04794 IUPAC Gold Book - pre-association 10.1351/goldbook.P04794 P04794 current 1 A step on the reaction path of some stepwise reactions in which the molecular entity C is already present in an encounter pair or encounter complex with A during the formation of B from A, e.g. In this mechanism the chemical species C may but does not necessarily assist the formation of B from A, which may itself be a bimolecular reaction with some other reagent. Pre-association is important when B is too short-lived to permit B and C to come together by diffusion. association https://goldbook.iupac.org/terms/view/A00472 bimolecular https://goldbook.iupac.org/terms/view/M03989 chemical species https://goldbook.iupac.org/terms/view/CT01038 diffusion https://goldbook.iupac.org/terms/view/D01716 encounter complex https://goldbook.iupac.org/terms/view/E02087 encounter pair https://goldbook.iupac.org/terms/view/E02090 microscopic diffusion control https://goldbook.iupac.org/terms/view/M03912 molecular entity https://goldbook.iupac.org/terms/view/M03986 reaction path https://goldbook.iupac.org/terms/view/R05174 reagent https://goldbook.iupac.org/terms/view/R05190 spectator mechanism https://goldbook.iupac.org/terms/view/S05810 stepwise reactions https://goldbook.iupac.org/terms/view/S05970 PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1152 (https://doi.org/10.1351/pac199466051077) https://goldbook.iupac.org/terms/view/P04794/html https://goldbook.iupac.org/terms/view/P04794/json https://goldbook.iupac.org/terms/view/P04794/plain Citation: 'pre-association' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.P04794 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-06-29T11:07:50+00:00