absolutely necessary, to document various procedures for study and evaluation. Only then can the Commission provide confident guidance to the chemical community.

One of the more interesting and complex problems that has concerned the Commission for a long time is that of describing the presence of principal characteristic groups and free valences at positions of parent hydrides where there is not a sufficient number of hydrogen atoms for the application of the usual substitutive nomenclature principles. This problem is also one of the least understood in organic nomenclature. Accordingly, the Commission has authorized this publication, a discussion and a comparison of three methods for accomplishing this fundamental operation. By so doing, the Commission hopes to provide a better understanding of each method, and at the same time, receive guidance for making a recommendation of a method to be used in the general literature, which may or may not be best for use in an index environment.

Rule C-315.1 in the 1979 edition of the IUPAC Organic Rules (ref. 1) provides a solution to this problem, but has been interpreted in two different ways, each of which has its effect on numbering priorities and names for certain spiro compounds and ring assembles. Chemical Abstracts Service has interpreted this rule in one way for its index nomenclature and the Beilstein Institute has interpreted it in another way.

Rule C-16.11 of the 1979 edition of the IUPAC Organic Rules (ref. 2) allows for hydro prefixes to be either nondetachable, i.e., they must always be cited directly in front of the name of a fully unsaturated parent structure, thus creating a parent hydride separate and distinct from the fully unsaturated analog, or detachable, i.e., cited as prefixes in front of the name of a fully unsaturated parent structure, but alphabetized among any substituent prefixes that may also be present CAS follows the latter method in its index nomenclature. Beilstein considers hydro prefixes as detachable as far as inversion of the name in an alphabetical index and numbering of the structure are concerned, but cites them after all substituent prefixes.

Nondetachability of hydro prefixes is a third method for solving this particular problem of substitutive nomenclature, but, in many cases, it seriously affects the numbering priorities for principal characteristic groups and free valences and even the formation of certain types of parent names, such as spiro parent hydrides and ring assembles having components with the maximum number of noncumulative double bonds.

Comments on this report should be sent to the author or to the Secretary of the IUPAC Commission on Nomenclature of Organic Chemistry, Prof. J. G. Traynham, Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803-1304, U.S.A.

Discussion

Structures in which principal characteristic groups or free valences are present at positions of heteromonocyclic or polycyclic parent hydrides having the maximum number of noncumulative double bonds where a sufficient number of hydrogen atoms for the application of the basic principles of substitutive nomenclature is not present are considered formally to have been derived from a hydrogenated derivative of the parent ring system. For example,

or or

are derived from 1,4-dihydronaphthalene

Three methods are commonly used to name such compounds, two of which are in use in index nomenclature systems. These methods are described briefly in Sections A, B, and C below, and are compared for a number of examples in Section D. The operations involved in each method are schematically compared in Section E.

A. Nondetachable-hydro-prefix method.

According to Rule C-16.11 (ref. 2), hydro prefixes may be detachable, that is, alphabetized along with substituent prefixes, if any; or nondetachable, that is, always placed directly in front of the name of the parent ring system and, as such, define a parent hydride that is different from the fully unsaturated parent hydride.

Example:

1,1-Dimethyl-1,4-dihydronaphthalene (hydro prefixes are treated here as nondetachable; the parent hydride is 1,4-dihydronaphthalene)

Note. If the hydro prefixes were treated as detachable, the parent hydride would be naphthalene and the name would be 1,4-dihydro-1,1-dimethylnaphthalene.

Note. In this sense, nondetachable hydro prefixes are different from operational suffixes, such as "-ene" and "-yne", each of which define a specific parent hydride, but is not given preference for low locants over principal characteristic groups.

	1,4-Dioxo-1,2,3,4-tetrahydronaphthalene-6-carboxylic acid (hydro prefixes are treated here as nondetachable; the parent hydride is 1,2,3,4-tetrahydronaphthalene)

Note. If the hydro prefixes were treated as detachable, the parent hydride would be naphthalene, and the name would be 5,6,7,8-tetrahydro-5,8-dioxonaphthalene-2-carboxylic acid.

