Tin(IV) chloride

Tin chloride, also known as tin tetrachloride or stannic chloride, is an inorganic compound of tin and chlorine with the formula SnCl₄. It is a colorless hygroscopic liquid, which fumes on contact with air. It is used as a precursor to other tin compounds. It was first discovered by Andreas Libavius and was known as spiritus fumans libavii.

Preparation

It is prepared from reaction of chlorine gas with tin at :

Structure

Anhydrous tin chloride solidifies at −33 °C to give monoclinic crystals with the P21/c space group. It is isostructural with SnBr₄. The molecules adopt near-perfect tetrahedral symmetry with average Sn–Cl distances of 227.9 pm.

Reactions

Tin chloride is well known as a Lewis acid. Thus it forms hydrates. The pentahydrate SnCl₄·5H₂O was formerly known as butter of tin. These hydrates consist of cis- molecules together with varying amounts of water of crystallization. The additional water molecules link together the molecules of through hydrogen bonds. A pentahydrate has also been crystallized. In cis-, the Sn-Cl bonds are 238.3 pm. Although the pentahydrate is the most common hydrate, lower hydrates have also been characterised.
Aside from water, other Lewis bases form adducts with SnCl₄. These include ammonia and organophosphines.
The ammonium salt of ²⁻ is formed from ammonium chloride. It is called "pink salt":
The analogous reaction with hydrochloric acid gives "hexachlorostannic acid".
Reaction of the tetrachloride with hydrogen fluoride gives tin tetrafluoride:
Tin chloride undergoes redistribution with tin bromide as assessed by ¹¹⁹Sn NMR and Raman spectroscopy. Equilibrium is achieved in seconds at room temperature. By contrast, halide exchange for related germanium and especially silicon halides is slower.

Applications

Precursor to organotin compounds

Anhydrous tin chloride is a major precursor in organotin chemistry. Upon treatment with Grignard reagents, tin chloride gives tetraalkyltin compounds:
Anhydrous tin chloride reacts with tetraorganotin compounds in redistribution reactions:
These organotin halides are useful precursors to catalysts and polymer stabilizers.

Organic synthesis

SnCl₄ is used in Friedel–Crafts reactions as a Lewis acid catalyst. For example, the acetylation of thiophene to give 2-acetylthiophene is promoted by tin chloride. Similarly, tin chloride is useful for nitrations.

Safety

Stannic chloride was used as a chemical weapon in World [War I], as it formed an irritating dense smoke on contact with air. It was supplanted by a mixture of silicon tetrachloride and titanium tetrachloride near the end of the war due to shortages of tin.