Tin(IV) 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.[1] It was first discovered by Andreas Libavius (1550–1616) and was known as spiritus fumans libavii.
It is prepared from reaction of chlorine gas with tin at 115C:
+ 2 →
Anhydrous tin(IV) 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(3) pm.[2]
Tin(IV) chloride is well known as a Lewis acid. Thus it forms hydrates. The pentahydrate SnCl4·5H2O was formerly known as butter of tin. They all consist of [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.[3] Although the pentahydrate is the most common hydrate, lower hydrates have also been characterised.[4]
Aside from water, other Lewis bases form adducts with SnCl4. These include ammonia and organophosphines. The complex [SnCl<sub>6</sub>]2− is formed with hydrochloric acid making hexachlorostannic acid.[1]
Anhydrous tin(IV) chloride is a major precursor in organotin chemistry. Upon treatment with Grignard reagents, tin(IV) chloride gives tetraalkyltin compounds:
SnCl4 + 4 RMgCl → SnR4 + 4 MgCl2Anhydrous tin(IV) chloride reacts with tetraorganotin compounds in redistribution reactions:
SnCl4 + SnR4 → 2 SnCl2R2These organotin halides are useful precursors to catalysts (e.g., dibutyltin dilaurate) and polymer stabilizers.[5]
SnCl4 is used in Friedel–Crafts reactions as a Lewis acid catalyst.[1] For example, the acetylation of thiophene to give 2-acetylthiophene is promoted by tin(IV) chloride.[6] Similarly, tin(IV) chloride is useful for the nitrations.[7]
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.[8]