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Titanium tetrachloride is the
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 chemist ...
with the
formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds of titanium dioxide () and hydrochloric acid, a reaction that was formerly exploited for use in smoke machines. It is sometimes referred to as "tickle" or "tickle 4" due to the phonetic resemblance of its molecular formula () to the word.


Properties and structure

is a dense, colourless distillable liquid, although crude samples may be yellow or even red-brown. It is one of the rare transition metal halides that is a liquid at room temperature, being another example. This property reflects the fact that molecules of weakly self-associate. Most metal chlorides are polymers, wherein the chloride atoms bridge between the metals. Its melting and
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
s are similar to those of . has a "closed" electronic shell, with the same number of electrons as the noble gas argon. The tetrahedral structure for is consistent with its description as a d0 metal center () surrounded by four identical ligands. This configuration leads to highly symmetrical structures, hence the tetrahedral shape of the molecule. adopts similar structures to and ; the three compounds share many similarities. and react to give mixed halides , where ''x'' = 0, 1, 2, 3, 4. Magnetic resonance measurements also indicate that halide exchange is also rapid between and . is soluble in toluene and chlorocarbons. Certain arenes form complexes of the type . reacts
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
ally with donor solvents such as THF to give hexacoordinated adducts. Bulkier ligands (L) give pentacoordinated adducts .


Production

is produced by the chloride process, which involves the reduction of titanium oxide ores, typically ilmenite (), with carbon under flowing chlorine at 900 °C. Impurities are removed by distillation. : The coproduction of is undesirable, which has motivated the development of alternative technologies. Instead of directly using ilmenite, "rutile slag" is used. This material, an impure form of , is derived from ilmenite by removal of iron, either using carbon reduction or extraction with
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
. Crude contains a variety of other volatile halides, including vanadyl chloride (), silicon tetrachloride (), and tin tetrachloride (), which must be separated.


Applications


Production of titanium metal

The world's supply of titanium metal, about 250,000 tons per year, is made from . The conversion involves the reduction of the tetrachloride with magnesium metal. This procedure is known as the Kroll process: : In the Hunter process, liquid sodium is the reducing agent instead of magnesium.


Production of titanium dioxide

Around 90% of the production is used to make the pigment titanium dioxide (). The conversion involves hydrolysis of , a process that forms hydrogen chloride: : In some cases, is oxidised directly with oxygen: :


Smoke screens

It has been used to produce smoke screens since it produces a heavy, white smoke that has little tendency to rise. "Tickle" was the standard means of producing on-set smoke effects for motion pictures, before being phased out in the 1980s due to concerns about hydrated HCl's effects on the respiratory system.


Chemical reactions

Titanium tetrachloride is a versatile reagent that forms diverse derivatives including those illustrated below.


Alcoholysis and related reactions

A characteristic reaction of is its easy hydrolysis, signaled by the release of HCl vapors and titanium oxides and
oxychloride In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula , where X = fluorine (F), ...
s. Titanium tetrachloride has been used to create naval smokescreens, as the hydrochloric acid aerosol and titanium dioxide that is formed scatter light very efficiently. This smoke is corrosive, however. Alcohols react with to give alkoxides with the formula (R = alkyl, ''n'' = 1, 2, 4). As indicated by their formula, these
alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...
s can adopt complex structures ranging from monomers to tetramers. Such compounds are useful in materials science as well as
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. A well known derivative is titanium isopropoxide, which is a monomer. Titanium bis(acetylacetonate)dichloride results from treatment of titanium tetrachloride with excess
acetylacetone Acetylacetone is an organic compound with the chemical formula . It is a colorless liquid, classified as a 1,3-diketone. It exists in equilibrium with a tautomer . These tautomers interconvert so rapidly under most conditions that they are tre ...
: : Organic amines react with to give complexes containing amido (-containing) and imido (-containing) complexes. With ammonia, titanium nitride is formed. An illustrative reaction is the synthesis of
tetrakis(dimethylamido)titanium Tetrakis(dimethylamino)titanium (TDMAT) is a chemical compound. The compound is generally classified as a metalorganic species, meaning that its properties are strongly influenced by the organic ligands but the compound lacks metal-carbon bonds. It ...
, a yellow, benzene-soluble liquid: This molecule is tetrahedral, with planar nitrogen centers. :


Complexes with simple ligands

is a
Lewis acid 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 sp ...
as implicated by its tendency to hydrolyze. With the ether THF, reacts to give yellow crystals of . With chloride salts, reacts to form sequentially , (see figure above), and . The reaction of chloride ions with depends on the counterion. and gives the pentacoordinate complex , whereas smaller gives . These reactions highlight the influence of electrostatics on the structures of compounds with highly ionic bonding.


Redox

Reduction of with aluminium results in one-electron reduction. The trichloride ( ) and tetrachloride have contrasting properties: the trichloride is a colored solid, being a
coordination polymer A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, o ...
, and is paramagnetic. When the reduction is conducted in THF solution, the Ti(III) product converts to the light-blue adduct .


Organometallic chemistry

The
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
of titanium typically starts from . An important reaction involves sodium
cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...
to give titanocene dichloride, . This compound and many of its derivatives are precursors to Ziegler–Natta catalysts. Tebbe's reagent, useful in organic chemistry, is an aluminium-containing derivative of titanocene that arises from the reaction of titanocene dichloride with trimethylaluminium. It is used for the "olefination" reactions. Arenes, such as react to give the piano-stool complexes (R = H, ; see figure above). This reaction illustrates the high Lewis acidity of the entity, which is generated by abstraction of chloride from by .


Reagent in organic synthesis

finds occasional use in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
, capitalizing on its
Lewis acid 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 sp ...
ity, its oxophilicity, and the electron-transfer properties of its reduced titanium halides It is used in the Lewis acid catalysed
aldol addition The aldol reaction is a means of forming carbon–carbon bonds in organic chemistry. Discovered independently by the Russian chemist Alexander Borodin in 1869 and by the French chemist Charles-Adolphe Wurtz in 1872, the reaction combines two carb ...
Mariappan Periasamy (2002): "New synthetic methods using the TiCl4-NR3 reagent system", '' Arkivoc'', p. 151-166. Key to this application is the tendency of to activate aldehydes (RCHO) by formation of adducts such as .


Toxicity and safety considerations

Hazards posed by titanium tetrachloride generally arise from its reaction with water that releases hydrochloric acid, which is severely corrosive itself and whose vapors are also extremely irritating. is a strong
Lewis acid 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 sp ...
, which exothermically forms adducts with even weak bases such as THF and water.


References


General reading

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External links


Titanium tetrachloride: Health Hazard InformationNIST Standard Reference DatabaseChemSub Online: Titanium tetrachloride
{{Chlorides Titanium(IV) compounds Chlorides Titanium halides Reagents for organic chemistry