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Chemistry International
Vol. 24, No. 6
November 2002


Aligning the Revised Red and Blue Books and Preparing for IUPAC Preferred Names

by Ture Damhus

Ture Damhus

At the 2001 IUPAC General Assembly in Brisbane, Australia, the former Commissions on Nomenclature of Organic and Inorganic Chemistry (CNOC and CNIC, respectively) began discussions on a project to avoid conflict between the preferred names for organic nomenclature and the revision of inorganic nomenclature. The project arose from the fact that CNOC was working on a revision of the "Blue Book" and its supplemental Guide–to become one revised Blue Book–and CNIC was working on a revision of the "Red Book." (See box below.)

Red Book
Nomenclature of Inorganic Chemistry (Recommendations 1990), Leigh, G. J. (Blackwell Science, 1990, ISBN 0-63202-4941)

Blue Book
Nomenclature of Organic Chemistry, Rigaudy, J. and Klesney, S. P. (Pergamon, 1979, ISBN 0-08022-3699)

Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), Panico, R.; Powell, W.H.; and Richer, J-C. (Blackwell Science, 1994, ISBN 0-63203-4882)

The revisions of these books should be completed in 2003.

The need for alignment of these two areas of nomenclature was clear given that the revised Blue Book had as a main feature the introduction of preferred names. That is, among the various names that can often be constructed for a given compound using the nomenclature principles endorsed by IUPAC, one name was to be designated the preferred one, ready to offer to users who would like to have always just one name. It turned out that a number of inorganic compounds were also represented with preferred names in the drafts of the revised Blue Book. This requires a bit more of an explanation.

The main principle of organic nomenclature as laid out in the Blue Book is that of naming compounds on the basis of parent compounds by applying standard operations, mainly substitution, whereby the parents are modified in order to arrive at the structure of the compound being named. This is substitutive nomenclature. (Examples include 'trichloroacetic acid', where the numerical prefix 'tri' together with the prefix 'chloro' indicates substitution of the three methyl group hydrogen atoms in acetic acid by chlorine, and 'butan-2-ol', obtained by modifying the parent name 'butane' by introducing the 'ol' suffix and the locant '2'.)

Many parent compounds are hydrides, such as butane and naphthalene, and a number of them are inorganic hydrides, i.e., they do not contain carbon, such as gallane (GaH3) and selane (SeH2). Other parent compounds are organic or inorganic oxoacids, such as acetic acid, CH3COOH, and phosphonic acid, PH(O)(OH)2. The parent names are important for the naming of organic derivatives and thus had a prominent plae in the drafts of the revised Blue Book. However, the former Commission on Nomenclature of Inorganic Chemistry felt that it was premature to designate the inorganic parent names themselves as preferred. The revised Red Book will not contain preferred names for the simple reason that the working group does not think the chemistry community is ready to select preferred names for inorganic compounds at large. The choice will often be difficult for inorganic compounds; compare examples given below.

The general problem of selecting parent structures and the desire to extend the naming principles of coordination chemistry to broader areas of inorganic chemistry had led inorganic chemists to favor additive nomenclature over substitutive nomenclature. Additive names specify compounds according to the way they are formally assembled from particular building blocks (central atoms and ligands).

A simple example may illustrate the general points made above. Take a neutral molecular compound like PCl5. Traditionally, it would be named 'phosphorus pentachloride'. This type of name will be called a compositional name (of binary type) in the revised Red Book. By systematic additive nomenclature, its name is 'pentachloridophosphorus'. (In the revised Red Book, ligands in additive names now carry the 'ido' ending rather than just the 'o' ending used before.) By substitutive nomenclature, starting from the parent hydride PH5, its name becomes 'pentachloro-l5-phosphane'. Now, which of the three names does one want to choose as the preferred one? The first name is what everybody uses. The next two belong to two different systematic approaches, one of which prevails in inorganic chemistry, the other in organic chemistry. Neither of the two systems can be extended to all of chemistry without creating sometimes extremely cumbersome and unfamiliar-looking names. Division lines have to be drawn through chemistry and most probably also through inorganic chemistry, and this will require more careful thinking than the Red Book group presently has time for. It is generally anticipated in IUPAC that a new project will be set up to address the issue of inorganic preferred names once the Red Book revision has been finished.

For organometallic compounds (compounds with a bond from a carbon atom to a metal atom), the question arises whether to use substitutive or additive nomenclature. Consider Pb(C2H5)4 . By substitutive nomenclature, the name is 'tetraethylplumbane', with reference to the parent hydride plumbane, PbH4, and the additive name is 'tetraethyllead' (or tetraethanidolead). Which one should be preferred? In the alignment project, there was support for a proposal made by W. H. Powell that prescribes where to draw the line between organometallic compounds that will have preferred substitutive names and those that will have preferred additive names. This proposal is being discussed among members of the Chemical Nomenclature and Structure Representation Division (Div. VIII) Advisory Committee and will probably be incorporated into the Blue Book and Red Book more or less in its present form.

