Titanium tetrachloride

Titanium tetrachloride is the inorganic compound with the formula TiCl₄. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl₄ is a volatile liquid. Upon contact with humid air, it forms spectacular opaque clouds of titanium dioxide (TiO₂) and hydrated hydrogen chloride. It is sometimes referred to as "tickle" or "tickle 4" due to the phonetic resemblance of its molecular formula (TiCl₄) to the word.[7][8]

Titanium tetrachloride

Names
IUPAC name Titanium(IV) chloride
Other names Titanium tetrachloride Tetrachlorotitanium
Identifiers
CAS Number	7550-45-0^Y
3D model (JSmol)	Interactive image
ChemSpider	22615^Y
ECHA InfoCard	100.028.584
EC Number	231-441-9
MeSH	Titanium+tetrachloride
PubChem CID	24193
RTECS number	XR1925000
UNII	8O3PJE5T7Q^Y
UN number	1838
CompTox Dashboard (EPA)	DTXSID8042476
InChI InChI=1S/4ClH.Ti/h4*1H;/q;;;;+4/p-4^Y Key: XJDNKRIXUMDJCW-UHFFFAOYSA-J^Y InChI=1/4ClH.Ti/h4*1H;/q;;;;+4/p-4/rCl4Ti/c1-5(2,3)4 Key: XJDNKRIXUMDJCW-FOGBWSKZAG
SMILES Cl[Ti](Cl)(Cl)Cl
Properties
Chemical formula	TiCl 4
Molar mass	189.679 g/mol
Appearance	Colourless liquid
Odor	penetrating acid odor
Density	1.726 g/cm^3
Melting point	−24.1 °C (−11.4 °F; 249.1 K)
Boiling point	136.4 °C (277.5 °F; 409.5 K)
Solubility in water	reacts (exothermic hydrolysis)[1]
Solubility	soluble in dichloromethane,[2] toluene,[3] pentane[4]
Vapor pressure	1.3 kPa (20 °C)
Magnetic susceptibility (χ)	−54.0·10^−6 cm^3/mol
Refractive index (nD)	1.61 (10.5 °C)
Viscosity	827 μPa s
Structure
Coordination geometry	Tetragonal
Molecular shape	Tetrahedral
Dipole moment	0 D
Thermochemistry
Std molar entropy (S^o298)	355 J·mol^−1·K^−1[5]
Std enthalpy of formation (ΔfH^⦵298)	−763 kJ·mol^−1[5]
Hazards[6]
Safety data sheet	MSDS
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	H314, H317, H318, H330, H335, H370, H372
GHS precautionary statements	P280, P301+330+331, P304+340, P305+351+338, P308+310
NFPA 704 (fire diamond)	0 3 2 W
Related compounds
Other anions	Titanium(IV) bromide Titanium(IV) fluoride Titanium(IV) iodide
Other cations	Hafnium(IV) chloride Zirconium(IV) chloride
Related compounds	Titanium(II) chloride Titanium(III) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Crystals of frozen titanium tetrachloride melting into the liquid

Properties and structure

TiCl₄ 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, VCl₄ being another example. This property reflects the fact that molecules of TiCl₄ weakly self-associate. Most metal chlorides are polymers, wherein the chloride atoms bridge between the metals. Its melting and boiling points are similar to those of CCl₄.

Ti⁴⁺ has a "closed" electronic shell, with the same number of electrons as the inert gas argon. The tetrahedral structure for TiCl₄ is consistent with its description as a d⁰ metal center (Ti⁴⁺) surrounded by four identical ligands. This configuration leads to highly symmetrical structures, hence the tetrahedral shape of the molecule. 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.[9]

TiCl₄ is soluble in toluene and chlorocarbons. Certain arenes form complexes of the type [(C₆R₆)TiCl₃]⁺. TiCl₄ reacts exothermically with donor solvents such as THF to give hexacoordinated adducts.[10] Bulkier ligands (L) give pentacoordinated adducts TiCl₄L.

Production

TiCl₄ is produced by the chloride process, which involves the reduction of titanium oxide ores, typically ilmenite (FeTiO₃), with carbon under flowing chlorine at 900 °C. Impurities are removed by distillation.

