Page 144 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN007C-307 June 29, 2001 19:40

Halogen Chemistry 221

CFCs and their partially hydrogenated derivatives are Larger polychlorinated molecules have been employed
identified by a series of numbers related to the formula of as insecticides—aldrin, DDT, chlordane, dieldrin, endrin,
the compound. From right to left, the first digit gives the gammexane, and isodrin, for example. One of the earli-
number of fluorine atoms, the second digit is one more est was DDT (dichlorodiphenyltrichloroethane), which is
than the number of hydrogen atoms, and the third digit is prepared by the condensation of chlorobenzene and chlo-
one less than the number of carbon atoms. Zero is always ral in the presence of sulfuric acid (H 2 SO 4 ):
omitted. Any remaining positions are assumed to be occu-
pied by chlorine atoms. Isomers may be given letter des- 2C 6 H 5 Cl + CCl 3 CHO → (p − ClC 6 H 4 ) 2 CHCCl 3 + H 2 O.
ignators. Examples of this numbering system are CFC-12 (79)
(CCl 2 F 2 ), CFC-113 (C 2 Cl 3 F 3 ), HFC-134a (CH 2 F CF 3 ), Other polychlorinated molecules function as herbi-
and HCFC-123 (C 2 HCl 2 F 3 ). If bromine is also present, cides: 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T
the compounds are called Halons. The first digit on the (2,4,5-trichlorophenoxyacetic acid), diquat, paraquat, and
right represents the number of bromine atoms, the sec- Teklon. Many of these compounds are no longer used be-
ond digit gives the number of chlorine atoms, the third cause their stability in the environment allows them to
gives the number of fluorine atoms, and the fourth gives accumulate in the fat of fish, birds, and mammals.
the number of carbon atoms. In this system, zero is des- Another example of industrially useful polychlorinated
ignated. Examples are Halon-1211 (CBrClF 2 ) and Halon- molecules that have been found to have long-term delete-
2402 (C 2 Br 2 F 4 ). rious environmental and health effects are the polychlo-
rinated biphenyls or PCBs: Aroclor, Clophen, Fenclor,
Kanechlor, Phenoclor, Pyralene, and Santotherm. (Com-
G. Other Halogenated Organics:
Uses and Hazards mercial PCBs are mixtures.) These substances have been
listed as carcinogens by the U.S. Environmental Protec-
Commercial applications for halogenated organic com- tion Agency, but they were once widely used as hydraulic
pounds have been mentioned throughout this article, CFCs fluids, plasticizers, adhesives, fire retardants, lubricants,
and their derivatives were specifically discussed in the pre- and in carbonless reproducing paper. They are still found
ceding section. Many other organic halogen compounds in older electrical capacitors and electrical transformers
have direct commercial applications, or are used in in- and are widely identified in the environment and chem-
dustry or the laboratory to synthesize other compounds ical dump sites. Newer transformers and capacitors use
with useful properties, such as dyes and medicinal agents. alternative fluids.
While it would be impossible to discuss thoroughly the Completely fluorinated polymers have low intermole-
wide range of industrial applications for halogenated or- cular forces, resulting in a low coefficient of friction
ganic compounds, some mention should be made of a few and excellent lubricating properties. The small fluorine
of the more important uses. Again, some of these com- atoms replace hydrogen atoms with very little distortion
pounds,especiallythosecontainingchlorine,arenolonger of the molecule and shield the carbon atoms from
employed as extensively as they once were because they chemical attack. As a result, polyfluorinated polymers
are toxic or their presence represents an environmental have exceptional thermal stabilities as well as lubricating
hazard. properties. Some of the most commercially important are
Small polychlorinated alkanes and alkenes have been the high-molecular-weight fluoroplastics and elastomers
widely used as solvents and fire retardants (carbon tetra- derived from tetrafluoroethylene, hexafluoropropylene,
chloride, CCl 4 , and several Halons: 1211, 1301, and 2402) vinylidene fluoride, and vinyl fluoride. The fluori-
and anesthetics (chloroform, CHCl 3 , mixed with acetone nated plastic, Teflon PTFE ® [polytetrafluoroethylene,
and ether). However, carbon tetrachloride can form phos- (CF 2 CF 2 ) n ], has a service temperature from –196
◦
gene (COCl 2 ), a very poisonous gas, when heated in the to 260 C and is used to pack bearings in motors.
®
presence of oxygen, and its use in fire extinguishers has Other important fluoropolymers include Teflon FEP (a
been discontinued. Chloroform, now known to cause liver plastic copolymer of tetrafluoroethylene and hexafluo-
damage has been replaced by the anesthetic halothane ropropylene), Kel-F ® [ (CF 2 CFCl) n ], and Viton ®
(CF 3 CHBrCl). Halothane has the advantages of non- (an elastomeric terpolymer of hexafluoropropylene,
flammability, high anesthetizing power, and a general lack tetrafluoroethylene, and vinylidene fluoride having
of postnarcotic effects. Toxic effects are possible if it is excellent thermal, chemical, and oxidative stability).
not properly administered. Ethyl chloride (CH 3 CH 2 Cl), The lower-molecular-weight fluoropolymers are liquid
once widely used for the manufacture of tetraethyllead, and are employed as lubricants. The monomer, carbon
is now used to some extent as an ethylating agent in the tetrafluoride (CF 4 ), is used as a degreasing agent for the
synthesis of dyes and fine chemicals. plasma etching of semiconductor devices.