In conclusion, the 'ane' hydride names are needed for a number of preferred organometallic names and, within inorganic chemistry, for constructing substitutive names that may or may not end up being preferred names in the future, depending on where the borderline is drawn between substitutive and additive nomenclature. Therefore, those parent hydride names will be given both in the Red Book and Blue Book. As an interesting aside, there was a problem in giving an 'ane' name to indium trihydride, InH3. The name 'indane' is occupied for a particular parent ring system in organic chemistry. The name 'indiane' would present problems when derived names containing the sequence ' indiene' could be misinterpreted as indicating the presence of two double bonds. The latter is maybe less of a problem, but nevertheless, the proposal being considered at present is the name 'indigane', which should not lead to confusion and which is in accord with the historical origin of the word 'indium' as related to an indigo-blue spectral line.

There are organic derivatives of inorganic parents that present a particular problem. Consider a compound like C2H5As(Cl)(OH)S. It could be named substitutively on the basis of the parent hydride AsH5, l5-arsane. It could also be named additively. However, neither of these options is chosen in practice. There is an extensive system of names for derivatives of certain inorganic oxoacids whereby this compound would be named 'ethylarsonochloridothioic acid'. This name is based on the substitution of a hydrogen atom by an ethyl group, and the replacement of the oxo group and one hydroxy group in the parent arsonic acid, HAs(O)(OH) 2, by sulfur and chloride, respectively. There are related oxoacids called

  • As(O)(OH)3 arsenic acid or arsoric acid
  • AsH(O)(OH)2 arsonic acid
  • As(OH)3 arsenous acid, arsorous acid
  • AsH(OH)2 arsonous acid
  • AsH2(O)OH arsinic acid
  • AsH2(OH) arsinous acid

which are all needed as parents when naming various organic derivatives according to this particular type of nomenclature called functional replacement nomenclature. The problem with these oxoacid names from the inorganic point of view is that they stem from an antiquated system for indicating oxidation states using the endings 'ous' and 'ic'. Compare with names like 'ferrous' and 'ferric', which were officially abandoned about 50 years ago (although they are difficult to get entirely rid of!) in favor of indications of oxidation state by Stock numbers (iron(II) oxide, etc.) or by charge numbers (iron(2+) oxide, etc.), because the 'ous' and 'ic' names were ambiguous. (The name 'ferric' only says it is a higher oxidation state than 'ferrous', but not which oxidation state.) There is no way to know what 'arsinous acid' is unless you are given a table of formulas to memorize it from, whereas the additive name 'dihydridohydroxidoarsenic', although a bit longer, immediately tells you that the structure is AsH2(OH). (But the latter name cannot be used as a parent name in functional replacement nomenclature.)

It was therefore not acceptable to institute the 'ous' and 'ic' names as preferred names for the oxoacids themselves. The agreement was made in the alignment project that these names are kept available as parent names for giving preferred names to the organic derivatives, but no decision was made regarding the status of the parent names themselves. Having said this, a number of names are of course in daily use all over chemistry (e.g., sulfuric acid, arsenic acid) and the Red Book team has no intention to prohibit the use of those well-entrenched names. They will be given in the revised Red Book alongside systematic additive names (which may or may not end up being the preferred ones later).

The Red Book chapter on substitutive nomenclature, in a draft form, was also reviewed in the alignment project. The process here consisted not just in naming specific compounds, but in inspecting the rules and ensuring they were consistent with the rules for substitutive nomenclature given in the Blue Book. And finally, a number of names of organic ligands needed for the Red Book were checked for consistency with the (new) Blue Book rules.

The formal name of this project, #2001-031-1-800, is "Alignment of nomenclature in areas of overlap between the preferred names for organic nomenclature and the revision of the nomenclature of inorganic chemistry." The project task group consisted of Henri Favre and Warren H. Powell, who represented the task group revising the Blue Book that will replace the 1979 Blue Book and the 1993 Guide; Gerry P. Moss, who represented the IUBMB-IUPAC Joint Commission on Biochemical Nomenclature; and Richard Hartshorn and myself (task group chairman), who represented the team working on a revision of the Red Book that will replace the 1990 version. At its meetings, the group also had participation from Neil G. Connelly, editor in chief of the revised Red Book; Herbert D. Kaesz, the last chairman of the former Commission on Nomenclature of Inorganic Chemistry where the Red Book revision project was initiated; and Alan D. McNaught (ADM), president of Division VIII on Chemical Nomenclature and Structure Representation. ADM also hosted the meetings at the facilities of The Royal Society of Chemistry in Cambridge.

I would like to thank Warren H. Powell and Alan McNaught for helpful comments on a draft of this article.

Ture Damhus <> is the former secretary of CNIC (2000-1001), a current member of Division VIII Advisory Committee, and also a member of the Interdivisional Committee on Terminology, Nomenclature, and Symbols. He works at Novozymes A/S, Denmark.


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