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

The coproduction of FeCl₃ 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 TiO₂, is derived from ilmenite by removal of iron, either using carbon reduction or extraction with sulfuric acid. Crude TiCl₄ contains a variety of other volatile halides, including vanadyl chloride (VOCl₃), silicon tetrachloride (SiCl₄), and tin tetrachloride (SnCl₄), 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 TiCl₄. The conversion involves the reduction of the tetrachloride with magnesium metal. This procedure is known as the Kroll process:[11]

2  Mg + TiCl₄ → 2 MgCl₂ + Ti

In the Hunter process, liquid sodium is the reducing agent instead of magnesium.

Production of titanium dioxide

Around 90% of the TiCl₄ production is used to make the pigment titanium dioxide (TiO₂). The conversion involves hydrolysis of TiCl₄, a process that forms hydrogen chloride:[11]

TiCl₄ + 2 H₂O → TiO₂ + 4 HCl

In some cases, TiCl₄ is oxidised directly with oxygen:

TiCl₄ + O₂ → TiO₂ + 2 Cl₂

Smoke screens

It has been used to produce smoke screens since it produces a heavy, white smoke that has little tendency to rise.[12]

Chemical reactions

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

Hydrolysis and related reactions

The most noteworthy reaction of TiCl₄ is its easy hydrolysis, signaled by the release of hydrogen chloride and the formation of titanium oxides and oxychlorides, as described above for the production of TiO₂. Titanium tetrachloride has been used to create naval smokescreens. The hydrogen chloride immediately absorbs water to form an aerosol of hydrochloric acid that efficiently scatter light. In addition, the highly refractive titanium dioxide is also an efficient light scatterer. This smoke is however corrosive.

Alcohols react with TiCl₄ to give the corresponding alkoxides with the formula [Ti(OR)₄]_n (R = alkyl, n = 1, 2, 4). As indicated by their formula, these alkoxides can adopt complex structures ranging from monomers to tetramers. Such compounds are useful in materials science as well as organic synthesis. A well known derivative is titanium isopropoxide, which is a monomer.

Organic amines react with TiCl₄ to give complexes containing amido (R₂N⁻-containing) and imido (RN²⁻-containing) complexes. With ammonia, titanium nitride is formed. An illustrative reaction is the synthesis of tetrakis(dimethylamido)titanium Ti(NMe₂)₄, a yellow, benzene-soluble liquid:[13] This molecule is tetrahedral, with planar nitrogen centers.[14]

4 LiNMe₂ + TiCl₄ → 4 LiCl + Ti(NMe₂)₄

Complexes with simple ligands

TiCl₄ is a Lewis acid as implicated by its tendency to hydrolyze. With the ether THF, TiCl₄ reacts to give yellow crystals of TiCl₄(THF)₂. With chloride salts, TiCl₄ reacts to form sequentially [Ti₂Cl₉]⁻, [Ti₂Cl₁₀]²⁻ (see figure above), and [TiCl₆]²⁻.[15] The reaction of chloride ions with TiCl₄ depends on the counterion. NBu₄Cl and TiCl₄ gives the pentacoordinate complex NBu₄TiCl₅, whereas smaller NEt⁺
₄ gives (NEt₄)₂Ti₂Cl₁₀. These reactions highlight the influence of electrostatics on the structures of compounds with highly ionic bonding.

Redox

Reduction of TiCl₄ with aluminium results in one-electron reduction. The trichloride (TiCl₃) and tetrachloride have contrasting properties: the trichloride is a solid, being a coordination polymer, and is paramagnetic. When the reduction is conducted in THF solution, the Ti(III) product converts to the light-blue adduct TiCl₃(THF)₃.

Organometallic chemistry

Main article: Organotitanium compound

The organometallic chemistry of titanium typically starts from TiCl₄. An important reaction involves sodium cyclopentadienyl to give titanocene dichloride, TiCl₂(C₅H₅)₂. 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 C₆(CH₃)₆ react to give the piano-stool complexes [Ti(C₆R₆)Cl₃]⁺ (R = H, CH₃; see figure above).[16] This reaction illustrates the high Lewis acidity of the TiCl⁺
₃ entity, which is generated by abstraction of chloride from TiCl₄ by AlCl₃.

