permanganate and allows it to dissolve in benzene, giving a purple
neutral solution that can oxidize
many organic compounds. They also
act as catalysts in certain reactions
involving organic salts by complexing with the positive metal cation
and thereby increasing its separation
from the organic anion, which shows
a consequent increase in activity.
Some of the uses of crown ethers depend on their selectivity for speciÜc
sizes of anions. Thus they can be
used to extract speciÜc ions from
mixtures and enrich isotope mixtures. Their selectivity also makes
them useful analytical reagents. See
also cryptands.
crucible A dish or other vessel in
which substances can be heated to a
high temperature. See also gooch
crucible.
crude oil See petroleum.
Crum Brown’s rule A rule that
predicts how substituents will enter
the benzene ring. If C 6 H 5 X is a compound with one substituent in the
benzene ring, it will produce the 1,3
(meta) disubstituted derivative if the
substance HX can be directly oxidized to HOX. If not, a mixture of 1,2
(ortho) and 1,4 (para) compounds will
be formed. The rule was proposed in
1892 by the British chemist Alexander Crum Brown (1838–1922).
cryogenic pump A *vacuum
pump in which pressure is reduced
by condensing gases on surfaces
maintained at about 20 K by means
of liquid hydrogen or at 4 K by
means of liquid helium. Pressures
down to 10
–8 mmHg (10
–6 Pa) can be
maintained; if they are used in conjunction with a *diffusion pump,
pressures as low as 10
–15 mmHg
(10
–13 Pa) can be reached.
cryohydrate A eutectic mixture of
ice and some other substance (e.g. an
ionic salt) obtained by freezing a solution.
cryolite A rare mineral form of
sodium aluminoÛuoride, Na 3 AlF 6 ,
which crystallizes in the monoclinic
system. It is usually white but may
also be colourless. The only important occurrence of the mineral is in
Greenland. It is used chieÛy to lower
the melting point of alumina in the
production of aluminium.
cryoscopic constant See depression of freezing point.
cryoscopy The use of *depression
of freezing point to determine relative molecular masses.
cryostat A vessel enabling a sample
to be maintained at a very low temperature. The *Dewar Ûask is the
most satisfactory vessel for controlling heat leaking in by radiation, conduction, or convection. Cryostats
usually consist of two or more Dewar
Ûasks nesting in each other. For example, a liquid nitrogen bath is often
used to cool a Dewar Ûask containing
a liquid helium bath.
cryptands Compounds with large
three-dimensional molecular structures containing ether chains linked
by three-coordinate nitrogen atoms.
Thus cryptands are macropolycyclic
polyaza-polyethers. For example, the
compound (2,2,2)-cryptand has three
chains of the form
–CH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 –.
These chains are linked at each end
by a nitrogen atom. Cryptands, like
the *crown ethers, can form coordination complexes with ions that can
Üt into the cavity formed by the open
three-dimensional structure, i.e. they
can ‘cryptate’ the ion. Various types
of cryptand have been produced having both spherical and cylindrical
cavities. The cryptands have the
crucible
150
c
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neutral solution that can oxidize
many organic compounds. They also
act as catalysts in certain reactions
involving organic salts by complexing with the positive metal cation
and thereby increasing its separation
from the organic anion, which shows
a consequent increase in activity.
Some of the uses of crown ethers depend on their selectivity for speciÜc
sizes of anions. Thus they can be
used to extract speciÜc ions from
mixtures and enrich isotope mixtures. Their selectivity also makes
them useful analytical reagents. See
also cryptands.
crucible A dish or other vessel in
which substances can be heated to a
high temperature. See also gooch
crucible.
crude oil See petroleum.
Crum Brown’s rule A rule that
predicts how substituents will enter
the benzene ring. If C 6 H 5 X is a compound with one substituent in the
benzene ring, it will produce the 1,3
(meta) disubstituted derivative if the
substance HX can be directly oxidized to HOX. If not, a mixture of 1,2
(ortho) and 1,4 (para) compounds will
be formed. The rule was proposed in
1892 by the British chemist Alexander Crum Brown (1838–1922).
cryogenic pump A *vacuum
pump in which pressure is reduced
by condensing gases on surfaces
maintained at about 20 K by means
of liquid hydrogen or at 4 K by
means of liquid helium. Pressures
down to 10
–8 mmHg (10
–6 Pa) can be
maintained; if they are used in conjunction with a *diffusion pump,
pressures as low as 10
–15 mmHg
(10
–13 Pa) can be reached.
cryohydrate A eutectic mixture of
ice and some other substance (e.g. an
ionic salt) obtained by freezing a solution.
cryolite A rare mineral form of
sodium aluminoÛuoride, Na 3 AlF 6 ,
which crystallizes in the monoclinic
system. It is usually white but may
also be colourless. The only important occurrence of the mineral is in
Greenland. It is used chieÛy to lower
the melting point of alumina in the
production of aluminium.
cryoscopic constant See depression of freezing point.
cryoscopy The use of *depression
of freezing point to determine relative molecular masses.
cryostat A vessel enabling a sample
to be maintained at a very low temperature. The *Dewar Ûask is the
most satisfactory vessel for controlling heat leaking in by radiation, conduction, or convection. Cryostats
usually consist of two or more Dewar
Ûasks nesting in each other. For example, a liquid nitrogen bath is often
used to cool a Dewar Ûask containing
a liquid helium bath.
cryptands Compounds with large
three-dimensional molecular structures containing ether chains linked
by three-coordinate nitrogen atoms.
Thus cryptands are macropolycyclic
polyaza-polyethers. For example, the
compound (2,2,2)-cryptand has three
chains of the form
–CH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 –.
These chains are linked at each end
by a nitrogen atom. Cryptands, like
the *crown ethers, can form coordination complexes with ions that can
Üt into the cavity formed by the open
three-dimensional structure, i.e. they
can ‘cryptate’ the ion. Various types
of cryptand have been produced having both spherical and cylindrical
cavities. The cryptands have the
crucible
150
c
www.AzShimi.ir www.AzShimi.com