permanganate and allows it to dis-
solve 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 complex-
ing 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 de-
pend on their selectivity for speciÜc
sizes of anions. Thus they can be
used to extract speciÜc ions from
mixtures and enrich isotope mix-
tures. Their selectivity also makes
them useful analytical reagents. See
also cryptands.crucibleA dish or other vessel in
which substances can be heated to a
high temperature. See also gooch
crucible.
crude oilSee petroleum.Crum Brown’s rule A rule that
predicts how substituents will enter
the benzene ring. If C 6 H 5 X is a com-
pound with one substituent in the
benzene ring, it will produce the 1,3
(meta) disubstituted derivative if the
substance HX can be directly oxi-
dized 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 Alexan-
der Crum Brown (1838–1922).cryogenic pumpA *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 10–8mmHg (10–6Pa) can be
maintained; if they are used in con-
junction with a *diffusion pump,
pressures as low as 10–15mmHg
(10–13Pa) can be reached.cryohydrateA eutectic mixture ofice and some other substance (e.g. an
ionic salt) obtained by freezing a so-
lution.cryoliteA 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 impor-
tant occurrence of the mineral is in
Greenland. It is used chieÛy to lower
the melting point of alumina in the
production of aluminium.cryoscopic constantSee depres-
sion of freezing point.cryoscopyThe use of *depression
of freezing point to determine rela-
tive molecular masses.cryostatA vessel enabling a sample
to be maintained at a very low tem-
perature. The *DewarÛask is the
most satisfactory vessel for control-
ling heat leaking in by radiation, con-
duction, or convection. Cryostats
usually consist of two or more Dewar
Ûasks nesting in each other. For ex-
ample, a liquid nitrogen bath is often
used to cool a DewarÛask containing
a liquid helium bath.cryptandsCompounds with large
three-dimensional molecular struc-
tures 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 coordi-
nation 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 hav-
ing both spherical and cylindrical
cavities. The cryptands have thecrucible 150c