If an acid or base is present in nearly constant concentration throughout a reaction
in solution (owing to buffering or the use of a large excess), it may be found to
increase the rate of that reaction and also to be consumed during the process. The
acid or base is then not a
catalyst and the phenomenon cannot be called
catalysis according to the well-established meaning of these terms in chemical kinetics, although
the
mechanism
of such a process is often intimately related to that of a catalysed reaction. It
is recommended that the term pseudo-
catalysis be used in these and analogous cases (not necessarily involving acids or bases).
For example, if a
Brønsted acid accelerates the
hydrolysis of an ester to a carboxylic acid and an alcohol, this is properly called acid
catalysis, whereas the
acceleration, by the same acid, of
hydrolysis of an amide should be described as pseudo-
catalysis by the acid: the '
acid pseudo-catalyst' is consumed during the reaction through formation of an ammonium ion. The terms
'
general acid pseudo-catalysis' and '
general base pseudo-catalysis' may be used as the analogues of
general acid catalysis and
general base catalysis. The term '
base-promoted', '
base-accelerated' or '
base-induced' is sometimes used for reactions that are pseudo-catalysed by bases. However, the
term '
promotion' also has a different meaning in other chemical contexts.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1153
