Stannic chloride, reaction + metal atoms

Hydrogen haHde addition to vinyl chloride in general yields the 1,1-adduct (50—52). The reactions of HCl and hydrogen iodide [10034-85-2], HI, with vinyl chloride proceed by an ionic mechanism, while the addition of hydrogen bromide [10035-10-6], HBr, involves a chain reaction in which a bromine atom [10097-32-2] is the chain carrier (52). In the absence of a transition-metal catalyst or antioxidants, HBr forms the 1,2-adduct with vinyl chloride (52). HF reacts with vinyl chloride in the presence of stannic chloride [7646-78-8], SnCl, to form 1,1-difluoroethane [75-37-6] (53). 

To 1 molecule of a mononitro-compound, i atoms of tin, or 3 molecules of stannous chloride, are therefore used. In calculating tire amount of the latter necessary for a reaction, it is to be remembered that the salt crystallises with two molecules of water (SnCl,+ 2H,0). If the reduction is to be effected by metallic tin, double the above quantity is frequently used, i.e. to 1 nitro-group, 3 atoms of tin. In this case, the tin is not converted into stannic chloride, but into stannous chloride ... 

The reduction of nitrobenzene to aniline is a typical oxidation-reduction reaction in which tin metal, Sn°, is oxidized to stannic ion, Sn, in the form of stannic chloride, SnCl4 hydrochloric acid serves as the source of protons. A plausible mechanism of this reaction is outlined in Scheme 21.3. Generally, the reduction occurs by a sequence of steps in which an electron is first transferred from a tin atom to the organic substrate to give an intermediate radical ion that is then protonated. The oxygen atoms on the nitro group are eventually removed as water molecules. It is left as an exercise to write a balanced equation for the overall reaction and to provide a mechanism for the reduction of N-phenylhydroxylamine (11) into aniline (see Exercises 13 and 14 at the end of this section). 

Tin hydride has been made by the reaction of hydrochloric acid with magnesium-tin alloy (1) or metallic tin (2) by reduction of stannous salts electrolytically (3), by metals (4, 5), or sodium borohydride (6) and by reduction of stannic chloride by atomic hydrogen (7) or metals (S). Perhaps the most practical method involves the reduction of stannic chloride with lithium aluminum hydride under nitrogen (9, 10, 11) containii 0 1% oxygen 12) at — 70° C. The presence of oxygen inhibits the decomposition, which ordinarily occurs at room temperature. This inhibition is at least partially due to the fact that metallic tin is a catalyst for the decomposition, but in the presence of oxygen becomes coated with a film of oxide, which is ineffective as a catalyst 12). 

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