2,2-Dimethyloctahydro-1H-inden-5-one (hydro prefixes are treated as nondetachable; the parent hydride is octahydro-1H-indene)

Note. If the hydro prefixes were treated as detachable, the parent hydride would be 5H-indene and the name would be octahydro-2,2-dimethyl-5H-inden-5-one.

Examples:

	4,4-Dimethyl-1,4-dihydronaphthalen-1-one
	4-Oxo-1,4-dihydronaphthalen-l-ylidene (the parent hydride is dihydronaphthalene)
	5-(Hydroxymethyl)-2,5-dimethyldecahydronaphthalene-2,8a-diol (the parent hydride is decahydronaphthalene)

B. The CAS "added hydrogen" method.

This method is actually simply a format variation of indicated hydrogen as it is used to define saturated ring positions in parent hydrides, as described by Rule A-21.6 of the IUPAC Organic Rules (ref. 3). Although the text of Rule C-315 (ref. 4) does not make such a differentiation, the format in various examples clearly shows the intent.

For the purposes under consideration in this report, the principles of the "added hydrogen" method are:

1. The presence of at least one hydrogen atom on a ring atom that is attached to adjacent ring atoms by single bonds only, and that results from the introduction of a free valence, a radical or ionic center, or a principal characteristic group into a fully unsaturated heteromonocyclic ring or a fused polycyclic ring system, is indicated by citing the capital italic letter H following the locant of the ring atom for each such position. This "added hydrogen" designation is enclosed in parentheses and inserted into the name immediately following the locant(s) for the free valences, radical or ionic centers, or principal characteristic groups.

Note. The same procedure is applied to a parent fused polycyclic component of a ring assembly or of a spiro system which must be considered as derived from a hydro derivative. It could also apply to bridged fused ring systems, when appropriate, but, at the present time, it is not so used in CAS index nomenclature.

3. When there is a choice, added hydrogen positions are assigned to ring atoms, angular or nonangular, with the lowest locants consistent with the structure of the compound, respecting the requirement that hydro prefixes must be used to saturate double bonds.

"Added hydrogen" removes the need for the use of nondetachable hydro prefixes to accommodate principal characteristic groups, free valences, spiro junctions, or ring assembles which must be considered as derived from hydro derivatives of rings or ring systems with the maximum number of noncumulative double bonds. It is, therefore, a very useful technique for an index nomenclature because it avoids scattering of closely related structures among a variety of index headings, and, at the same time, allows the index heading compound to represent at least a formally correct structure, even if it does not itself exist as such.

C. The Beilstein "extended indicated hydrogen method".

This method extends the use of indicated hydrogen as defined by Rule A-21.6 of the IUPAC Organic Rules (ref 3) to denote saturated ring positions remaining after principal characteristic groups or free valences have been added to the parent hydride structure. This method is called the "restricted indicated hydrogen" method by Beilstein chemists because it "restricts" the use of indicated hydrogen to the minimum.

For the purposes under consideration in this report, the principles of the extended indicated hydrogen method are as follows:

1. The presence of at least one hydrogen atom on a ring atom that is attached to adjacent ring atoms by single bonds only and that results from the introduction of a free valence, a radical or ionic center, or a principal characteristic group into a fully unsaturated heteromonocyclic ring or fused polycyclic ring system is indicated by citing the capital italic letter H following the locant of the ring atom for each such position in front of the name of the parent structure. Indicated hydrogen is not cited when the saturated skeletal atom occurs between two "blocking" positions, that is, positions that, because of substitution or the nature of the skeletal atom, cannot accommodate a double bond, respecting the requirement that hydro prefixes must be used to saturate double bonds.

Note. The same procedure is applied to a bridged fused parent hydride and to a ring assembly or a spiro ring system with component rings or ring systems containing the maximum number of noncumulative double bonds.

