curs in all atomic nuclei. See also hydron.
protonic acid An *acid that forms
positive hydrogen ions (or, strictly,
oxonium ions) in aqueous solution.
The term is used to distinguish ‘traditional’ acids from Lewis acids or from
Lowry–Brønsted acids in nonaqueous
solvents.
proton number See atomic number.
Prout’s hypothesis The hypothesis 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 number of protons in an atom and noninteger 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. Cyclooctatetraene (C 8 H 8 ), for instance, has a ring
of eight carbon atoms with conjugated double bonds, but the ring is
not planar and the compound acts
like an alkene, undergoing addition
reactions.
pseudo-axial See ring conformations.
pseudoephedrine See ephedrine.
pseudo-equatorial See ring conformations.
pseudohalogens A group of compounds, 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 containing such ions as CN
– and SCN
– .
pseudo order An order of a chemical reaction that appears to be less
than the true order because of the
experimental conditions used.
Pseudo orders occur when one reactant is present in large excess. For example, a reaction of substance A
undergoing hydrolysis may appear to
be proportional only to [A] because
the amount of water present is so
large.
psilocin See psilocybin.
psilocybin A hallucinogen, similar
in effect to psi*mescaline, found in
certain species of mushroom. It is accompanied by a related, more active,
compound psilocin. Both are classiÜed as class A drugs in the UK. They
can be detected by the Marquis test
or the Froehde test.
PTFE See polytetrafluoroethene.
ptyalin An enzyme that digests carbohydrates (see amylase). It is present
in mammalian saliva and is responsible for the initial stages of starch digestion.
p-type semiconductor See semiconductor.
pullulan A water-soluble polysaccharide composed of glucose units
that are polymerized in such a way
as to make it viscous and impermeable to oxygen. Pullulan is used in
443
pullulan
p
www.AzShimi.ir www.AzShimi.com
protonic acid An *acid that forms
positive hydrogen ions (or, strictly,
oxonium ions) in aqueous solution.
The term is used to distinguish ‘traditional’ acids from Lewis acids or from
Lowry–Brønsted acids in nonaqueous
solvents.
proton number See atomic number.
Prout’s hypothesis The hypothesis 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 number of protons in an atom and noninteger 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. Cyclooctatetraene (C 8 H 8 ), for instance, has a ring
of eight carbon atoms with conjugated double bonds, but the ring is
not planar and the compound acts
like an alkene, undergoing addition
reactions.
pseudo-axial See ring conformations.
pseudoephedrine See ephedrine.
pseudo-equatorial See ring conformations.
pseudohalogens A group of compounds, 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 containing such ions as CN
– and SCN
– .
pseudo order An order of a chemical reaction that appears to be less
than the true order because of the
experimental conditions used.
Pseudo orders occur when one reactant is present in large excess. For example, a reaction of substance A
undergoing hydrolysis may appear to
be proportional only to [A] because
the amount of water present is so
large.
psilocin See psilocybin.
psilocybin A hallucinogen, similar
in effect to psi*mescaline, found in
certain species of mushroom. It is accompanied by a related, more active,
compound psilocin. Both are classiÜed as class A drugs in the UK. They
can be detected by the Marquis test
or the Froehde test.
PTFE See polytetrafluoroethene.
ptyalin An enzyme that digests carbohydrates (see amylase). It is present
in mammalian saliva and is responsible for the initial stages of starch digestion.
p-type semiconductor See semiconductor.
pullulan A water-soluble polysaccharide composed of glucose units
that are polymerized in such a way
as to make it viscous and impermeable to oxygen. Pullulan is used in
443
pullulan
p
www.AzShimi.ir www.AzShimi.com