Subject silicon halides

Salts giving an alkaline reaction may be corrosive to the irons, and while neutral solutions can be handled safely there is usually little point in using high-silicon irons for these relatively innocuous solutions. The irons are useful in handling acidic solutions, subject to the restrictions already referred to regarding the halide, sulphite and phosphate ions. 

SiHCl3 (boiling point 31.8°C) is subjected to fractional distillation and removed from Fe, B and A1 halides. Finally pure SiHCl3 is converted into silicon by chemical vapour deposition according to the reaction ... 

The presence of group 4 metals P to a carbenium ion has a dramatic effect on the stability of the carbenium ion, a remarkable phenomenon known as the P-effect.12 29, 30 The silicon P-effect was discovered in 1937 by Ushakov and Itenberg,31 who noted the exceptional reactivity of P-silyl-substituted halides R3SiCH2CH2X towards elimination (Scheme 1). The effect has since been the subject of many mechanistic32-36 and theoretical studies37-39 and several reviews.10-12,29... 

You have now seen how enols and enolates react with electrophiles based on hydrogen (deuterium), carbon, halogens, silicon, sulfur, and nitrogen. What remains to be seen is how new carbon-carbon bonds can be formed with alkyl halides and carbonyl compounds in their normal electrophilic mode. These reactions are the subject of Chapters 26-29. We must first look at the ways aromatic compounds react with electrophiles. You will see similarities with the behaviour of enols. 

Process Reaction and its application to the preparation of silicone precursors (6-9), plus a corresponding industrial preparative method for tetraal-kyllead compounds (10), spurred further investigations into the use of direct reactions between solid metals and liquid alkyl halides to synthesize alkylmetal derivatives, generating an extensive literature on this subject (3.4.11). 

The stereochemistry and mechanistic implication for nucleophilic substitution at silicon continue to receive attention, and have been the subject of a comprehensive reviewThe nucleophiles HMPA, DMSO, DMF, and PhaPO catalyze the hydrolysis of triorganochlorosilanes and also their racemization. Stable 1 1 ionic adducts of HMPA with MeaSiBr have been isolated in the past and suggested to be possible intermediates in the racemization process. However, it has now been shown that such ionic 1 1 adducts are not usually formed and that the reported example was a special case which should not be generalized into a mechanism involving displacement of halides by the nucleophile. The previously accepted mechanism involving the formation of five- and six-coordinate species still seems more feasible. 

Purification methods have been developed which usually involve conversion of silicon into volatile silanes which are then subjected to fractional distillation, and converted back to silicon. The Siemens process, Equations 3.2 and 3.3, involves the production of SiHCl3 by reacting silicon with HCl. Following fractional distillation, the halide is reduced with hydrogen at about 1 000 °C to produce rods of ultra-high purity silicon (Figure 3.2). 

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