curs in all atomic nuclei. See also hy-
dron.
protonic acid An *acid that forms
positive hydrogen ions (or, strictly,
oxonium ions) in aqueous solution.
The term is used to distinguish ‘tradi-
tional’ acids from Lewis acids or from
Lowry–Brønsted acids in nonaqueous
solvents.
proton number See atomic num-
ber.
Prout’s hypothesis The hypothe-
sis put forward by the British
chemist William Prout (1785–1850)
in 1815 that all atomic weights are
integer multiples of the atomic
weight of hydrogen and hence that
all atoms are made out of hydrogen.
Subsequent work on atomic weights
in the 19th century showed that this
hypothesis is incorrect (with chlorine
having an atomic weight of 35.5
being a glaring example of this). The
understanding of atomic structure
that emerged in the 20th century,
with atomic number being the num-
ber of protons in an atom and non-
integer atomic weights being due to
mixtures of isotopes, has vindicated
the spirit of Prout’s hypothesis.
A
- William Prout’s original paper
prussic acid See hydrogen
cyanide.
pseudoaromatic (antiaromatic) A
compound that has a ring of atoms
containing alternating double and
single bonds, yet does not have the
characteristic properties of *aromatic
compounds. Such compounds do not
obey the Hückel rule. Cyclooctate-
traene (C 8 H 8 ), for instance, has a ring
of eight carbon atoms with conju-
gated double bonds, but the ring is
not planar and the compound acts
like an alkene, undergoing addition
reactions.
pseudo-axialSee ring conforma-
tions.pseudoephedrineSee ephedrine.pseudo-equatorialSee ring con-
formations.pseudohalogensA group of com-
pounds, including cyanogen (CN) 2
and thiocyanogen (SCN) 2 , that have
some resemblance to the halogens.
Thus, they form hydrogen acids
(HCN and HSCN) and ionic salts con-
taining such ions as CN–and SCN–.pseudo order An order of a chemi-
cal reaction that appears to be less
than the true order because of the
experimental conditions used.
Pseudo orders occur when one reac-
tant is present in large excess. For ex-
ample, a reaction of substance A
undergoing hydrolysis may appear to
be proportional only to [A] because
the amount of water present is so
large.psilocinSee psilocybin.
psilocybinA hallucinogen, similar
in effect to psi*mescaline, found in
certain species of mushroom. It is ac-
companied by a related, more active,
compound psilocin. Both are clas-
siÜed as class A drugs in the UK. They
can be detected by the Marquis test
or the Froehde test.PTFESee polytetrafluoroethene.ptyalinAn enzyme that digests car-
bohydrates (see amylase). It is present
in mammalian saliva and is responsi-
ble for the initial stages of starch di-
gestion.p-type semiconductorSee semi-
conductor.pullulanA water-soluble polysac-
charide composed of glucose units
that are polymerized in such a way
as to make it viscous and imperme-
able to oxygen. Pullulan is used in443 pullulan
p