Uranium tetrachloride is an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
, a salt of uranium and chlorine, with the formula UCl
4. It is a hygroscopic olive-green solid. It was used in the electromagnetic
isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
(EMIS) process of
uranium enrichment
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 ...
. It is one of the main starting materials for
organouranium chemistry.
Synthesis and structure
Uranium tetrachloride is synthesised generally by the reaction of
uranium trioxide
Uranium trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO3, is a ...
(UO
3) and
hexachloropropene. Solvent UCl
4 adducts can be formed by a simpler reaction of UI
4 with
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride g ...
in organic solvents.
According to
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
the uranium centers are eight-coordinate, being surrounded by eight chlorine atoms, four at 264 pm and the other four at 287pm.
Chemical properties
Dissolution in protic solvents is more complicated. When UCl
4 is added to water the uranium
aqua ion is formed.
:UCl
4 + H
2O →
2O)">(H2O)sup>4+ + 4Cl
−
The aqua ion
2O)">(H2O)sup>4+, (x is 8 or 9) is strongly hydrolyzed.
:
2O)">(H2O)sup>4+
2O)(OH)">(H2O)(OH)sup>3+ + H
+
The
pKa for this reaction is ca. 1.6,
[IUPAC SC-Database]
A comprehensive database of published data on equilibrium constants of metal complexes and ligands so hydrolysis is absent only in solutions of acid strength 1 mol dm
−3 or stronger (pH < 0). Further hydrolysis occurs at pH > 3. Weak chloro complexes of the aqua ion may be formed. Published estimates of the log K value for the formation of
Clsup>3+(aq) vary from −0.5 to +3 because of difficulty in dealing with simultaneous hydrolysis.
[
With alcohols, partial ]solvolysis
In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chiral reactant affords the racemate. Sometimes however, the ste ...
may occur.
:UCl4 + ROH UCl(OR) + HCl
Uranium tetrachloride dissolves in non-protic solvents such as tetrahydrofuran
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is m ...
, acetonitrile
Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile ( hydrogen cyanide is a simpler nitrile, but the cyanide anion is not class ...
, dimethyl formamide
Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the ma ...
etc. that can act as Lewis base
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any s ...
s. Solvates of formula UCl4L are formed which may be isolated. The solvent must be completely free of dissolved water, or hydrolysis will occur, with the solvent, S, picking up the released proton.
:UCl4 + H2O + S UCl3(OH) + SH+ +Cl−
The solvent molecules may be replaced by other ligand in a reaction such as
:UCl4 + 2Cl− → 6">Cl6sup>2−.
The solvent is not shown, just as when complexes of other metal ions are formed in aqueous solution.
Solutions of UCl4 are susceptible to oxidation by air, resulting in the production of complexes of the uranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula . It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may ...
ion.
Applications
Uranium tetrachloride is produced commercially by the reaction of carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemic ...
with pure uranium dioxide
Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear react ...
UO2 at 370 °C. It has been used as feed in the electromagnetic isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...
(EMIS) process of uranium enrichment
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 ...
. Beginning in 1944, the Oak Ridge Y-12 Plant converted UO3 to UCl4 feed for the Ernest O. Lawrence's Alpha Calutrons. Its major benefit being the uranium tetrachloride used in the calutrons is not as corrosive as the uranium hexafluoride
Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
used in most other enrichment technologies This process was abandoned in the 1950s. In the 1980s, however, Iraq
Iraq,; ku, عێراق, translit=Êraq officially the Republic of Iraq, '; ku, کۆماری عێراق, translit=Komarî Êraq is a country in Western Asia. It is bordered by Turkey to the north, Iran to the east, the Persian Gulf and ...
unexpectedly revived this option as part of its nuclear weapons program. In the enrichment process, uranium tetrachloride is ionized into a uranium plasma.
The uranium ions are then accelerated and passed through a strong magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. After traveling along half of a circle, the beam is split into a region nearer the outside wall, which is depleted, and a region nearer the inside wall, which is enriched in 235U. The large amounts of energy required in maintaining the strong magnetic fields as well as the low recovery rates of the uranium feed material and slower more inconvenient facility operation make this an unlikely choice for large scale enrichment plants.
Work is being done in the use of molten uranium chloride–alkali chloride mixtures as reactor fuels in molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
s. Uranium tetrachloride melts dissolved in a lithium chloride
Lithium chloride is a chemical compound with the formula Li Cl. The salt is a typical ionic compound (with certain covalent characteristics), although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorid ...
–potassium chloride
Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...
eutectic have also been explored as a means to recover actinides
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
from irradiated nuclear fuel
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
s through pyrochemical nuclear reprocessing
Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, t ...
.[Olander, D. R. and Camahort, J. L. (1966), Reaction of chlorine and uranium tetrachloride in the fused lithium chloride-potassium chloride eutectic. AIChE Journal, 12: 693–699. ]
Safety
Like all water soluble uranium salts, uranium tetrachloride is nephrotoxic (poisonous to the kidney) and can cause severe renal damage and acute renal failure if ingested.
References
{{DEFAULTSORT:Uranium Tetrachloride
Uranium(IV) compounds
Chlorides
Actinide halides
Nuclear materials