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composite mechanism

Also contains definition of: negative feedback

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:
  1. The kinetic equation for the reaction does not correspond to its stoichiometry.
  2. 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:
  1. Reactions occurring in parallel, such as:
    C01210-1
    C01210-2
    are called parallel reactions or simultaneous reactions. When there are simultaneous reactions there is sometimes competition, as in the scheme:
    C01210-3
    C01210-4
    where B and C compete with one another for A.
  2. Reactions occurring in forward and reverse directions are called opposing reactions:
    C01210-5
  3. Reactions occurring in sequence, such as
    C01210-6
    are known as consecutive reactions.
  4. 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
    The intermediate Y may catalyse the reaction C01210-7 (positive feedback) or it may inhibit it (negative feedback).
  5. Chain reactions
Source:
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 161
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IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.
Last update: 2014-02-24; version: 2.3.3.
DOI of this term: doi:10.1351/goldbook.C01210.
Original PDF version: http://www.iupac.org/goldbook/C01210.pdf. The PDF version is out of date and is provided for reference purposes only. For some entries, the PDF version may be unavailable.
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