Question

The dissolution of stannous hydroxide in excess base produces

Answer 1

Tin(II) chloride (stannous chloride) is a white crystallinesolid with the formula SnCl2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent (in acid solution), and in electrolytic baths for tin-plating. Tin(II) chloride should not be confused with the other chloride of tin; tin(IV) chloride or stannic chloride (SnCl4).

Structure

SnCl2 has a lone pair, such that the molecule in the gas phase is bent. In th

Therefore if clear solutions of tin(II) chloride are to be used, hydrochloric acid must be added in order to maintain the equilibrium towards the left-hand side (using Le Chatelier's principle). Solutions of SnCl2 are also unstable towards oxidation by the air:

6 SnCl2( aq) + O2(g) + 2 Water (molecule)( l) → 2 SnCl4(aq) + 4 Sn(OH)Cl(Solid|s)

This can be prevented by storing the solution over lumps of tin metal.

There are many such cases where tin(II) chloride acts as a reducing agent, reducing silver and gold salts to the metal, and iron(III) salts to iron(II), for example:

SnCl2(aq) + 2 FeCl3(aq) → SnCl4(aq) + 2FeCl2(aq)

Solutions of tin(II) chloride can also serve simply as a source of Sn2+ ions, which can form other tin(II) compounds via precipitation reactions, for example brown (or black) tin(II) sulfide:

SnCl2(aq) + Na2S(aq) → SnS(s) + 2 NaCl(aq)

If alkali is added to a solution of SnCl2, a white precipitate of hydrated tin(II) oxide forms initially; this then dissolves in excess base to form a stannite salt such as sodium stannite:

SnCl2(aq) + 2 NaOH(aq) → SnO·H2O(s) + 2NaCl(aq)SnO·H2O(s) + NaOH(aq) → NaSn(OH)3(aq)

Anhydrous SnCl2 can be used to make a variety of interesting tin(II) compounds in non-aqueous solvents. For example, the lithium salt of 4-methyl-2,6-di-tert-butylphenol reacts with SnCl2 in THF to give the yellow linear two-coordinate compound Sn(OAr)2 (Ar = aryl).

Tin(II) chloride also behaves as a Lewis acid, forming complexes with ligands such aschloride ion, for example:

SnCl2(aq) + CsCl(aq) → CsSnCl3(aq)

Most of these complexes are pyramidal, and since complexes such as SnCl3 have a fulloctet, there is little tendency to add more than one ligand. The lone pair of electrons in such complexes is available for bonding, however, and therefore the complex itself can act as aLewis base or ligand. This seen in theferrocene-related product of the following reaction :

SnCl2 + Fe(η5-C5H5)(CO)2HgCl → Fe(η5-C5H5)(CO)2SnCl3 + Hg

SnCl2 can be used to make a variety of such compounds containing metal-metal bonds, for example:

SnCl2 + Co2(CO)8 → (CO)4Co-(SnCl2)-Co(CO)4

Preparation

Anhydrous SnCl2 is prepared by the action of dry hydrogen chloride gas on tin metal. The dihydrate is made by a similar reaction, using hydrochloric acid:

Sn( s) + 2 HCl( aq) → SnCl2(aq) + H2(g)

The water is then carefully evaporated from the acidic solution to produce crystals of SnCl2·2H2O. This dihydrate can bedehydrated to anhydrous using acetic anhydride.

Uses

A solution of tin(II) chloride containing a little hydrochloric acid is used for the tin-plating of steel, in order to make tin cans. An electric potential is applied, and tin metal is formed at the cathode via electrolysis.

It is used as a catalyst in the production of the plastic polylactic acid (PLA).

Tin(II) chloride also finds wide use as areducing agent. This is seen in its use for silvering mirrors, where silver metal is deposited on the glass:

Sn2+(aq) + 2 Ag+ → Sn4+(aq) + 2 Ag(s)

A related reduction was traditionally used as an analytical test for Hg2+(aq). For example, if SnCl2 is added dropwise into a solution ofmercury(II) chloride, a white precipitate ofmercury(I) chloride is first formed; as more SnCl2 is added this turns black as metallic mercury is formed. Stannous chloride can be used to test for the presence of goldcompounds. SnCl2 turns bright purple in the presence of gold.

This very reaction between stannous chloride and gold was used in episode #215: Clueless of House, M.D. to affirm gold poisoning.

In organic chemistry, SnCl2 is mainly used in the Stephen reduction, whereby a nitrile is reduced (via an imidoyl chloride salt) to animine which is easily hydrolysed to analdehyde. The reaction usually works best with aromatic nitriles Aryl-CN. A related reaction (called the Sonn-Müller method) starts with an amide, which is treated withPCl5 to form the imidoyl chloride salt.



The Stephen reduction is less used today, because it has been mostly superseded bydiisobutylaluminium hydride reduction.