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Vol. 34 No. 3
May-June 2012

Nomenclature Notes | A column about the Principles of Chemical Nomenclature—A Guide to IUPAC Recommendations, 2011 Edition

On the Various Nomenclature Systems

In the first “Nomenclature Notes” (March-April 2012 CI), it was stated that IUPAC nomenclature is composed of a set of different nomenclatures, which do not necessarily use the same conventions and methods. IUPAC bodies such as the Interdivisional Committee on Terminology, Nomenclature and Symbols (ICTNS) are trying to remove these inconsistencies, but, nevertheless if you are trying to decipher an IUPAC name, or to construct one, it is necessary to realize which type of nomenclature is being used, which means recognizing the type of compound that is being treated. An ideal IUPAC name should be unique, but should also convey the structure of a given compound.

At the lowest level, compositional nomenclature simply lists the constituents of a compound in some prescribed order, generally alphabetical, and says nothing about the structure. It is really another way of presenting an entry in a formula index. Substitutive nomenclature is a principal IUPAC nomenclature system and the preferred method for naming organic compounds. This relies on selecting a suitable unsubstituted parent compound from which the compound at issue can be considered as ultimately derived, and then modifying the parent name in a series of formal operations. For example, a compound such as C2H5Cl would be considered to be derived from the parent compound, ethane, C2H6, which is then substituted by removing a hydrogen atom to produce C2H5, an ethyl radical, and then adding a chlorine atom to yield C2H5Cl, called chloroethane.

This is a particularly simple case, but the principles used for naming in this way are all very similar, though as the complexity of the compound being treated increases, more and more complex parent compounds and many more rules are required.

An alternative name for chloroethane is ethyl chloride, and this is an example of binary nomenclature which developed from inorganic chemistry and its original systematization by Lavoisier and his colleagues. Binary nomenclature groups constituents of compounds into two classes, positive and negative. This works well for salts, such as sodium chloride, but more complex organic materials may require the employment of at least two classes of nomenclature, for example, substitutive to name a formal cation such as chloroethyl, and binary to name the related ester, chloroethyl acetate. Functional class nomenclature is a system used for organic compounds when substitutive nomenclature is less appropriate, for example with organic acids and anhydrides.

A more recent, though similarly historical, system is additive nomenclature, a second principal IUPAC nomenclature system, with its origins in inorganic coordination chemistry. In this approach, the name of a compound considered to be a coordination complex, such as Na3[CoCl6], is derived by identifying the three cations and the single anion from which it is composed. The latter is a complex anion, and it is then divided formally into its constituent central cation and its associated six chloride anions or ligands. Finally, the individual names are then assembled to give the compound name trisodium hexachloridocobaltate. In this system, organic ligands would receive substitutive names. Then, to be used in an additive context, organometallic compounds almost always require such a nomenclature mixture. The additive system may be used to name some compounds which would not usually be considered to be coordination complexes.

The third major class of IUPAC nomenclature is polymer nomenclature. It is generally impossible to specify a polymer molecule exactly, as we attempt to do in the cases treated so far, because the chain lengths and end groups are rarely accurately known. One approach is to use a source-based system, because one generally knows the monomer from which the polymer was produced. This gives names such as poly(buta-1,3-diene), but it lacks structural information. A structure-based name for this material might be poly(1-vinylethane-1,2-diyl), but this represents only the units of the polymer chain, not the end groups, so it may be equally incomplete.

The 2011 edition of Principles of Chemical Nomenclature includes full descriptions of all these nomenclature systems, and of several others which are not necessarily due to IUPAC, with details of how to apply them. It also contains instructions for how to decipher IUPAC names to identify the structures that such names are intended to convey. Principles of Chemical Nomenclature also introduces what gives the promise of being a unique class of identifier applicable to a wide class of compound, the IUPAC International Chemical Identifier, or InChI.

Jeffery Leigh is the editor and contributing author of Principles of Chemical Nomenclature—A Guide to IUPAC Recommendations, 2011 Edition (RSC 2011, ISBN 978-1-84973-007-5). Leigh is emeritus professor at the University of Sussex and has been active in IUPAC nomenclature since 1973.

www.iupac.org/project/2006-029-1-800

Next in this series will be a review of non-IUPAC nomenclatures.


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