(IUPAC Recommendations 1994)



Membership of the Commission
List of General Symbols
Synopsis of the 2nd Edition, 1994
General Remark on the 1994 Revision
Arrangement, Abbreviations and Symbols


(IUPAC Recommendations 1994)

Prepared for publication by


Département de Chimie Organique, Université de Genève, CH-1211 Genève 4, Suisse

*Membership of the Commission during the period (1987-93) the report was drafted was as follows:

Chairman: 1987-93 P. Müller (Switzerland); Secretary; 1987-91 M. P. Doyle (USA); 1991-93 W. Drenth (Netherlands). Titular Members: P. N. I. Ahlberg (Sweden, 1987-91); E. A. Halevi (Israel, 1987-89); J. M. McBride (USA, 1987-93); V. Minkin (USSR, 1991-93); O. M. Nefedov (USSR, 1987-91); M. Oki (Japan, 1987-91); J. Shorter (UK, 1989-93); Y. Takeuchi (Japan, 1991-93); Z. Rappoport (Israel, 1991-93); Associate Members: P. N. I. Ahlberg (Sweden, 1991-93); T. A. Albright (USA, 1987-91); W. Drenth (Netherlands, 1987-91); E. A. Halevi (Israel, 1989-93); R. A. Y. Jones (UK, 1987-89); V. I. Minkin (USSR, 1987-91); O. M. Nefedov (USSR, 1991-93); C. Perrin (USA, 1991-93); D. Raber (USA, 1991-93); K. Schwetlick (GDR, 1987-89); Y. Takeuchi (Japan, 1987-91); P. Van Brandt (Belgium, 1987-93); J. R. Zdysiewicz (Australia, 1989-93); National Representatives: J.-L. Abboud Mas (Spain, 1991-93); E. Baciocchi (Italy, 1989-93); M. V. Bhatt (India, 1989-91); X. Jiang (China, 1987-93); J. J. E. Humeres Allende (Brazil, 1991-93); P. Laszlo (Belgium, 1987-91); D. J. McLennan (New Zealand, 1987-91); Z. Rappoport (Israel, 1987-91); R. Sabbah (France, 1989-93); B. E. Smart (USA, 1987-91); J. A. Silva Cavaleiro (Portugal, 1991-93); J. Suh (Republic of Korea, 1989-93); O. Tarhan (Turkey, 1989-93); T. T. Tidwell (Canada, 1991-93); M. Tisler (Yugoslavia, 1987-93); J. Zavada (Czechslovakia, 1987-89); J. Zdysiewicz (Australia, 1987-89).

Membership of the Working Party 1987-93:

Chairman: P. Müller (Switzerland): Members: P. N. I. Ahlberg (Sweden), M. P. Doyle (USA), W. Drenth Netherlands), E. A. Halevi (Israel), R. A. Y. Jones (UK), J. M. McBride (USA), V. Minkin (USSR), M. Oki (Japan). Y. Takeuchi (Japan), J. R. Zdysiewicz (Australia).

List of General Symbols

A pre-exponential factor in Arrhenius equation (see energy of activation).

[alpha] order of reaction with respect to reactant A, or Brønsted coefficient (see order of reaction, Brønsted relation).

[beta] order of reaction with respect to reactant B, or Brønsted coefficient (see order of reaction, Brønsted relation).

Cp heat capacity (at constant pressure).

e base of natural logarithms (e = 2.718).

Ea energy of activation (see energy of activation).

G Gibbs energy.

H enthalpy.

h Planck constant.

K Kelvin (unit of thermodynamic temperature).

K0 equilibrium constant.

k rate constant (see order of reaction).

kB Boltzmann constant.

ln natural logarithm.

lg decadic logarithm.

[lambda] wavelength.

[nu] frequency.

0 (superscript) relating to thermodynamic standard state.

p pressure.

R gas constant.

S entropy.

s second (unit of time).

[Sigma] summation.

T thermodynamic temperature.

t time.

v rate of reaction (see rate of reaction).

[double dagger] (superscript) relating to transition state.

[resonance arrow] See delocalization.

[X] amount concentration of X.

See also IUPAC QUANTITIES (1988).

