A @P04794@ mechanism in which one of the @M03986@, C, is already present in an @E02090@ with A during formation of B from A, but does not assist the formation of B, e.g.
The formation of B from A may itself be a @M03989@ reaction with some other @R05190@. Since C does not assist the formation of A, it is described as being present as a spectator, and hence such a mechanism is sometimes referred to as a spectator mechanism.
See also:
microscopic diffusion control
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1165 [Terms] [Paper]