Titanium tetrachloride
Encyclopedia : T : TI : TIT : Titanium tetrachloride
| Titanium tetrachloride | |
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| 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 formation ΔfH°liquid | -804.16 kJ/mol |
| Standard molar entropy S°liquid | 221.93 J·K−1·mol−1 |
| Safety data | |
| EU classification | Corrosive |
| R-phrases | R14, R34 |
| S-phrases | S1/S2, S7/S8, S26, S36/S37/S39, S45 |
| NFPA 704 |   
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| Supplementary data page | |
| Structure and properties | n, εr, 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 infoboxInfobox disclaimer and references] | |
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 [(C6R6)TiCl3]+. 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
Reduction of TiCl4 using magnesium metal produces titanium metal; this is in fact the final step of the Kroll process.
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: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 TiCl4-xBrx, 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 [Ti2Cl9]-, [Ti2Cl10]2-, and [TiCl6]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 [Ti(C6(CH3)6)Cl3]+, 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 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
Further reading
External links
- [International Chemical Safety Card 1230]
- [European Chemicals Bureau]
- [NIST Standard Reference Database]
- [Sumitomo Titanium Corporation information sheet]
- For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].
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