This method also removes the need for the use of nondetachable hydro prefixes to accommodate principal characteristic groups, free valences, spiro junctions, or ring assemblies which must be considered as derived from hydro derivatives of rings or ring systems with the maximum number of cumulative double bonds. It is, therefore, a very useful technique for an index nomenclature because it avoids scattering of closely related structures among a variety of index headings.

D. A comparison of the above three methods.

1.		A. 1,2-Dihydronaphthalen-1-one B. 1(2H)-Naphthalenone C. 2H-Naphthalen-l-one
2.		A. 1,2-Dihydroquinoline-1-carboxylic acid B. Quinoline-1(2H)-carboxylic acid C. 2H-Quinoline-1-carboxylic acid
3.		A. 1,2-Dihydro-l-pyridyl B. 1(2H)-Pyridinyl C. 2H-1-Pyridyl
4.		A. 4a,8a-Dihydronaphthalen-4a-ylium B. Naphthalen-4a(8aH)-ylium C. 8aH-Naphthalen-4a-ylium
5.		A. 1,4,5,6-Tetrahydropyrimidine-4,6-dione B. Pyrimidine-4,6(1H,5H)-dione C. 1H-Pyrimidine-4,6-dione
6.		A. 2,3-Dihydro-1H-cyclopenta[a]naphthalene-1,2-dione B. 1H-Benz[e]indene-1,2(3H)-dione C. 3H-Cyclopenta[a]naphthalene-1,2-dione

Note. B. is a CAS index name. According to Rule C-14.11(d) of the IUPAC Organic Rules (ref 5), the name should be based on the ring system cyclopenta[a]naphthalene and, by applying method B, the "added hydrogen" method, the name would be 1H-cyclopenta[a]naphthalene-1,2(3H)-dione.

7.		A. 2,3-Dihydro-1H-isoindol-2-yl-1-ylidene B. 1H-Isoindol-2(3H)-yl-1-ylidene (not 2H-isoindol-2-yl-1(3H)-ylidene) C. 3H-Isoindol-2-yl-1-ylidene
8.		A. 4a,5,6,6a-Tetrahydro-2H,4H-benzo[1,2-b:4,3-c']dipyran-5,6-dione B. 4,4a-Dihydro-2H,5H-benzo[1,2-b:4,3-c']dipyran-5,6(6aH)-dione C. 4,4a-Dihydro-2H,6H-benzo[l,2-b:4,3-c']dipyran-5,6-dione
9.		A. 1,3-Dioxo-2,3-dihydro-1H-benz[de]isoquinolin-2-yl B. 1,3-Dioxo-1H-benz[de]isoquinolin-2(3H)-yl C. 1,3-Dioxo-1H,3H-benz[de]isoquinolin-2-yl
10.		A. 1,2,3,4-tetrahydroquinolin-2-ylidene B. 3,4-dihydro-2(1H)-quinolinyhdene C. 3,4-dihydro-1H-quinolin-2-ylidene
11.		A. 1,2,3,4,4a,7-Hexahydronaphthalen-7-one B. 5,6,7,8-Tetrahydro-2(4aH)naphthalenone C. 5,6,7,8-Tetrahydro-4aH-naphthalen-2-one
12.		A. 1,2,3,4,4a,5-Hexahydronaphthalene-4a-carboxylic acid B. 1,3,4,5-Tetrahydro-4a(2H)naphthalenecarboxylic acid C. 1,3,4,5-Tetrahydro-2H-naphthalene-4a-carboxylic acid

Note. A 4a(1H)-isomer is not consistent with the structure of the compound.

