Crookes, Sir William (1832–1919)
British chemist and physicist, who in
1861 used *spectroscopy to discover
*thallium and in 1875 invented the
radiometer. He also developed an improved vacuum tube (Crookes’ tube)
for studying gas discharges. Crookes
was also involved in industrial chemistry and realised the importance of
nitrogen Üxation for fertilizers.
crossed-beam reaction A chemical reaction in which two molecular
beams are crossed; one beam is regarded as the incident beam of gas
and the other as the target gas. This
technique enables a great deal of information to be gained about the
chemical reaction since the states of
both the target and projectile molecules can be controlled. The incident beam is characterized by its
incident beam Ûux, I, which is the
number of particles per unit area per
unit time. The scattered molecules
can be detected Ürst by ionizing
them and then detecting the ions
electrically or using spectroscopy if
changes in the vibrational or rotational states of molecules in a reaction are of interest.
cross linkage A short side chain of
atoms linking two longer chains in a
polymeric material.
crown See ring conformations.
crown ethers Organic compounds
with molecules containing large
rings of carbon and oxygen atoms.
The crown ethers are macrocyclic
polyethers. The Ürst to be synthesized was the compound 18-crown-6,
which consists of a ring of six
–CH 2 –CH 2 –O– units (i.e. C 12 H 24 O 6 ).
The general method of naming
crown ethers is to use the form ncrown-m, where n is the number of
atoms in the ring and m is the number of oxygen atoms. Substituted
crown ethers can also be made. The
crown ethers are able to form
strongly bound complexes with
metal ions by coordination through
the oxygen atoms. The stability of
these complexes depends on the size
of the ion relative to the cavity available in the ring of the particular
crown ether. Crown ethers also form
complexes with ammonium ions
(NH 4 +) and alkyl ammonium ions
(RNH 3 +). They can be used for increasing the solubility of ionic salts
in nonpolar solvents. For example,
dicyclohexyl-18-crown-6 complexes
with the potassium ion of potassium
149
crown ethers
c
18-crown-6
dicyclohexyl-18-crown-6 complex
oxygen
CH 2 group
metal ion
Crown ethers
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British chemist and physicist, who in
1861 used *spectroscopy to discover
*thallium and in 1875 invented the
radiometer. He also developed an improved vacuum tube (Crookes’ tube)
for studying gas discharges. Crookes
was also involved in industrial chemistry and realised the importance of
nitrogen Üxation for fertilizers.
crossed-beam reaction A chemical reaction in which two molecular
beams are crossed; one beam is regarded as the incident beam of gas
and the other as the target gas. This
technique enables a great deal of information to be gained about the
chemical reaction since the states of
both the target and projectile molecules can be controlled. The incident beam is characterized by its
incident beam Ûux, I, which is the
number of particles per unit area per
unit time. The scattered molecules
can be detected Ürst by ionizing
them and then detecting the ions
electrically or using spectroscopy if
changes in the vibrational or rotational states of molecules in a reaction are of interest.
cross linkage A short side chain of
atoms linking two longer chains in a
polymeric material.
crown See ring conformations.
crown ethers Organic compounds
with molecules containing large
rings of carbon and oxygen atoms.
The crown ethers are macrocyclic
polyethers. The Ürst to be synthesized was the compound 18-crown-6,
which consists of a ring of six
–CH 2 –CH 2 –O– units (i.e. C 12 H 24 O 6 ).
The general method of naming
crown ethers is to use the form ncrown-m, where n is the number of
atoms in the ring and m is the number of oxygen atoms. Substituted
crown ethers can also be made. The
crown ethers are able to form
strongly bound complexes with
metal ions by coordination through
the oxygen atoms. The stability of
these complexes depends on the size
of the ion relative to the cavity available in the ring of the particular
crown ether. Crown ethers also form
complexes with ammonium ions
(NH 4 +) and alkyl ammonium ions
(RNH 3 +). They can be used for increasing the solubility of ionic salts
in nonpolar solvents. For example,
dicyclohexyl-18-crown-6 complexes
with the potassium ion of potassium
149
crown ethers
c
18-crown-6
dicyclohexyl-18-crown-6 complex
oxygen
CH 2 group
metal ion
Crown ethers
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