composite mechanism
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:
    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.
  2. Reactions occurring in forward and reverse directions are called opposing reactions:
  3. Reactions occurring in sequence, such as
    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:
    The intermediate Y may catalyse the reaction C01210-7 (positive feedback) or it may inhibit it (negative feedback).
  5. Chain reactions
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 161