A reaction that involves more than one elementary reaction
is said to occur by a composite mechanism. The terms complex mechanism
mechanism, and step-wise
mechanism are also commonly used. There are two main kinds of evidence for a composite
- 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
PAC, 1996, 68, 149
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
on page 161