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220 Halogen Chemistry

electron-withdrawing properties of the halogens. Such Finally, CFC accumulation in the lower atmosphere could
species readily form organometallic compounds by hydro- also contribute to global warming through the greenhouse
gen–metal exchange. Bromine and iodine on the ring are effect.
easily replaced by active metals to form organometallics. In 1989, almost all nations of the world agreed to phase
Chlorine reacts very slowly, while fluorine is inert to out CFC production by the year 2000. Then, following
substitution by metals. increasingly alarming evidence for ozone depletion, this
Halogens on alkyl side chains generally resemble halo- agreement was amended in 1990 and again in 1992, set-
gens in alkyl halides. As with alkyl halides, preparation ting an advanced deadline for complete, rather than par-
is typically carried out by (1) substitution of side-chain tial phase-out of CFC production. December 31, 1995,
hydroxyl groups or (2) addition of HX or X 2 to side-chain marked the end of production of virtually all CFCs in the
olefins. Free-radical reaction with halogen molecules, X 2 , industrialized world. There are two exceptions to a CFC
yields a mixture of products and works best for chlorina- ban in the United States: (1) use as a propellant in in-
tion or bromination. halers for asthma patents, and (2) use in the manufacture
If the halogen is attached to the carbon which links of methyl chloroform to clean O-ring seals in the Space
the alkyl group to the ring system (e.g., Ar CX 2 CH 3 ), Shuttle.
the halogen is somewhat more susceptible to nucleophilic HCFCs (hydrochlorof luorocarbons) and HFCs (hydro-
attack. The degree of reactivity is intermediate between f luorocarbons) have been developed as reasonable, but
that of the corresponding alkyl and allyl halide. When probably temporary, substitutes for CFCs. The hydrogen
halogens are not attached to the carbon atom adjacent to atoms in these substitutes make them subject to oxida-
the benzene ring, the reactivity of the halogen is essentially tion in the lower atmosphere, thus theoretically less able
that of the analogous alkyl halide. to carry chlorine up to stratospheric ozone. However, the
identity and effects of the Cl-containing decomposition
products of HCFCs are still incompletely known, and a
F. CFCs and Their Substitutes
portion of HCFC emissions and their degradation products
The acronym CFC (chlorof luorocarbon) is used to des- may still reach the stratosphere, giving these compounds
ignate perhalogenated derivatives of methane (CH 4 ) and some potential for ozone depletion. While HCFCs are re-
ethane (CH 3 CH 3 ) that contain both chlorine and fluo- placing CFCs in a number of commercial applications,
rine. As a class, these compounds are unusually stable with HCFCs are also scheduled for total phase-out by 2030.
very low toxicities. Such properties—plus high densities The chemistry required to make HCFC and HFC substi-
and large heats of vaporization, low boiling points, vis- tutes for CFCs is typically two to four times more complex
cosities, and surface tensions, nonflammability, and lack than for CFC production. While simple HCFCs and HFCs
of odor—make CFCs uniquely suitable as refrigerants, can be prepared by halogen exchange,
propellants, blowing agents, and solvents.
HF
CFCs have been among the most effective and widely CHCl 3 −→ CHCl 2 F + CHClF 2 + CHF 3 , (78)
used synthetic organic compounds in modern society.
However, in a series of satellite observations made from the one-carbon products have unacceptably low boiling
1987 to 1994, the Cl-containing decomposition products points. To match CFCs in physical properties requires
of these compounds wer shown to play a significant role in substitute compounds that contain two carbon atoms. The
reducing the earth’s stratospheric ozone level. On a global two-carbon requirement tends to increase production costs
basis, the ozone layer has shrunk an average of about 3% for a number of reasons. First, more steps, generally in-
since 1978, with the greatest depletions occurring in the volving halogen addition/elimination as well as substitu-
polar regions, especially over the Antarctic. tion, are usually required. This produces more complex
Loss of stratospheric ozone has two major detrimen- mixtures, increasing separation difficulties. To make mat-
tal effects. First, ozone acts as a natural shield against ters worse, hydrogen fluoride, which is present in both
biologically damaging ultraviolet radiation from the sun. addition and exchange reactions used to prepare the final
Epidemiological studies suggest that each 1% depletion product, tends to form hard-to-manage azeotropes with
of ozone might lead to a 1–3% increase in skin can- many HFCs. Finally, because HFCs and HCFCs are de-
cer among fair-skinned people. Other effects include in- signed to be less stable than CFCs, they tend to decom-
creased susceptibility to eye injuries and suppression of pose on catalyst surfaces, deactivating the catalyst. In two-
the immune system. Second, the absorption of solar en- carbon HCHC systems, a significant amount of HCl may
ergybystratosphericozoneisamajorsourceofheatforthe be eliminated on some catalyst surfaces, producing toxic
upper atmosphere. Alteration of stratospheric temperature side products which must also be removed from the final
could significantly alter global wind systems and climate. product.