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