Titanium tetrachloride

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Titanium tetrachloride
General
Systematic name	Titanium tetrachloride Titanium(IV) chloride
Molecular formula	TiCl₄
Molar mass	189.71 g/mol
Appearance	colorless fuming liquid
CAS number
Properties
Density and phase	1.730 g/ml, liquid
Solubility in water	Decomposes
Melting point	-24 °C
Boiling point	136.4 °C
Viscosity	? cP at ? °C
Structure
Molecular shape	Tetrahedral
Dipole moment	zero
Thermodynamic data
Standard enthalpy of	-804.16 kJ/mol
Standard molar entropy S°_liquid	221.93 J·K⁻¹·mol⁻¹
Safety data
EU classification	Corrosive
R-phrases	,
S-phrases	/, /, , //,
NFPA 704
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Titanium(IV) fluoride Titanium(IV) bromide Titanium(IV) iodide
Other cations	Zirconium(IV) chloride Hafnium(IV) chloride
Related compounds	Titanium(II) chloride Titanium(III) chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Titanium tetrachloride or titanium(IV) chloride is the compound TiCl₄. TiCl₄ is an important intermediate in the separation of titanium from its ores and in the production of titanium as well as many chemicals. This chemical compound is remarkable in many ways, not the least of which is that it is an unusual example of a liquid metal halide and it fumes spectacularly in air. TiCl₄ is also called, light-heartedly, 'Tickle' or 'Tickle-four' by those that use this chemical.

Properties and structure

TiCl₄ is a dense, colourless (or pale yellow) distillable liquid, although crude samples can be reddish-brown. It is one of the rare transition metal chlorides that is liquid at room temperature, VCl₄ being another example. This distinctive property arises from the fact that TiCl₄ is molecular, that is each TiCl₄ behaves independently. Most metal chlorides are polymers, where the chloride atoms bridge between the metals. The attraction between the individual TiCl₄ molecules is weak, primarily van der Waals forces, and these weak interactions result in low melting and boiling points akin to that for CCl₄.

TiCl₄ is tetrahedral, which is consistent with its description as Ti⁴⁺ surrounded by four Cl^- ligands. Ti⁴⁺ has a "closed" electronic shell, with the same number of electrons as the inert gas argon. This configuration leads to highly symmetrical structures, hence the tetrahedron.

Typical of a non-polar species, TiCl₄ is soluble in toluene and chlorocarbons. Evidence exists that certain arenes form complexes of the type ^*⁺. TiCl₄ reacts exothermically with donor solvents such as THF to give hexacoordinated adducts. Bulkier ligands (L) give pentacoordinated derivatives TiCl₄L.

The main problem with handling TiCl₄, aside from its tendency to release corrosive hydrogen chloride, is the formation of titanium oxides and oxychlorides that cement stoppers and syringes.

Production

TiCl₄ is produced by the Kroll process, which involves the reduction of titanium oxide ores, typically ilmenite or rutile, with carbon under flowing chlorine at 900 °C. Impurities are removed using a distillation furnace to afford pure TiCl₄.

2 FeTiO₃ + 7 Cl₂ + 6 C → 2 TiCl₄ + 2 FeCl₃ + 6 CO

TiCl₄ is inexpensive, thus it is typically purchased for laboratory operations.

Applications

Production of titanium metal

TiO₂ + 2Cl₂(g) + C → TiCl₄(l) + CO₂(g)

Reduction of TiCl₄ using magnesium metal produces titanium metal; this is in fact the final step of the Kroll process.

2Mg + TiCl₄(l) → 2MgCl₂(l) + Ti(s)

Production of titanium dioxide

Around 90% of the TiCl₄ production is used to make pigment; titanium(IV) oxide (TiO₂). Key is the reaction of TiCl₄ with water to form hydrochloric acid: TiCl₄ + 2 H₂O → TiO₂ + 4 HCl.

Smoke-screens

In the past titanium tetrachloride has also been used to create naval smokescreens. When sprayed into the air, TiCl₄ rapidly reacts with atmospheric moisture:

TiCl₄ + 2 H₂O → TiO₂ + 4 HCl

The hydrogen chloride immediately absorbs more water to form tiny droplets of hydrochloric acid, which (depending on humidity) may absorb still more water, to produce large droplets that efficiently scatter light. In addition, the intensely white titanium dioxide is also an efficient light scatterer. Due to the corrosiveness of this smoke, however, TiCl₄ is no longer used.

Chemistry

Organometallic and inorganic chemistry

TiCl₄ adopts similar structures to TiBr₄ and TiI₄; the three compounds share many similarities. TiCl₄ and TiBr₄ react to give mixed halides TiCl_4-xBr_x, where x = 0, 1, 2, 3, 4. Magnetic resonance measurements also indicate that halide exchange is also rapid between TiCl₄ and VCl₄.

TiCl₄ is a superb and versatile Lewis acid, as indicated by its tendency to hydrolyze, which implicates the intermediacy of TiCl₄(H₂O). With THF, TiCl₄ forms yellow crystals of TiCl₄(THF)₂. With Cl^- donors, TiCl₄ reacts to form sequentially ^*^2-. Interestingly, the reaction of chloride ions with TiCl₄ depends on the counterion. NBu₄Cl reacts with TiCl₄ to give the pentacoordinate complex NBu₄TiCl₅, whereas smaller NEt₄ reacts to give (NEt₄)₂Ti₂Cl₁₀. These reactions highlight the influence of electrostatic forces on the structures of compounds with highly ionic bonding.

Much of the extensive organometallic chemistry of titanium starts from TiCl₄. Its most important reaction is with sodium cyclopentadienyl to give titanocene dichloride, TiCl₂(C₅H₅)₂. This compound is used in organic synthesis (Tebbe's reagent). Arenes, such as C₆(CH₃)₆ reacts to give ^*⁺, which is a piano-stool complex. This reaction illustrates the extraordinary Lewis acidity of the TiCl₃⁺ entity, which is derived from TiCl₄ using the even stronger chloride-abstracting agent AlCl₃.

TiCl₄ reacts with four equivalents LiNMe₂ to give Ti(NMe₂)₄, a yellow, benzene-soluble liquid. This molecule is tetrahedral, with planar nitrogen centers.

Reagent in organic synthesis

It is widely used in organic chemistry as a Lewis acid, for example in the Mukaiyama aldol condensation. Key to this application is the tendency of TiCl₄ to interact with aldehydes, RCHO, to give adducts such (RCHO)TiCl₄OC(H)R.

Olefin polymerisation

This compound and many of its derivatives are important precursors to Ziegler-Natta catalysts.

Reduction

Reduction of TiCl₄ yields TiCl₃. E.g. reduction of TiCl₄ with aluminium in THF results in the light-blue THF-adduct TiCl₃(THF)₃.

Toxicity and safety considerations

Vapors of TiCl₄ can damage eyes, skin, mucous membranes, and the lungs. Given the tendency of TiCl₄ to hydrolyze, most of these hazards can be attributed to the effect of hydrogen chloride. TiCl₄ is a very aggressive Lewis acid, exothermically forming adducts with even weak bases such as THF and explosively with water, again releasing HCl.

References

External links

Titanium compounds | Chlorides | Metal halides | Reagents for organic chemistry

Titan(IV)-chlorid | Tetracloruro di titanio | 塩化チタン(IV)

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