binary compounds. Examples are sodium sulfate
(Na 2 SO 4 ), ammonium chloride (NH 4 Cl), and ammo-
nium phosphate ((NH 4 ) 3 PO 4 ).
pseudocatalysis 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 phe-
nomenon cannot be called CATALYSISaccording to the
well-established meaning of these terms in chemical
kinetics, although the MECHANISMof such a process is
often intimately related to that of a catalyzed reaction.
It is recommended that the term pseudocatalysis be
used in these and analogous cases (not necessarily
involving acids or bases). For example, if a BRONSTED
ACID accelerates the hydrolysis of an ester to a car-
boxylic 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 pseudocatalyst” is con-
sumed during the reaction through formation of an
ammonium ion. The terms general acid pseudocatalysis
and general base pseudocatalysiscan be used as the
analogs of GENERAL ACID CATALYSISand GENERAL BASE
CATALYSIS.
The terms base-promoted, base-accelerated, or
base-inducedare sometimes used for reactions that are
pseudocatalyzed by bases. However, the term promo-
tionalso has a different meaning in other chemical con-
texts.
pseudo-first-order rate coefficient SeeORDER OF
REACTION.
pseudomolecular rearrangement The use of this
awkwardly formed term is discouraged. It is synony-
mous with “intermolecular rearrangement.”
See alsoMOLECULAR REARRANGEMENT.
pseudopericyclic A concerted TRANSFORMATION is
pseudopericyclic if the primary changes in bonding
occur within a cyclic array of atoms at one (or more)
sites where nonbonding and bonding ATOMIC ORBITALs
interchange roles.
A formal example is the enolBecause the pi and sigma atomic orbitals that inter-
change roles are orthogonal, such a reaction does not
proceed through a fully CONJUGATED TRANSITION
STATEand is thus not a pericyclic reaction and therefore
is not governed by the rules that express ORBITAL SYM-
METRYrestrictions applicable to pericyclic reactions.
See alsoCONCERTED PROCESS.pseudo-unimolecular A term sometimes used syn-
onymously with PSEUDO-FIRST ORDERbut is inherently
meaningless.
See alsoMOLECULARITY; ORDER OF REACTION.pterin 2-Amino-4-hydroxypteridine.See alsoMOLYBDOPTERIN.p-type semiconductor A semiconductor that has an
excess of conducting holes created by adding trace
amounts of other elements to the original pure semi-
conductor crystal; a semiconductor doped with Group
III elements.pyrolysis THERMOLYSIS, usually associated with
exposure to a high temperature.
See alsoFLASH-VACUUM PYROLYSIS.pyrolysis 223