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Halogen Chemistry 203

The commercial production of iodine depends on the 2F 2 + 2H 2 O → O 2 + 4F + 4H , (13)
+
−
source of the element. From natural brines, the process
although small amounts of hydrogen peroxide (H 2 O 2 ),
is similar to bromine recovery: displacement of iodide
oxygen difluoride (OF 2 ), and ozone (O 3 ) may be formed
−
ion (I ) by chlorine, blowout of the impure iodine, and
as well. In the case of chlorine, the reaction analogous to
repurification by sublimation. Other processes employ io-
Eq. (13) tends to be slow, and the initial result is dispro-
dide precipitation by silver nitrate (AgNO 3 ), separation
portionation (self-oxidation and reduction):
on ion-exchange resins, or oxidation of iodide by sulfuric
+
+
acid (somewhat similar to the original procedure used by Cl 2 + H 2 O → HOCl + H + Cl . (14)
Courtois).
The hypochlorous acid (HOCl) that is formed decomposes
When the naturally occurring source is iodate, the pro-
slowly to give oxygen. Bromine does not react with water
cess typically employs sodium bisulfite reduction to pro-
under acid conditions. In the case of iodine, the partial
duce iodide:
pressure of oxygen in the air is sufficient to oxidize iodide
−
−
−
−
IO + 3HSO → I + 3HSO . (11) ion back to the elemental form [analogous to Eq. (13) in
3 3 4
reverse].
More iodate is added and the solution is acidified:
If base is present, disproportionation is rapid for chlo-
−
−
+
5I + IO + 6H → 3I 2 + 3H 2 O. (12) rine, bromine, and iodine:
3
−
−
−
The iodine precipitates and is purified by sublimation. 2OH + X 2 → XO + X + H 2 O. (15)
Elemental iodine has many commercial uses. Much is Further disproportionation of the XO ion to give XO −
−
3
incorporated into a wide variety of organic compounds may follow:
that find application as catalysts for the manufacture of
−
−
3XO → 2X + XO − (16)
synthetic rubber, as supplements for animal feed, and as 3
−
−
stabilizers, dyes, antiseptics, and photographic chemicals. Reaction (16) is rapid for IO , less rapid for BrO , and
−
even slower for ClO . Consequently, at room temperature,
a basic solution of chlorine gives ClO , while bromine
−
G. Chemical Reactions
gives a mixture of BrO and BrO , and iodine gives IO ,
−
−
−
3 3
Most metals combine directly with all of the halogens Increasing the temperature favors the formation of XO .
−
3
(X 2 ) and react particularly readily with fluorine. Many At 50 to 80 C in base, Br 2 is quantitatively converted to
◦
−
nonmetals also react. The reactivity of fluorine is so great BrO . The formation of ClO in base also becomes fairly
−
3 3
◦
that almost all of these reactions can be initiated at or- rapid above 75 C.
dinary temperatures and will proceed with the evolution Crystalline hydrates or clathrates of chlorine
of considerable heat and light until the fluorine has been (Cl 2 ·7.3H 2 O) and bromine (Br 2 ·8.5H 2 O) are formed
completely consumed. Some metals, such as aluminum, by freezing water in the presence of the halogens. The
nickel, iron, and copper, form protective fluoride coatings hydrogen-bonded, open arrangement of water molecules
that block further reaction unless the temperature is raised. which forms on freezing acts as a trap for the halogen
The reactivity of the halogen decreases with atomic molecules. Iodine also combines with starch to give a
number. Consequently, a halogen of lower atomic num- deep blue color, due to the formation of another complex
−
ber will displace, or oxidize, a halide ion (X ) of higher clathrate in which iodine molecules are trapped within
atomic number [Eq. (7)], both when the ion is in solution the helical structure of the starch molecule. Formation of
and in a crystal lattice. Fluorine (and chlorine to a lesser this blue color is used as a sensitive test for the presence
extent) will also react with both metals and nonmetals to of iodine.
produce higher states of oxidation than do bromine and The halogens are generally more soluble in organic sol-
iodine. Thus, the nonmetal sulfur is converted to SF 6 by vents than in water. Carbon tetrachloride and chloroform
fluorine, to SCl 2 by chlorine, and to S 2 Br 2 by bromine. readily extract chlorine, bromine, and iodine from water
Compounds with S I bonds are known, but sulfur iodides to give yellow-, red-, and violet-colored solutions, respec-
cannot be prepared by direct reaction of the elements. The tively.
highest thermally stable states resulting from the halo- Chlorine, bromine, and especially iodine react with
genation of the metal rhenium are ReF 7 , ReCl 6 , ReBr 5 , Lewis bases such as benzene, alcohols, ethers, ketones,
and ReI 4 . and amines to give highly colored solutions that are char-
All of the halogens are soluble to some extent in water, acteristic of the base and halogen. Part of the energy as-
but they may also react to give a number of products de- sociated with the halogen-base bond is attributable to a
pending on the temperature and the amount of acid or base transfer of charge between the donor atom and the halo-
present in the solution. Fluorine rapidly oxidizes water to gen, thus giving the name charge-transfer complex to the
give mainly oxygen and fluoride ion, species which is produced.