2nd Edition, 1994

Synopsis. This glossary contains definitions and explanatory notes for terms used in the context of research and publications in physical organic chemistry. Its aim is to provide guidance on physical organic chemical terminology with a view to achieve a far-reaching consensus on the definitions of useful terms and on the abandonment of unsatisfactory ones. As a consequence of the development of physical organic chemistry, and of the increasing use of physical organic terminology in other fields of chemistry, the 1994 revision of the Glossary is much expanded in comparison to the previous edition, and it also includes terms from cognate fields. A few definitions have been refined, some others totally revised in the light of comments received from the scientific community.

General Remarks

The "Glossary of Terms Used In Physical Organic Chemistry" was published in provisional form in 1979 (IUPAC PHYSICAL ORGANIC GLOSSARY (1979)) and in revised form in 1983. A historical account of procedure and progress up to that stage was outlined in the provisional publication. The 1983 revision incorporated modifications agreed by the IUPAC Commission III.2 (Physical Organic Chemistry), partly in response to comments received since the provisional publication. The terms defined in the 1983 Glossary are incorporated in the IUPAC "COMPENDIUM OF CHEMICAL TERMINOLOGY" (1987) (Victor Gold Book).

The present revision was undertaken with the long-term objective of compiling a compendium of organic chemical terminology. For this reason the criteria for inclusion of terms were much expanded. The criteria for the 1983 edition were that the meaning of a term was either not given in non-scientific dictionaries or standard textbooks or differed significantly from the definition in such reference works, or that the term was associated with some ambiguity or uncertainty with respect to its meaning. The present compendium tries to be comprehensive, but without including trivial terms and without redefining IUPAC approved terms. In particular, terms from cognate fields which are used by physical organic chemists, are included. The terminology for the various spectroscopic methods of interest to organic chemists is not treated in the present document, except for some terms from NMR spectroscopy, but may be the subject of a separate study. As a matter of policy, named reactions (such as Diels-Alder) and symbolic representation of reaction mechanisms (SN1, etc.) have been excluded with a few exceptions. These topics have been treated in separate reports of this Commission (IUPAC REACTION MECHANISMS (1989), IUPAC TERMINOLOGY FOR TRANSFORMATIONS (1989)).

The work has been coordinated with that of other Commissions within the Division of Organic Chemistry, which deal with terminology, (e.g., IUPAC PHOTOCHEMICAL GLOSSARY (1988), and its revision (1992), Commission III.3; IUPAC STEREOCHEMICAL TERMINOLOGY (1993), joint project of Commissions III.1 and III.2, and IUPAC CLASS NAMES (1993), joint project of Commissions III.1 and III.2), and with that of Commission I.5. Discrepancies in the views between the Commissions have been eliminated. Whenever there was overlap or disagreement between the 1983 glossary and one of the more recent ones, the best available definitions were incorporated in the present text with a reference to their origin. Otherwise this glossary was not expanded into areas where other Commissions or working parties are already active. It is hoped that a merged compendium will be published after approval of the various glossaries.

The aim of the Glossary is to provide guidance on physical-organic chemical terminology with a view to achieving a far-reaching consensus on the definitions of useful terms and the abandonment of unsatisfactory ones. The Commission is anxious to emphasize that it cannot (and would not wish to) impose rules or restrictions which might hinder rather than help the precise formulation of new ideas.

Generally speaking, operational definitions were preferred to definitions couched in terms of theoretical models. We have tried to avoid taking sides on issues of scientific, as distinct from semantic, disagreement.

The Commission is pleased to acknowledge the generous contributions of many scientists who helped by proposing or defining new terms, or by criticizing existing ones. The following have contributed to this revision: I.P. Beletskaya, T.W. Bentley, C.F. Bernasconi, E. Buncel, C. Eaborn, J.F. Gal, S. Hoz
W.P. Jencks, N. Kallay, N.J. Leonard, R.M. Magid, P.C. Maria, R.T. Myers, C. Reichardt, J.D. Roberts, M.F. Ruasse, H. Zollinger

The Commission thanks Mrs. M. Wyss and Dr. S. Motallebi for their help in the preparation of the manuscript of this Glossary.