13.		A. Tetrahydro-2H-pyran-3-one B. Dihydro-2H-pyran-3(4H)-one C. Dihydro-4H-pyran-3-one
14.		A. 2,3,3a,4,5,9b-Hexahydro-1H-cyclopenta[a]napththalene-3,5-dione B. 1,3a,4,9b-Tetrahydro-3H-benz[e]inden-3,5(2H)-dione C. 1,3a,4,9b-Tetrahydro-2H-cyclopenta[a]naphthalene-3,5-dione

Note. B. is a CAS index name. According to Rule C-14.11(d) of the IUPAC Organic Rules (ref. 5), the name should be based on the ring system cyclopenta[a]naphthalene and, applying method B, the "added hydrogen" method, the name would be 1,3a,4,9b-tetrahydro-3H-cyclopenta[a]naphthalene-3,5(2H)-dione

15.		A. 2,3,3a,4-Tetrahydro-1H-indene-1,4-dione B. 3,3a-Dihydro-1H-indene-1,4(2H)-dione C. 3,3a-Dihydro-2H-indene-1,4-dione
16.		A. 6,8,9,10-Tetrahydro-2H,4H-benzo[1,2-b:4,3-c']dipyran-2,6-dione B. 9,10-Dihydro-2H,4H-benzo[1,2-b:4,3-c']dipyran-2,6(8H)-dione C. 9,10-Dihydro-4H,8H-benzo[1,2-b:4,3-c']dipyran-2,6-dione
17.		A. 1,2,3,4-Tetrahydropyrazine-2,3-dione B. 1,4-Dihydropyrazine-2,3-dione C. 1,4-Dihydropyrazine-2,3-dione
18.		A. 9,10-Dihydroanthracene-9,10-dione B. Anthracene-9,10-dione C. Anthracene-9,10-dione

19.		A. 2,3,4,5,5a,9a-Hexahydro-1H-1,4-benzodiazepine-2,5-dione B. 3,4,5a,9a-Tetrahydro-1H-1,4-benzodiazepine-2,5-dione C 3,4,5a,9a-Tetrahydro-1H-1,4-benzodiazepine-2,5-dione
20.		A. Decahydronaphthalene-4a,8a-diol B. Octahydronaphthalene-4a,8a-diol C. Octahydronaphthalene-4a,8a-diol
21.		A. 2,3-Dihydro-1H-phenalene-1,3-dione B. 1H-Phenalene-1,3(2H)-dione C. Phenalene-1,3-dione
22.		A. 5,8-Dihydro-1H-cyclopenta[b]naphthalene-1,5,8-trione B. 1H-Benz[f]indene-1,5,8-trione C. Cydopenta[b]naphthalene-1,5,8-trione

Note. B is a CAS index name. According to Rule C-14.1 1(d) of the IUPAC Organic Rules (ref. 5), the name should be based on the ring system cyclopenta[a]naphthalene, and the same name as C would result by application of method B, the "added hydrogen" method.

23.		A. 1,2,3,4,9,10-Hexahydroanthracene-1,9,10-trione B. 3,4-Dihydroanthracene-1,9,10(2H)-trione C. 3,4-Dihydro-2H-anthracene-1,9,10-trione
24.		A. 6,8-Dihydro-1H,5H-benzo[ij]quinolizine-6,8-dione B. 1H,5H-Benzo[ij]quinolizine-6,8-dione C. 1H-Benzo[ij]quinolizine-6,8-dione
25.		A. 1,2,3,4,4a,8a-Hexahydronaphthalene-4a,8a-diyl B. 1,2,3,4-Tedahydronapthalene-4a,8a-diyl C. 1,2,3,4-Tetrahydronapthalene-4a,8a-diyl
26.		A. 1,2,3,4-Tetrahydropyrazine-1,4-diyl B. 2,3-Dihydropyrazine-1,4-diyl C. 2,3-Dihydropyrazine-1,4-diyl

E. Comparing the methods

The order of the operations involved in the above methods is directly associated with decreasing priorities for numbering structures.

Method A	Method B	Method C
Nondetachableable hydro prefixes	Added hydrogen	Extended indicated hydrogen
Indicated hydrogen	Indicated hydrogen	Principal characteristic groups
Hydro prefixes	Principal characteristic groups (added hydrogen)	Indicated hydrogen
Principal characteristic groups	Hydro and substituent prefix groups	Hydro prefixes
Substituent prefix groups	-----	Substituent prefix groups

In the above comparison, Method B is the only one that is completely consistent with Rule C-15.11 of the IUPAC Organic Rules (ref 6).

