Dictionary of Chemistry [6th Ed.]

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 im-

proved vacuum tube (Crookes’ tube)

for studying gas discharges. Crookes

was also involved in industrial chem-

istry and realised the importance of

nitrogenÜxation for fertilizers.

crossed-beam reaction A chemi-

cal reaction in which two molecular

beams are crossed; one beam is re-

garded as the incident beam of gas

and the other as the target gas. This

technique enables a great deal of in-

formation to be gained about the

chemical reaction since the states of

both the target and projectile mol-

ecules can be controlled. The inci-

dent 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 detectedÜrst by ionizing

them and then detecting the ions

electrically or using spectroscopy if

changes in the vibrational or rota-

tional states of molecules in a reac-

tion 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. TheÜrst to be synthe-

sized 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 n-

crown-m, where n is the number of

atoms in the ring and m is the num-

ber 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 avail-

able 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 in-

creasing 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