Title: spectator mechanism
Long Title: IUPAC Gold Book - spectator mechanism
DOI: 10.1351/goldbook.S05810
Status: current

Definition
A pre-association mechanism in which one of the molecular entities, C, is already present in an encounter pair with A during formation of B from A, but does not assist the formation of B, e.g.  The formation of B from A may itself be a bimolecular reaction with some other reagent. Since C does not assist the formation of A, it is described as being present as a spectator, and hence such a mechanism is sometimes referred to as a spectator mechanism.

Related Terms
- bimolecular: https://goldbook.iupac.org/terms/view/M03989
- encounter pair: https://goldbook.iupac.org/terms/view/E02090
- microscopic diffusion control: https://goldbook.iupac.org/terms/view/M03912
- molecular entities: https://goldbook.iupac.org/terms/view/M03986
- pre-association: https://goldbook.iupac.org/terms/view/P04794
- reagent: https://goldbook.iupac.org/terms/view/R05190

Source
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1165 (https://doi.org/10.1351/pac199466051077)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/S05810/html
- json: https://goldbook.iupac.org/terms/view/S05810/json
- xml: https://goldbook.iupac.org/terms/view/S05810/xml

Citation: Citation: 'spectator mechanism' 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.S05810
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Accessed: 2026-07-23T16:19:38+00:00