Term: composite mechanism
https://doi.org/10.1351/goldbook.C01210

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: C01210-1.pngC01210-2.png are called parallel reactions or simultaneous reactions. When there are simultaneous reactions there is sometimes competition, as in the scheme: C01210-3.pngC01210-4.png where B and C compete with one another for A.Reactions occurring in forward and reverse directions are called opposing reactions: C01210-5.pngReactions occurring in sequence, such as C01210-6.png 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: C01210-6.pngThe intermediate Y may catalyse the reaction C01210-7.png (positive feedback) or it may inhibit it (negative feedback).Chain reactions

Related Terms:
1) elementary reaction (http://doi.org/10.1351/goldbook.E02035).
2) composite (http://doi.org/10.1351/goldbook.CT07545).
3) stoichiometry (http://doi.org/10.1351/goldbook.S06026).
4) simultaneous reactions (http://doi.org/10.1351/goldbook.S05680).
5) competition (http://doi.org/10.1351/goldbook.C01198).
6) sequence (http://doi.org/10.1351/goldbook.ST06775).
7) consecutive reactions (http://doi.org/10.1351/goldbook.C01276).
8) positive feedback (http://doi.org/10.1351/goldbook.P04767).
9) opposing reactions (http://doi.org/10.1351/goldbook.O04302).
10) complex mechanism (http://doi.org/10.1351/goldbook.C01205).
11) indirect (http://doi.org/10.1351/goldbook.I03011).
12) step-wise (http://doi.org/10.1351/goldbook.S05970).
13) parallel reactions (http://doi.org/10.1351/goldbook.P04403).
14) chain reactions (http://doi.org/10.1351/goldbook.C00960).

Images:
1) Not defined (https://goldbook.iupac.org/img/inline/C01210-1.png) 
2) Not defined (https://goldbook.iupac.org/img/inline/C01210-2.png) 
3) Not defined (https://goldbook.iupac.org/img/inline/C01210-3.png) 
4) Not defined (https://goldbook.iupac.org/img/inline/C01210-4.png) 
5) Not defined (https://goldbook.iupac.org/img/inline/C01210-5.png) 
6) Not defined (https://goldbook.iupac.org/img/inline/C01210-6.png) 
7) Not defined (https://goldbook.iupac.org/img/inline/C01210-7.png) 

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)

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. https://doi.org/10.1351/goldbook.C01210

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