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Halogen Chemistry 199

acid (HF). For this achievement, among others, Moissan TABLE I Important Natural Sources for the Halogens
was awarded the Nobel Prize for Chemistry in 1906. The
Element Source/remarks
first purely chemical synthesis of elemental fluorine was
not achieved until 100 years after Moissan’s electrochem- Fluorine Fluorite or fluorspar (CaF 2 ), cryolite (Na 3 AlF 6 ),
ical synthesis. In 1986, K. O. Christe utilized the principle fluorapatite (calcium hydroxyphosphate where
some hydroxide is replaced by fluoride)
that certain unstable transition metal fluorides can be sta-
Chlorine Normal ocean water, other natural brines, halite
bilized in the form of their corresponding MF 2− anions,
6 (NaCl), sylvite (KCl), carnallite (MgCl 2 , KCl,
6H 2 O), kainite (MgSO 4 , KCl, 3H 2 O)
50% aqueous HF
2KMnO 4 + 2KF + 10HF + 3H 2 O 2 −−−−−−−→ Bromine Normal ocean water, other natural brines, bromide
impurities in carnallite and in some other less
2K 2 MnF 6 (s) + 8H 2 O + 3O 2 , (2)
abundant minerals such as bischofite (MgCl 2 ,
6H 2 O) and tackhydrite (CaCl 2 , 2MgCl 2 , 12H 2 O)
to produce elemental fluorine in high yield by a very sim-
Iodine Some natural brines (especially from oil wells),
ple reaction,
lautarite [Ca(IO 3 ) 2 ], dietzeite [7Ca(IO 3 ) 2 ,
K 2 MnF 6 + 2SbF 5 → 2KSbF 6 + [MnF 4 ] (3) 8CaCrO 4 ], seaweed (Laminaria family)
Astatine None, produced artificially
2[MnF 4 ] → 2MnF 3 + F 2 (g). (4)
The underlying principle behind this synthesis is the dis-
placement of a weaker Lewis acid, MnF 4 , from its MnF 2− 235
6 ments such as U. The total amount of astatine present
salt by the stronger Lewis acid, SbF 5 . Since the liberated on earth at any given time is estimated to be less than
MnF 4 isthermodynamicallyunstable,itspontaneouslyde- one ounce, making astatine the rarest naturally occurring
composes to the lower fluoride, MnF 3 , with the simulta- terrestrial element.
neous evolution of elemental fluorine. Table II compares the distribution of the halogens in
Astatine (Greek astatos, “unstable”) has no stable or various locations on earth, and on the moon and in the
long-lived isotopes. It was first prepared by D. R. Corson, solar system. A few entries deserve special comment.
K. R. MacKenzie, and E. Segr´e in 1940 by alpha bom- Airborne particulates typically contain increased con-
bardment of Bi-209 in a cyclotron: centrations of halogens near the ocean, in polluted areas,
209 4 211 1 and in regions of volcanic activity. Fluoride concentra-
2
0
83 Bi + He → 85 At + 2 n . (5)
tions are increased near aluminum and steel mills and
Some 27 isotopes of astatine are now known. The longest phosphate-fertilizer plants. Bromide concentrations were
lived has a half-life of only 8.1 h, and the largest prepara- onceincreasednearurbanareasasaresultoftheburningof
tions of the element to date have given less than a micro- gasoline additives (ethylene dibromide, a lead scavenger),
gram. but environmental legislation has resulted in a decline of
this chemical’s utilization. The enrichment of iodine in
marine air as compared to sea water has been attributed
B. Occurrence
to concentration of the element in organic surface films.
Except for astatine, the halogens occur rather widely in na- Iodine-enriched material then enters the atmosphere in the
−
ture in the form of the halide (X ) anion, with iodine also form of spray.
−
occurring as iodate (IO ). Due to the great solubility of Exceptformethylchloride(CH 3 Cl),mostoftheorganic
3
most halide salts, large quantities of chloride and bromide gases containing halogens are man-made. The chloroflu-
are concentrated in the ocean and in other natural brines. orocarbons (Section IX.F), which are particularly stable
There are also more than 70 minerals containing a halogen and can remain in the atmosphere for between 50 and 100
as the sole or principal anionic constituent, although only a years, have been implicated in the depletion of the earth’s
few of these are common. Most of the halogen-containing ozone layer and in global warming (the greenhouse effect).
minerals are formed in one of four ways: (1) saline deposi- Inorganic gases containing fluoride are detected only near
tion by evaporation of sea water or other natural brines; (2) polluted areas and in regions of volcanic activity. Hydro-
hydrothermal deposition; (3) surface alterations of silver, gen chloride is frequently found as a major component of
copper, lead, or mercury ores; and (4) deposition by sub- acid rain.
limation near regions of geothermal or volcanic activity. In the ocean, the halogens form part of a group of el-
Table I summarizes some of the more important terrestrial ements classified as biounlimited. Elements in this group
sources for the halogens. have concentration ratios that are constant in samples
Minute amounts of astatine have been detected in na- from all locations, from surface level to deep water. By
ture as a steady-state population produced from the decay measuring the concentration of one element in this group,
of certain long-lived, naturally occurring radioactive ele- the composition of all other biounlimited components can