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:
  • 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 C01210-7.png (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 (