Tin(IV) chloride, also known as tin tetrachloride or stannic chloride, is an inorganic compound of tin and chlorine with the formula SnCl<sub>4</sub>. 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 (1550–1616) and was known as spiritus fumans libavii.
Preparation
It is prepared from reaction of chlorine gas with tin at :
:
Structure
alt=Space-filling model of anhydrous SnCl4.|left|thumb|151x151px|Space-filling model of anhydrous SnCl<sub>4</sub>.
200px|left|thumb|Structure of solid SnCl<sub>4</sub>.
Anhydrous tin(IV) chloride solidifies at −33 °C to give monoclinic crystals with the P21/c space group. It is isostructural with SnBr<sub>4</sub>. The molecules adopt near-perfect tetrahedral symmetry with average Sn–Cl distances of 227.9(3) pm.
Reactions
Tin(IV) chloride is well known as a Lewis acid. Thus it forms hydrates. The pentahydrate SnCl<sub>4</sub>·5H<sub>2</sub>O was formerly known as butter of tin. These hydrates consist of cis-[SnCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] molecules together with varying amounts of water of crystallization. The additional water molecules link together the molecules of [SnCl<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>] 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 SnCl<sub>4</sub>. These include ammonia and organophosphines.
The ammonium salt of [SnCl<sub>6</sub>]<sup>2−</sup> is formed from ammonium chloride. It is called "pink salt":
Applications
Precursor to organotin compounds
Anhydrous tin(IV) chloride is a major precursor in organotin chemistry. Upon treatment with Grignard reagents, tin(IV) chloride gives tetraalkyltin compounds:
:SnCl<sub>4</sub> + 4 RMgCl → SnR<sub>4</sub> + 4 MgCl<sub>2</sub>
Anhydrous tin(IV) chloride reacts with tetraorganotin compounds in redistribution reactions:
:SnCl<sub>4</sub> + SnR<sub>4</sub> → 2 SnCl<sub>2</sub>R<sub>2</sub>
These organotin halides are useful precursors to catalysts (e.g., dibutyltin dilaurate) and polymer stabilizers.
Organic synthesis
SnCl<sub>4</sub> is used in Friedel–Crafts reactions as a Lewis acid catalyst. Similarly, tin(IV) chloride is useful for nitrations.
Safety
Stannic chloride was used as a chemical weapon in World War I, as it formed an irritating (but non-deadly) 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.
References
External links
- International Chemical Safety Card 0953