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

Halogen Chemistry 209

Polyhalonium cations are also known, with iodine- The compound is a powerful oxidizing and fluorinating
containing cations being the most numerous and best char- agent, reacting with many metals to give oxides and
acterized. There is no evidence for simple salts contain- fluorides. It is readily hydrolyzed by base, reacts slowly
+
+
ing an X ion; however, X (X = Cl, Br) as well as the with water, and liberates other halogens from their acids
3
+
mixed species XY and XYZ (X, Y, Z = Cl, Br, or I) or salts.
+
2
can be formed by reaction of halogens or interhalogens
with strong oxidants. The halogen or interhalogen may
B. Chlorine Oxides
also serve as the oxidant. Thus, gold dissolves in BrF 3
+
to give BrF and AuF ions. Solutions containing these Chlorine forms a number of well-known oxides
−
2 4
cations are frequently highly colored. Penta- and hep- (Table VII) which are generally unstable and prone to
taatomic cations are also known (ClF , BrF , ClF , and explosion. Only ClO 2 and Cl 2 O are employed to any
+
+
+
4 4 6
IF , for example). extent commercially, where they are used to make chlori-
+
6
The halogens, X 2 (X = Cl, Br, I), dissolve in strongly nated solvents and chemicals for water treatment, or func-
oxidizing solvents such as oleum (a solution of SO 3 in tion as important bleaching agents for wood pulp and
concentrated H 2 SO 4 , also called fuming sulfuric acid) to textiles.
+
+
+
give cationic species X . Both Br and I can be iso- Dichlorine oxide (Cl 2 O) is prepared commercially by
2 2 2
+
lated as salts, but the presence of the Cl ion has, so far, several methods, including the reaction of Cl 2 gas with
2
only been inferred from spectroscopic data. Other halogen moist sodium carbonate:
+
+
+
cations include X (X = Cl, Br, I), I , and X (X = Br and
4
3
5
I only). 2Cl 2 + 2Na 2 CO 3 + H 2 O → 2NaHCO 3
+ 2NaCl + Cl 2 O(g). (36)
The gas is not used directly because of a tendency to ex-
V. HALOGEN OXIDES
plode when heated or sparked. However, since Cl 2 O is the
anhydride of hypochlorous acid (HOCl), it is easily con-
A. Oxygen Fluorides
verted to hypochlorite salts, such as Ca(OCl) 2 , by treat-
Compounds of oxygen and fluorine are more properly ment with aqueous base. These salts are more stable and
called oxygen fluorides, because the electronegativity of safer to handle.
fluorine is greater than that of oxygen. Several oxygen flu- Chlorine dioxide (ClO 2 ) may be prepared by reduction
orides are known (Table VII), the most stable being OF 2 of sodium chlorate with sulfur dioxide in strongly acidic
and the least stable, O 4 F 2 . Reasonably stable fluorinated solution:
peroxides have also been characterized.
OF 2 , a colorless gas at room temperature, can be pre- 2NaClO 3 + SO 2 + H 2 SO 4 → 2ClO 2 + 2NaHSO 4 . (37)
pared by passing F 2 gas rapidly through a 2% aqueous
It also has a tendency to explode if present in concentrated
sodium hydroxide solution:
amounts and must be carefully prepared at the point of
2F 2 + 2NaOH → OF 2 + 2NaF + H 2 O. (35) use. ClO 2 is soluble in water, forming solutions which
−
decompose to chloride and chlorate (ClO ) when exposed
3
−
to light. In base, a mixture of chlorite (ClO ) and chlorate
TABLE VII Halogen Oxides 2
is formed.
Fluorine Chlorine Bromine Iodine a As a gas, chlorine dioxide decomposes thermally and in
the presence of ultraviolet light to produce the short-lived
OF 2 Cl 2 O Br 2 O
chlorine oxide radical, ClO. This same radical (a species
O 2 F 2
Cl 2 O b 3 with an unpaired electron) is also produced by the pho-
ClO 2 BrO 2 tolysis of chlorofluorocarbons such as CFCl 3 and CF 2 Cl 2
Br 2 O 3 and has been implicated in reactions leading to depletion
O 4 F c 2 Cl 2 O 4 I 2 O 4 of ozone in the earth’s upper atmosphere (Section IX.F).
I 2 O 5
Cl 2 O d 6
C. Bromine and Iodine Oxides
Cl 2 O 7
I 4 O 9
There are only three reasonably well-established oxides of
a Decompose on heating.
b Explodes even below 0 C. bromine. All three have low thermal stabilities and have
◦
c Unstable even at −150 C. not been extensively investigated. Of the oxides of iodine
◦
d Dimer in equilibrium with ClO 3 . (Table VII), I 2 O 5 is the most stable and useful.