This version for the World Wide Web was prepared by Charles L. Perrin ( and G. P. Moss (

Arrangement, Abbreviations and Symbols

The arrangement is simply alphabetical (a space or hyphen is ignored); terms beginning with Greek letters following those beginning with Latin ones. Italicized words in the body of a definition, as well as those cited at the end, point to relevant cross-references. No distinction is made between singular and plural in cross-referencing. Literature references should direct the reader either to the original literature where the term was originally defined, or to pertinent references where it is used. The appearance of a term in quotation marks in the body of a definition indicates that no further information will be found under that heading. Capitalized names indicate references.

A cross (+) against the title of an entry implies that the Commission recommends discontinuation of the term.

In accordance with general practice Kekulé structures are normally used in this Glossary to represent the formulae of aromatic compounds. A single hexagon with a full circle inside is used in cases where delocalization is emphasized. A full circle is also used in the case of monocyclic aromatic ions, but fused aromatics are represented by Kekulé structures, because in this case each circle would imply the presence of 6 pi electrons in each hexagon. Thus naphthalene with two circles would appear to have 12 pi electrons, while it has only 10 in reality. Dashed lines indicate delocalization of positive or negative charge or of the spin of an unpaired electron. Curved singly headed arrows represent formal movement of electron pairs, while doubly headed arrows between formulae symbolize resonance.

In accordance with previous IUPAC recommendations (IUPAC QUANTITIES (1988)) the symbol [double dagger] to indicate transition states ("double dagger") is used as a prefix to the appropriate quantities, e.g. [Delta][double dagger]G rather than the more often used [Delta]G[double dagger].

Relative positions are indicated by post-slashed Arabic numerals. For example, 1/4/dibromo-addition may refer to addition at positions 9 and 10 of anthracene, or at positions 2 and 5 of hexa-2,4-diene (IUPAC TERMINOLOGY FOR TRANSFORMATIONS (1989)).


*IUPAC CLASS NAMES (1993). IUPAC: Organic Chemistry Division. Glossary of Class Names of Organic Compounds and Reactive Intermediates Based on Structure. IDCNS and public review; now published in Pure Appl. Chem., 67, 1307-1375 (1995).

*IUPAC COMPENDIUM (1987). IUPAC: Compendium of Chemical Terminology, (GOLD, V., LOENING, K. L., MCNAUGHT, A. D., and SEHMI, P., Eds.), Blackwell, Oxford.

*IUPAC PHOTOCHEMICAL GLOSSARY (1988). IUPAC: Organic Chemistry Division: Commission on Photochemistry. Glossary of Terms Used in Photochemistry. Pure Appl. Chem., 60, 1055-1106.

*IUPAC PHOTOCHEMICAL GLOSSARY (1992). IUPAC: Organic Chemistry Division: Commission on Photochemistry. Glossary of Terms Used in Photochemistry. Draft 1, provisional.

IUPAC PHYSICAL ORGANIC GLOSSARY (1979). IUPAC: Organic Chemistry Division: Commission on Physical Organic Chemistry. Glossary of Terms Used in Physical Organic Chemistry. Provisional. Pure Appl. Chem., 51, 1725-1801.

*IUPAC QUANTITIES (1988). IUPAC: Physical Chemistry Division: Commission on Physicochemical Symbols, Terminology and Units."Quantities, Units and Symbols in Physical Chemistry", MILLS, I. CVITAS, T., HOMANN, K., KALLAY, N., and KUCHITSU, K., Eds.), Blackwell, Oxford.

IUPAC REACTION MECHANISMS (1989). IUPAC: Organic Chemistry Division: Commission on Physical Organic Chemistry. System for Symbolic Representation of Reaction Mechanisms. Pure Appl. Chem., 61, 23-56.

*IUPAC STEREOCHEMICAL TERMINOLOGY (1993). IUPAC: Organic Chemistry Division: Basic Terminology of Stereochemistry. IDCNS and public review.

IUPAC TERMINOLOGY FOR TRANSFORMATIONS (1989). IUPAC: Organic Chemistry Division: Commission on Physical Organic Chemistry. Terminology for Organic Chemical Transformations. Pure Appl. Chem., 61, 725-768.

Continue with terms starting with A.

Return to home page for Glossary of terms used in Physical Organic Chemistry.