Preferred Label : composite mechanism;
IUPAC 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:
ol li The kinetic equation for the reaction does not correspond to its stoichiometry.
/li li 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. /li /ol There are many types of composite mechanisms, for example:
li 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. /li li Reactions occurring
in forward and reverse directions are called opposing reactions: /li li Reactions
occurring in sequence, such as are known as consecutive reactions. /li li 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). /li li Chain reactions
/li /ol ;
Origin ID : C01210;
See also
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:
ol li The kinetic equation for the reaction does not correspond to its stoichiometry.
/li li 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. /li /ol There are many types of composite mechanisms, for example:
li 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. /li li Reactions occurring
in forward and reverse directions are called opposing reactions: /li li Reactions
occurring in sequence, such as are known as consecutive reactions. /li li 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). /li li Chain reactions
/li /ol
