Title: composite mechanism
Long Title: IUPAC Gold Book - composite mechanism
DOI: 10.1351/goldbook.C01210
Status: current

Definition
A reaction that involves more than one elementary reaction is said to occur by a composite mechanism. The terms complex mechanism, indirect mechanism, and step-wise mechanism are also commonly used. There are two main kinds of evidence for a composite mechanism: The kinetic equation for the reaction does not correspond to its stoichiometry.There is experimental evidence, direct or indirect, for intermediates of such a nature that it is necessary to conclude that more than one elementary reaction is involved. There are many types of composite mechanisms, for example: Reactions occurring in parallel, such as:  are called parallel reactions or simultaneous reactions. When there are simultaneous reactions there is sometimes  competition, as in the scheme:  where B and C compete with one another for A.Reactions occurring in forward and reverse directions are called opposing reactions: Reactions occurring in sequence, such as  are known as consecutive reactions.Reactions are said to exhibit feedback if a substance formed in one step affects the rate of a previous step. For example, in the scheme: The intermediate Y may catalyse the reaction  (positive feedback) or it may inhibit it (negative feedback).Chain reactions

Related Terms
- Chain reactions: https://goldbook.iupac.org//terms/view/C00960
- complex mechanism: https://goldbook.iupac.org//terms/view/C01205
- composite: https://goldbook.iupac.org//terms/view/CT07545
- consecutive reactions: https://goldbook.iupac.org//terms/view/C01276
- elementary reaction: https://goldbook.iupac.org//terms/view/E02035
- indirect: https://goldbook.iupac.org//terms/view/I03011
- opposing reactions: https://goldbook.iupac.org//terms/view/O04302
- parallel reactions: https://goldbook.iupac.org//terms/view/P04403
- sequence: https://goldbook.iupac.org//terms/view/ST06775
- simultaneous reactions: https://goldbook.iupac.org//terms/view/S05680
- step-wise: https://goldbook.iupac.org//terms/view/S05970
- stoichiometry: https://goldbook.iupac.org//terms/view/S06026

Source
- PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 161 (https://doi.org/10.1351/pac199668010149)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/C01210/html
- json: https://goldbook.iupac.org/terms/view/C01210/json
- xml: https://goldbook.iupac.org/terms/view/C01210/xml

Citation: Citation: 'composite 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.C01210
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Accessed: 2026-04-20T17:19:26+00:00