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206 Halogen Chemistry

+
−
3HF → H 2 F + HF , (26) UO 2 + CCl 4 → UCl 4 + CO 2 . (29)
2
The range of applicability of the method, the reaction con-
occurs to a greater extent than the analogous reaction for
ditions, and the products obtained from the reaction will
water. Most inorganic fluorides dissolve in HF to give vary from halogen to halogen.
−
HF ions, which have an anomalously high conductance
2
and make the resulting solutions quite conductive. Inor-
B. Classification
ganic compounds other than fluorides generally react with
the solvent to produce the corresponding fluoride. Liquid Binary halides can be grouped into two very broad cate-
HF is also used in biochemical research to solubilize cer- gories: ionic and covalent. In ionic halides, the halogen ac-
tain carbohydrates and complex proteins that dissolve with quires the electronic configuration of the nearest noble gas
only minor chemical changes. by appropriating one electron from atom M. Both atoms
Acid–base and oxidation–reduction reactions in HF thenbecomechargedions.Incovalenthalides,thebonding
have been thoroughly studied. As an acid, HF will readily electrons are more shared than transferred, and the degree
protonate water, alcohols, carboxylic acids, and other or- of charge separation between M and X is characteristically
ganic solutes having unshared pairs of electrons. Because less. There is an almost continuous gradation in the nature
the tendency for oxidation of fluoride to fluorine is very of the bonds formed in inorganic halides, from nearly com-
low, many inorganic and organic compounds can be flu- pletely ionic to nearly completely covalent. This gradation
−
orinated by the electrochemical insertion of F using the is reflected in the chemical and physical characteristics of
HF solvent system. the resulting compounds.
The other hydrogen halides, with lower boiling points, The ionic halides can be distinguished by their higher
shorter liquid ranges, and negligible self-dissociation, are boiling and melting points, lower volatilities, and greater
less useful as solvents. Considerably lower dielectric con- degree of conductivity when fused. For the same metal
stants, as compared to HF, also mean that these com- cation, boiling and melting points generally decrease
pounds do not, in general, solubilize ionic compounds down the group (fluorid > chloride > bromide > iodide),
very readily. Nevertheless, work with HCl has indicated reflecting a gradual increase in covalent character. Most
that this solvent system provides a convenient medium for ionic halides dissolve in water to form hydrated metal ions
the preparation of a number of chlorinated or protonated and halide ions. For the same metal cation, halide solu-
inorganic and organic products. bility generally increases down the group (fluoride <
chloride < bromide < iodide), with dissolution of the
smaller fluoride ions being less favored because of a
III. OTHER INORGANIC HALIDES higher crystal lattice energy (the energy holding the ions
together in the solid state).
A. Preparation The covalent halides are often called acid halides be-
cause they react with water to give hydroxy compounds
The halogens form compounds of the type M a X b with
that are acidic. Reactions with other Lewis bases (elec-
most elements (M) in the Periodic Table. The method of
tron donors) such as alcohols, ammonia, and substituted
synthesis depends on the properties of the desired product.
amines also occur. Covalent halides are generally more
If hydrolysis (reaction with water) is not a problem, then
soluble in nonpolar solvents such as benzene and carbon
dissolution of the element [Eq. (24)] or its oxide,
tetrachloride.
MgO + 2HCl → MgCl 2 + H 2 O, (27) There are several factors which can be used to predict
the degree of ionic or covalent character in a halide. First,
carbonate [Eq. (25)], or hydroxide in aqueous hydrohalic ionic character is markedly increased by an increase in the
acid usually provides a convenient route to the appropriate electronegativity difference between M and X. The halides
halide. If the product is unstable in water, direct reaction of the Group IA and IIA metals, which have especially low
of the element M with the halogen, electronegativities, as well as those of the lanthanides and
actinides, tend to be ionic. Halides of metals with higher
S + 3F 2 → SF 6 , (28)
electronegativities, such as those of many transition met-
or with the anhydrous hydrogen halide [Eq. (24)] may be als, exhibit more covalent character. Halides of nonmetals,
employed. which have the highest electronegativities, are predomi-
There are numerous alternative routes, including halo- nantly covalent.
gen displacement [as in Eq. (7)] and high-temperature Ionic character is also affected by the total charge
halogenation of metal oxides in the presence of carbon present on M and the size of the halogen. If a given
or carbon tetrachloride: metal can have more than one oxidation state, the lowest