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Halogen Chemistry 215

D. Pseudo-halogens B. Chemistry
Pseudo-halogens are not true halogens, but other diatomic Five oxidation states for astatine (−1, 0, +1, +5, and
molecules which contain two relatively electronegative +7) have been definitely established, and a + 3 state has
units (X) that are symmetrically bound together, e.g., been postulated. The aqueous potentials (V, 0.1 M H )
+
cyanogen (NC CN), oxocyanogen (NCO OCN), and are estimated to be as follows (compare Fig. 1):
thiocyanogen (NCS SCN). Like true halogens, the
+0.3 +1.0 +1.5 >+1.6
−
pseudo-halogens reduce to anions X − (CN , OCN ,
−
−
SCN ) and to molecular pseudohalides (e.g., CH 3 CN). At ←−−− At(0) ←−−− HAtO ←−−− AtO ←−−−− AtO .
−
−
−
4
3
They form compounds among themselves (NC SCN)
(51)
and with the halogens. Analogs to the polyhalide ions,
[I(SCN) 2 ] , are also known. In aqueous solution, astatine is usually found in the free
−
In common with the halogens, pseudo-halogens add (0) state. However, unlike the other halogens, astatine(0)
to carbon–carbon double bounds, react with a number is not diatomic. It can be volatilized from solution and ex-
of metals to give salts containing X − ions, and form tracted (like iodine) into organic solvents such as benzene
complexes with metal ions [Co(NCS) 4 ] . Not all com- and carbon tetrachloride. Coextraction with halogens, X 2 ,
2−
parisons are strictly analogous, however. The pseudo- into chloroform produces interhalogen compounds, AtX
−
halogen hydrides, HX, are very weak acids by compar- (X = Cl, Br, I). Reaction with halide ion, X , yields poly-
−
−
ison to the hydrohalic acids (pK a : HCl, −7.4; HCN, halide ions, AtX (X = Br, I) and AtIBr , which can be
2
8.9), and the stabilities of the metal complexes formed extracted into isopropylether.
−
by the halogens and the pseudohalogens tend to differ The astatide ion (At ) is formed by reducing free as-
widely. tatine using moderately strong reducing agents (zinc with
−
acid or SO 2 ). The astatate ion (+5), AtO , is obtained
3
by reaction of lower states of astatine with more powerful
IV
oxidants such as Ce ,S 2 O , and IO . XeF 2 in alkaline
2−
−
8 4
VIII. ASTATINE solution is used to oxidize astatine to perastatate (+7).
A few organic compounds of astatine have been charac-
A. Preparation, Isolation, and Purification terized. Most are prepared by displacement of iodide from
the desired compound. Astobenzene, C 6 H 5 At, is formed
Astatine is the heaviest member of the halogen family.
by reaction of iodobenzene and AtI,
With no stable or long-lived isotopes, it must be synthe-
sized artificially through nuclear reactions. The short half- C 6 H 5 I + AtI → C 6 H 5 At + I 2 , (52)
lives of even the longest-lived isotopes make it extremely
or by reaction between iodobenzene vapor and astatide
difficult to obtain weighable amounts of the element. Al-
ion,
though the identification of some astatine compounds can
−
−
be made directly by mass spectrometry, most investiga- C 6 H 5 I + At → C 6 H 5 At + I . (53)
tions of the chemistry of this element have been carried
A number of normal and iso-alkyl astatides have been
out by tracer techniques using extremely dilute solutions prepared by reaction between the vapor of the analogous
(10 −8 M or less). alkyl iodide and At ion at elevated temperatures. Demer-
−
The chemistry of astatine is usually studied using 211 At.
curiation reactions have produced aromatic aminoacids,
This isotope is prepared by the alpha-bombardment of steroids, and imidazoles containing astatine.
209
Bi using energies in the range of 26 to 28 MeV [Eq. (5)].
The isotope 211 At has a half-life of 7.2 h and decays either
by electron capture (59%) or by alpha-emission (41%). It IX. ORGANIC HALOGEN COMPOUNDS
is removed from the target by distillation in a stream of
nitrogen at a temperature of 200 to 600 C and purified A. General Survey
◦
by redistillation. Alternatively, the target is dissolved in
perchloric acid containing a little iodine, the Bi is removed Organic halogen compounds are derivatives of organic
by precipitation with phosphate, and the aqueous solution compounds in which one or more hydrogen atoms have
of AtI is used as is or the AtI is extracted into an organic been replaced by an equal number of halogen atoms (F,
solvent such as carbon tetrachloride or chloroform. Cl, Br, or I). Almost any class of organic compounds (e.g.,
Further extractions are carried out to obtain the purified alcohols, ketones, carboxylic acids) can contain halogen
product. The isotope 211 At is assayed by counting the atoms. Very few halogenated compounds occur naturally.
alpha-particles or X-rays resulting from its nuclear Most are produced synthetically in the laboratory or as a
decay. result of industrial processes.