Reagent in organic synthesis

TiCl₄ finds occasional use in organic synthesis, capitalizing on its Lewis acidity, its oxophilicity, and the electron-transfer properties of its reduced titanium halides[17] It is used in the Lewis acid catalysed aldol addition[18] Key to this application is the tendency of TiCl₄ to activate aldehydes (RCHO) by formation of adducts such as (RCHO)TiCl₄OC(H)R.

Toxicity and safety considerations

Hazards posed by titanium tetrachloride generally arise from the release of hydrogen chloride (HCl). TiCl₄ is a strong Lewis acid, exothermically forming adducts with even weak bases such as THF and explosively with water, releasing HCl.

References

1. Eremenko, B. V.; Bezuglaya, T. N.; Savitskaya, A. N.; Malysheva, M. L.; Kozlov, I. S.; Bogodist, L. G. (2001). "Stability of Aqueous Dispersions of the Hydrated Titanium Dioxide Prepared by Titanium Tetrachloride Hydrolysis". Colloid Journal. 63 (2): 173–178. doi:10.1023/A:1016673605744.
2. "titanium(IV) chloride, 1M soln. in dichloromethane". Alfa Aesar. Alfa Aesar. Retrieved 7 March 2018.
3. "Titanium(IV) chloride solution 1.0 M in toluene". Sigma-Aldrich. Retrieved 7 March 2018.
4. Butts, Edward H De. "patent US3021349A".
5. Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). Houghton-Mifflin. p. A23. ISBN 978-0-618-94690-7.
6. "Classifications - CL Inventory". echa.europa.eu.
7. American Chemistry Council – "Titanium Tetrachloride: Stepping Stone to Amazing Technology"
8. "Archived copy". Archived from the original on 2014-03-19. Retrieved 2013-04-10.CS1 maint: archived copy as title (link) Iowa State University – "Chemistry Material Safety Data Sheets"
9. Webb, S. P.; Gordon, M. S. (1999). "Intermolecular Self-Interactions of the Titanium Tetrahalides TiX₄ (X = F, Cl, Br)". J. Am. Chem. Soc. 121 (11): 2552–2560. doi:10.1021/ja983339i.
10. Manzer, L. E. (1982). Tetrahydrofuran Complexes of Selected Early Transition Metals. Inorganic Syntheses. 21. pp. 135–40. doi:10.1002/9780470132524.ch31. ISBN 978-0-470-13252-4.
11. Völz, Hans G.; et al. (2006). "Pigments, Inorganic". Inorganic Pigments. Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a20_243.pub2. ISBN 978-3527306732.
12. The Royal Navy at War (DVD). London: Imperial War Museum. 2005.
13. Bradey, D. C.; Thomas, M. (1960). "Some Dialkylamino-derivatives of Titanium and Zirconium". J. Chem. Soc.: 3857–3861. doi:10.1039/JR9600003857.
14. M. E. Davie; T. Foerster; S. Parsons; C. Pulham; D. W. H. Rankin; B. A. Smart (2006). "The Crystal Structure of Tetrakis(dimethylamino)titanium(IV)". Polyhedron. 25 (4): 923–929. doi:10.1016/j.poly.2005.10.019.
15. Creaser, C. S.; Creighton, J. A. (1975). "Pentachloro- and Pentabromotitanate(IV) ions". Dalton Trans. (14): 1402–1405. doi:10.1039/DT9750001402.
16. Calderazzo, F.; Ferri, I.; Pampaloni, G.; Troyanov, S. (1996). "η⁶-Arene Derivatives of Titanium(IV), Zirconium(IV) and Hafnium(IV)". J. Organomet. Chem. 518 (1–2): 189–196. doi:10.1016/0022-328X(96)06194-3.
17. Gundersen, L.-L.; Rise, F.; Undheim, K. (2004). "Titanium(IV) chloride". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York, NY: J. Wiley & Sons.
18. Mariappan Periasamy (2002): "New synthetic methods using the TiCl4-NR3 reagent system", ARKIVOC, p. 151-166.

General reading

• Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego, CA: Academic Press. ISBN 978-0-12-352651-9.
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

External links

• Titanium tetrachloride: Health Hazard Information
• NIST Standard Reference Database
• ChemSub Online: Titanium tetrachloride