Even though the use of nondetachable hydro prefixes is included in the IUPAC Organic Rules (Rule C-16.11, ref 2), their use as described by Method A would require that they be considered for numbering preference before all substituent groups, including principal characteristic groups, which is in opposition to Rule C-15.11 (ref 6).

Method C is not consistent with Rule C-15.11 in the IUPAC Organic Rules (ref. 6) and would be consistent with the priority of principal characteristic groups over subtractive suffixes, such as "-ene", "-yne", etc., for numbering only if indicated hydrogen and hydro prefixes as described in Method C were considered equivalent to these subtractive suffixes.

The comparison above is illustrated schematically for the given compound in the following diagram.

Compound:

Step	Method
	A. Nondetachable Hydro Prefixes IUPAC	B. Added Hydrogen CAS	C. Extended Indicated Hydrogen Beilstein
1.	Draw skeletal graph, including bridges, spiro and ring assembly attachments
2.	Add heteroatom nodes and number, if possible 4,11a-Epithiopyrrolo[3',4':3,4]pyrido[2,1-b][1,3]benzothiazole
3.	Add the maximum number of noncumulative double bonds consistent with bridges and allowing, where possible, for the presence of spiro atoms and principal characacteristic groups		Add principal characteristic groups 4,11a-Epithiopyrrolo[3'4':3,4]pyrido- [2,1-b]benzothiazole-1,3,5-trione
4.	Add "extra" hydrogen atoms and cite as indicated hydrogen 1H-4,11a-Epithiopyrroio[3',4':3,4]pyrido[2,1-b]benzothiazole		Add maximum number of noncumulative double bonds
5.	Add hydrogen atoms consistent with the structure of the compound and cite as prefixes 2,3,3a,4,5,11b-Hexahydro-1H-4,11a-epithio- pyrrolo[3',4':3,4]pyrido[2,1-b]benzothiazole	Add principal characteristic groups and "added hydrogen" consistent with the structure of the compound 1H-4,11a-Epithiopyrrolo[3',4':3,4]pyrido- [2,1-b]benzothiazole-1,3,5(2H,4H)-trione	Add "extra" hydrogen atoms and cite as "indicated hydrogen" 4H-4,11a-Epithiopyrrolo[3',4':3,4]pyrido- [2,1-b]benzothiazole-1,3,5-trione
6.	Add principal characteristic groups and other substituent groups and cite as suffixes and alphabetized prefixes, respectively 2,4-Diphenyl-2,3,3a,4,5,11b-hexahydro- 1H-4,11a-epithiopyrrolo[3',4':3,4]pyrido- [2,1-b]benzothiazole-1,3,5-trione	Add hydrogen atoms and other substituent groups and cite as alphabetized prefixes 3a,11b-Dihydro-2,4-diphenyl-1H-4,11a- epithiopyrrolo[3',4,:3,4]pyrido[2,1-b]- benzothiazole-1,3,5(2H,4H)-trione	Add hydrogen atoms and cite as prefixes 3a,11b-Dihydro-4H-4,11a-epithio- pyrrolo[3',4':3,4]pyrido[2,1-b]benzo- thiazole-1,3,5-trione
7.			Add other substituents and cite as alphabetized prefixes 2,4-Diphenyl-3a,11b-dihydro-4H- 4,11a- epithiopyrrolo[3',4':3,4]pyrido- [2,1-b]benzothiazole-1,3,5-trione

References

(1) International Union of Pure and Applied Chemistry. Organic Chemistry Division. Commission on Nomenclature of Organic Chemistry, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F and H, 1979 ed., Pergamon Press, Oxford, 559 pp., Rule C-315.1, p. 172.

(2) ibid., Rule C-16.11, p. 108.

(3) ibid., Rule A-21.6, p. 25.

(4) ibid., Rule C-315, pp. 172-173.

(5) ibid., Rule C-14.11(d), p. 102.

(6) ibid., Rule C-15.11, p. 105.