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Halogen Chemistry 213

Most commercial perchloric acid is used to manufac- rigid systems, the reaction is specific for cis-functional
ture perchlorate salts. Perchlorates of almost all metals are groups, making periodates extremely useful in natural
known, and except for potassium, rubidium, and cesium, product, carbohydrate, and nucleic acid chemistry.
they are quite soluble in water. The perchlorates of greatest Unlike perchlorates, periodates form many stable com-
−
industrial importance are NH 4 ClO 4 (a solid rocket propel- lexes with transition metals in which the IO unit is co-
6
lent), Mg(ClO 4 ) 2 (an electrolyte in dry-cell batteries), and ordinated to the metal through two oxygens rather than
KClO 4 (used in fireworks and fiares). only one. Coordination involving periodate sometimes
Perchlorate salts have been widely used in laboratory stabilizes unusual metal oxidation states such as Ni(IV),
studiestoadjustthetotalelectrolyteconcentrationbecause Cu(III), and Ag(III).
the anion was thought to be noncoordinating. Thus, addi-
tion of perchlorates in almost any amount could be made VII. OTHER INORGANIC
without affecting the identity of complex ions also present HALOGEN COMPOUNDS
in the solution. This assumption is now known to be incor-
rect, and coordinated perchlorate has been identified in a A. Oxyfluorides
number of cases involving both main group and transition
There are a number of halogen oxyfluorides (Table VIII)
metal ions. The donor ability of perchlorate is quite weak,
in which X (Cl, Br, or I) is bonded to both O and F,
however, and the use of this ion as an inert electrolyte in
forming compounds of the general formula, F n XO m . The
aqueous solution is still widespread.
center halogen is always X, and the chemistry of these
Perbromate can be prepared only using the most pow-
compounds resembles that of the halogen fluorides, XF n .
erful oxidizing agents, such as fluorine gas and XeF 2 , or
Most are very powerful oxidizing and fluorinating agents
electrolysis. The best synthesis involves oxidation of bro-
and react with fluoride donors and acceptors to give mixed
mate by F 2 gas in 5 M sodium hydroxide under carefully
controlled conditions: oxyfluoro anions and cations.
Five chlorine oxyfluorides have been definitely char-
−
−
−
−
BrO + F 2 + 2OH → BrO + 2F + H 2 O. (47)
3 4 acterized (Table VIII). Their stabilities range from FClO,
Solutions of HBrO 4 can be concentrated up to 6 M with- which decomposes with a half-life of about 25 at room
out decomposition and are stable for prolonged periods. temperature, to FClO 3 (perchloryl fluoride), which is ther-
◦
Some perbromate salts have been isolated and appear to be mally stable to about 400 C.
reasonably stable. Even NH 4 BrO 4 can be heated to around The highly unstable FClO is prepared by hydrolysis of
ClF 3 or photolysis of a mixture of ClF 3 and ozone in Ar at
◦
170 C.
very low temperatures. F 3 ClO is prepared by fluorination
Periodates are made by oxidation of iodide, iodine, or
of Cl 2 O using F 2 gas in the presence of NaF or some
iodate in aqueous solution. The industrial process involves
other metal fluoride. It is a very powerful oxidizing and
the electrolytic oxidation of NaIO 3 , or oxidation of sodium
fluorinating agent, reacting slowly at room temperature,
chlorate using chlorine gas.
but rapidly when heated or irradiated with ultraviolet light.
−
IO + 6OH + Cl 2 → IO 5− + 2Cl + 3H 2 O. (48)
−
−
3 6 FClO 2 is prepared by fluorination of ClO 2 using ClF 3 :
The product is actually dihydrogen orthoperiodate, 6NaClO 3 +4ClF 3 → 6FClO 2 +6NaF+2Cl 2 +3O 2 . (49)
Na 3 H 2 IO 6 , the parent acid being H 5 IO 6 .
The chemistry of periodates is complicated by the num- It is also a good fluorinating agent and a moderately strong
ber of species which are formed by deprotonation, de- oxidizing agent, but it tends to explode in the presence of
hydration, and aggregation of the parent acid, H 5 IO 6 . In strong reducing agents.
aqueous solution, periodic acid exists primarily as the IO −
4
ion plus lesser amounts of several orthoperiodates such TABLE VIII Neutral Halogen Oxyfluorides
−
as H 5 IO 6 (the parent acid), H 4 IO (deprotonation of the Oxidation
6
parent), and H 3 IO 2− (further deprotonation). An increase state a Chlorine Bromine Iodine
6
in pH results in progressive deprotonation, dehydration,
+3 FClO
and dimerization, the principal species being IO , H 4 IO ,
−
−
4 6 +5
3−
H 3 IO ,H 2 IO , and H 2 I 2 O . FClO 2 FBrO 2 FIO 2
2−
4−
6 6 10 F 3 ClO F 3 BrO F 3 IO
Most periodate salts, other than those with the IO ion,
−
4 +7
contain an IO unit of some sort. Periodates are good ox- FClO 3 FBrO 3 FIO 3
−
6
idizing agents, especially in acid solution. They are com- F 3 ClO 2 F 3 IO 2
F 5 IO
monly used in organic chemistry to cleave adjacent func-
tional groups such as alcohols, ketones, and amines. In a Central halogen, Cl, Br, or I.