In contrast to such systematic names, there are traditional names, semisystematic or trivial, which are widely used for a core group of common compounds. Examples are "acetic acid", "benzene", "cholesterol", "styrene", "formaldehyde", "water", "iron". Many of these names are also part of general nonscientific language and are thus not confined to use within the science of chemistry. They are useful, and in many cases indispensable (consider the alternative systematic name for cholesterol, for example). Little is to be gained, and certainly much to be lost, by replacing such names. Therefore, where they meet the requirements of utility and precision, and can be expected to continue to be widely used by chemists and others, they are retained and, for the most part, preferred in this Guide.
Semisystematic names also exist, such as "methane", "propanol", and "benzoic acid", which are so familiar that few chemists realize that they are not fully systematic. They are retained, and indeed, in some cases there are no better systematic alternatives.
It is important to recognize that the rules of systematic nomenclature need not necessarily lead to a unique name for each compound, but must always lead to an unambiguous one. Lucidity in communication often requires that the rules be applied with different priorities. A comparative discussion of the compounds , , , and , might be easier to follow if they were all named as propenes, even though with the last three, the benzene ring, amino, and hydroxy groups may have seniority over the double bond for citation as a parent or as a suffix. In other cases, a set of rules that generates clear and efficient names for some compounds can lead to clumsy and nearly unrecognizable names for others, even closely related ones. To force the naming of all compounds into the Procrustean bed of one set of rules would not serve the needs of general communication, and the Commission believes that the majority of organic chemists would not accept such a policy for general communication. This situation can be illustrated by a compound that most chemists would probably name as "pentaphenylethane", instinctively, whereas the application of a principle favouring rings over chains leads to a name such as "ethanepentaylpentabenzene". The first name is certainly more easily recognized than the second. Another example is compound (1), which would be named "benzoazete" by application of fusion nomenclature principles, but von Baeyer nomenclature would give the name "7-azabicyclo[4.2.0]octa-1,3,5,7-tetraene". On the other hand, von Baeyer nomenclature is most useful with compounds such as "bicyclo[4.3.2]undecane" (2).
In view of the foregoing considerations, this Guide to IUPAC Nomenclature of Organic Compounds often presents alternative sets of rules, equally systematic, wherever available and justifiable, to enable a user to fit the name to a particular need.
Lastly, the Commission recognizes that for certain types of compounds, there is significant disagreement among chemists in different fields as to what should be the preferred nomenclature. This situation leads to an apparent lack of decisiveness in some of the recommendations in this document. This is unavoidable, because long experience has taught that formulating rules not having general support is a futile exercise; such rules will be widely ignored. Therefore, the Commission's policy is to offer critically examined alternatives, some of which may be new proposals, and to observe how they are accepted and used. If one of the alternatives subsequently becomes preferred to an overwhelming extent by the community of chemists, a future edition of recommendations can reflect that fact.
In this Guide, some practices of the Chemical Abstracts Service and/or of the Beilstein Institute have been mentioned. This is done only for informational purposes, and such instances are not necessarily recommendations of the Commission. The Commission recognized that there are circumstances that require a preferred (i.e., unique) name. These include comprehensive indexing (such as for the volume indexes to Chemical Abstracts) in order to avoid an intolerable amount of cross-indexing and multiple entries. This need is being met in a particular way by Chemical Abstracts Service as in-house procedures designed to place compounds with the same parent skeleton together while at the same time minimizing the number of rules. The Chemical Abstracts Index Guide treats the majority of compounds, but is not complete. There are a number of other in-house procedures applied elsewhere, such as in Beilstein (not yet explicitly published). Specialty files, such as steroids and carbohydrates, have different bases. The result is not always compatible with ease of recognition, ease of generation, or conciseness. Unique names are also very important in legal situations, with manifestation in patents, export-import regulations, health and safety information, etc. These needs are being addressed by other Commission projects. In this Guide, the primary aim is to provide directions for arriving at an unambiguous name, although some guidance is given about establishing preferences (see, for example, Table 10 and Table 16).
The rules given in the Nomenclature of Organic Chemistry, commonly known as the "Blue Book", emphasize the generation of unambiguous names in accord with the historical development of the subject, because the need for a "unique" name was not perceived to be compelling by earlier generations of chemists. The so-called information explosion of recent decades is a major factor in changing this perception. The present matrix of rules, however, cannot easily be overlaid with a simple set of principles for selecting a preferred name among the systematic alternatives, and to declare a preference by arbitrary fiat in each situation would surely lead to widespread rejection. Therefore, the Commission has initiated projects to formulate a comprehensive guide for selecting unique names that will, insofar as feasible, have good recognition value and general acceptance among chemists; the results will be presented in later publications. Further projects, with longer-range objectives of systematizing nomenclature of organic compounds, are also under way.
Finally, those who use this Guide should be aware that the nomenclature in these recommendations is independent of the orientation of the graphic structure (except as stated in the recommendations for nomenclature of fused-ring structures) and of conformation. Furthermore, the nomenclature does not indicate nor imply an electronic structure or spin multiplicity.
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