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

214 Halogen Chemistry

The synthesis of F 3 ClO 2 involves a sequence of has oxidation states ranging from +2to +8. No stable
fluorine-transfer reactions starting with FClO 2 .F 3 ClO 2 is compounds of He, Ne, or Ar have ever been found. Some
extremely reactive, but forms a stable anion, [F 2 ClO 2 ] − evidence for a difluoride of radon has been obtained from
by fluoride transfer to acceptors such as BF 3 . The re- radiochemical tracer studies. Of the other halides, XeCl 2 ,
maining chlorine oxyfluoride, FClO 3 , can be prepared by XeBr 2 , and XeCl 4 have been detected as the nuclear de-
the electrolysis of sodium perchlorate in anhydrous HF. cay products of their 129 I analogs using M¨ossbauer spec-
However, the most convenient route for industrial-scale troscopy.
manufacture involves the fluorination of a perchlorate The three xenon fluorides, XeF 2 , XeF 4 , and XeF 6 , can
salt using antimony pentafluoride (SbF 5 ) in the presence all be prepared by direct reaction of xenon and fluorine
of HOSO 2 ForHF: gases under pressure, but conditions must be carefully
controlled to obtain the desired product. Xenon tetrafluo-
KClO 4 +2HF+SbF 5 → FClO 3 +KsbF 6 +H 2 O. (50)
ride is produced in nearly quantitative amounts when a 1:5
By comparison to the other chlorine oxyfluorides, perchlo- mixture of Xe and F 2 is heated to 400 C under 6 atm pres-
◦
ryl fluoride is a mild fluorinating agent and has been used sure. Xenon difluoride is formed by the same procedure if
for the preparation of fluorinated steroids. FClO 3 also re- xenon is in excess. The preparation of XeF 6 requires 1:20
◦
acts with phenyl lithium to form the unstable perchloryl volume mixtures of xenon and fluorine at 250–300 C and
benzene, C 6 H 5 ClO 3 , and lithium fluoride. 50–60 atm pressure.
Bromine oxyfluorides have not been as thoroughly stud- The reaction of Xe and F 2 can also be induced at room
ied as their chlorine analogs, but appear to be less ther- temperature by any source of energy capable of disso-
mally stable and somewhat more reactive chemically. ciating the F 2 molecule, such as ultraviolet light, ioniz-
Bromyl fluoride, FBrO 2 , and perbromyl fluoride, FBrO 3 , ing radiation, and electrical discharges. Pure XeF 2 can be
hydrolyze readily in water and even attack glass. prepared by the simple action of sunlight on a glass bulb
The iodine oxyfluorides tend to be polymeric, rather containing a mixture of xenon and fluorine.
than monomeric. Unlike chlorine, no oxyfluoride com- The reactivity of the xenon fluorides increases with the
poundwithiodineinthe3+oxidationstateisknown;how- number of fluorine atoms in the molecule. XeF 2 is stable
ever, iodine oxide pentafluoride, F 5 IO, (but not F 5 ClO) has in water if acid is present, although the resulting solution
been characterized. F 5 IO does not hydrolyze readily and is a powerful oxidizing agent. When base is added, hydrol-
is obtained when IF 7 reacts with water, silica, glass, or ysis produces Xe, HF, and O 2 . XeF 4 hydrolyzes instantly
I 2 O 5 . in water, and XeF 6 is so reactive that it fluorinates silica
and cannot be handled in glass containers. The hydroly-
sis of the two higher fluorides produces XeO 3 , a highly
B. Derivatives of Oxyacids
explosive compound.
Replacement of hydrogen in oxyacids such as perchloric, XeF 2 is a mild fluorinating agent and useful for the ad-
fluorosulfuric, or nitric, by a halogen yields compounds dition of fluorine to carbon–carbon double bonds and for
of the type XOClO 3 , XOSO 2 F, and XONO 2 where X aromatic fluorination. XeF 4 is more reactive, but some-
−
equals F, Cl, Br, and I in the +1 oxidation state. In ad- what less useful. XeF 2 is a good F donor, readily form-
dition, derivatives such as X(ONO 2 ) 3 and X(OSO 2 F) 3 as ing [XeF] and the [Xe 2 F 3 ] dimer. Only XeF 6 is a good
+
+
well as metal (M) complexes of the type M[X(ONO 2 ) 2 ], F − acceptor, reacting with Group 1 fluorides to form
M[X(ONO 2 ) 4 ], and M[X(OSO 2 F) 4 ] are known, where X compounds with the formula MXeF 7 (M = Rb, Cs) and
equal Br and I in the 3+ oxidation state. M 2 XeF 8 (M = Na, K, Rb, Cs).
Most of these compounds must be prepared at low tem- Three stable oxyfluorides have been characterized:
peratures and have a tendency to explode. In general, the XeOF 4 and XeO 2 F 2 with Xe in the +6 oxidation state,
thermal stability decreases with increasing atomic number and XeO 3 F 2 with Xe in the +8 oxidation state. These are
of the halogen. The halogen nitrates are less stable than obtained by controlled hydrolysis of xenon fluorides or
the perchlorates, while the fluorosulfates form the most by fluorination of xenon trioxide. The oxyfluoro anions,
−
−
stable derivatives of all. [XeO 3 F] and [(XeOF 4 ) 3 F] result when the hydrolysis
products of XeO 3 and XeOF 4 are treated with F .
−
Krypton difluoride, KF 2 , is obtained by passing an elec-
C. Compounds of the Noble Gases
◦
tric discharge through Kr and F 2 at −183 C, or by irra-
Prior to 1962, the noble gases were thought to be com- diation of these two gases using high-energy electrons or
pletely inert because of the stability of their electron con- protons. KF 2 is a volatile, white solid that decomposes
figurations. A variety of fluoride, oxide, and oxyfluoride slowly at room temperature. Radon has only a short half-
compounds and ions are now known for xenon (with a life ( 222 Rn, 3.825 days), and evidence for RnF 2 and RnF +
less extensive series for krypton), in which the noble gas is obtained from tracer studies.