Glossary of Class Names of Organic Compounds
and Reactive Intermediates Based on Structure

Commission on Nomenclature of Organic Chemistry;
Peter A. S. Smith, Convenor of the Working Group

Commission on Physical Organic Chemistry;
Paul Müller, Convenor of the Working Group


This is a glossary of terms used to denote classes of compounds, substituent groups and reactive intermediates, in contrast to individual compounds. The overwhelming majority of the terms refer to organic compounds, but a few classes that may be considered inorganic are included for convenience. The principal criterion for inclusion is that the class be definable by structure.

A class is a set of compounds sharing a common structural feature to which is attached a variable part (or parts) defining a specific compound of the class. The common feature is often a functional group to which one or a small number of variable parts are attached (e.g. aldehydes, ketones). The variable parts are in this glossary represented by hydrocarbyl groups (q.v.). The most simple representation of a class is thus defined. However membership of one class need not exclude membership of another class, and certain substitutions, as e.g. with heterocyclyl groups (q.v.) linked through a carbon atom in the R group, may be allowed. For instance, ethylamine as well as [1-(furan-2-yl)ethyl]amine and (2-methoxyethyl)amine belong to the class amines. The common feature may also be a sizeable part of the molecular structure, or a repetitive part of the molecular structure, possibly but not necessarily devoid of functionalities (e.g. steroids, terpenoids); quite often the attached variable groups may be many and of any functionality.

Classes defined by use, characteristics, or origin are excluded. Class names that are plurals of systematic names for parent compounds are occasionally cited. Some terms are included that are not actually class names, but are commonly encountered components of class names. These are mostly prefixes, and defining their meanings permits a host of class names beginning with them to be dispensed with.

There is some overlap with other IUPAC documents, especially the 'Glossary of Terms Used in Physical-Organic Chemistry'. Because the viewpoints are sometimes different, the wording of the definitions may in a few cases differ but most are reproduced verbatim.

The recommended class names, which are signalled by an asterisk, respect IUPAC nomenclature as presented in the 'Blue Book' (Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, 1979 edition), the 'Revised Nomenclature for Radicals, Ions, Radical Ions and Related Species, Recommendations 1992' and A Guide to IUPAC Nomenclature of Organic Chemistry, Recommendations 1993, (including Revisions, Published and hitherto Unpublished, to the 1979 Edition of Nomenclature of Organic Chemistry).

Some class names are explicitly defined in the Blue Book, for others the meaning is implicit. Many more class names are to be found in the chemical literature. A selection of the more important ones appears in this glossary. Some class names occurring mainly in the older literature have been included too. The compilers have considered customary usage and the need for clearly defined limits of meaning. Some compromises were necessary, but the definitions have survived scrutiny by the Commissions and consultants.

Recommended class names carry a reference to an official IUPAC publication. The status of the remaining entries varies from unreserved acceptance to outright rejection. Class names not derived from an official source may carry a reference to other literature (a preference for the cited text over others is not implied). The definition of rejected class names is for information only; reasons for rejection are usually cited or indicated, and a cross-reference to the preferred class name is given.

A compound may belong to two or more different classes, e.g. it may be a ketone as well as an ester. Quite often composite names are then used, one class being denoted by an adjective, the second by a noun. The adjective and noun are separated by a space, thus oxo ester. However in the name of a specific compound there is no space between the prefix, indicating a specific substituent, and the name of the parent. Contrast e.g. [alpha]-amino carboxylic acids (class name) and 2-aminopentanoic acid (specific compound name).


An asterisk (*) on an entry indicates that the definition is based on the contents of an official IUPAC source. Italicisation marks a relevant entry used in the definition of another entry. In formulae, charges are not circled and non-bonding electrons are not shown. However, in carbenes and related molecules, one electron pair is shown as a reminder of their lower-than-standard valency state. The dipolar bonds in sulfoxides, sulfones, etc., are represented by formal double bonds. In the formulae to be found in the definitions, the symbol R means any hydrocarbyl group (q.v.) or H, unless its meaning is specifically qualified. When more than one R appears, it is to be understood that the R's may be the same or different, unless otherwise indicated. Two or more R's may be linked so as to form a ring, without changing the definition (e.g. R2C=O includes cyclopentanone as well as pentan-3-one and pentan-2-one). M indicates a metal or metal ion, Ar means an aryl group and Ph a phenyl group.

The abbreviations used for some frequently cited sources are as follows:

BNRD: Biochemical Nomenclature and Related Documents, Portland Press, London, 1992.

2-Carb: Nomenclature of Carbohydrates, revised 1994, in preparation.[Now published]

CBN: Commission on Biochemical Nomenclature. IUPAC-IUB

CNIC: Commission on Nomenclature of Inorganic Chemistry, IUPAC.

CNOC: Commission on Nomenclature of Organic Chemistry, IUPAC.

CPOC: Commission on Physical Organic Chemistry, IUPAC.

EJB: European Journal of Biochemistry.

GNOC: A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993, (including Revisions, Published and hitherto Unpublished, to the 1979 Edition of Nomenclature of Organic Chemistry), Blackwell Scientific Publications, Oxford, 1993.

GTPOC: Glossary of Terms Used in Physical Organic Chemistry, PAC 66, 1077-1184 (1994).

JCBN: Joint Commission on Biochemical Nomenclature, IUBMB-IUPAC.

NOC: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F and H, Pergamon Press, Oxford, 1979.

PAC: Pure and Applied Chemistry.

RNRI: Revised Nomenclature for Radicals, Ions, Radical Ions and Related Species, Recommendations 1993, PAC 65, 1357-1455 (1993).

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