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 thick clouds of titanium dioxide (TiO₂) and hydrochloric acid, a reaction that was formerly exploited for use in smoke machines. It is sometimes referred to as "tickle" or "tickle 4", as a phonetic representation of the symbols of its molecular formula (TiCl₄).

TiCl₄ is a dense, colourless 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 point is similar to that of CCl₄.

Ti⁴⁺ has a "closed" electronic shell, with the same number of electrons as the noble 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.

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. Bulkier ligands (L) give pentacoordinated adducts TiCl₄L.

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.

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.

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:

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