Halosilane

Hydrogen hahdes also react freely with elemental sihcon at moderate temperatures to yield halosilanes (26—31). 

The properties and applications of commercially important hydride functional silanes, ie, compounds having a Si—H bond halosilanes, ie, compounds having a Si—X bond and organosilanes, ie, compounds having a Si—C bond, are discussed hereia. Compounds having Si—OSi bonds are called sdoxanes or sihcones. Those having a Si—OR bond are called siUcon esters. Sdoxanes and siUcon esters are discussed elsewhere ia the Eniyclopedia (see Silicon COMPOUNDS, SILICON ESTERS SILICON COMPOUNDS, SILICONES). 

Dkect synthesis is the preparative method that ultimately accounts for most of the commercial siUcon hydride production. This is the synthesis of halosilanes by the dkect reaction of a halogen or haUde with siUcon metal, siUcon dioxide, siUcon carbide, or metal sihcide without an intervening chemical step or reagent. Trichlorosilane is produced by the reaction of hydrogen chloride and siUcon, ferrosiUcon, or calcium sihcide with or without a copper catalyst (82,83). Standard purity is produced in a static bed at 400—900°C. 

Only three inorganic halosilanes are produced on a large iadustrial scale, ie, tetrachlorosilane [10026-04-7] tetrafluorosilane [7783-61-17, and trichlorosilane. 

The heat of reaction of tetrachlorosilane with an excess of water is 290.0 kJ /mol (69.3 kcal/mol). The reaction of tetraduorosilane with excess water contrasts with the other halosilanes, because it leads to formation of hexaduorosiUcic acid and a hydrous siUca. 

The reaction of halosilanes with alcohols proceeds analogously. Dihinctional amines react with tetrahalosilanes to yield tetrakis(dialkylamino)silanes. 

Reduction of halosilanes with hydtides leads to the formation of hydtide functional silanes and is considered in that section. 

Organometalhcs. Halosilanes undergo substitution reactions with alkali metal organics, Grignard reagents, and alkylaluininums. These reactions lead to carbon—siUcon bond formation. 

Halosilanes can also be produced by displacement of amines with a hydrogen hahde (173). 

Mixed halosilanes are produced by warming a mixture of tetrahydrosilanes. 

Health and Safety. Halosilane vapors react with moist air to produce the respective hydrohalogen acid mist. Federal standards have not set exposure to halosilanes, but it is generally beheved that there is no serious risk if vapor concentrations are maintained below a level that produces an irritating concentration of acid mist. The exposure threshold limit value (TLV) for HCl is 5 ppm, expressed as a ceiling limit. Because most people experience odor and irritation at or below 5 ppm, HCl is considered to have good warning properties. 

R2ASNCS, react similarly to the phosphorus halides (76RCR25). Halosilanes do not react with thiiranes. 

Halosilanes also undergo facile elimination when treated with methoxide ioa... 

Halosilanes are highly volatile as well as less aggressive, and thus, separation of the resulting polymers from inorganic salts is not required. 

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

An alternative method for preparing phosphinous amides makes a profit on the high affinity between silicon and halogen atoms. This is the driving force of the reactions between halophosphanes and Ar-(trimethylsilyl)anilines, AT-(tri-methylsilyl)amides or AT-(trimethylsilyl)ureas and thioureas, as represented in the Scheme 6. In these processes the desired P-N bond and an halosilane are simultaneously formed [53,58-60]. 

Reduction of the very bulky halosilanes with alkali metals (Scheme 2.42,... 

Silicon substituents can be introduced into alkenes and alkynes by hydrosilation.70 This reaction, in contrast to hydroboration, does not occur spontaneously, but it can be carried out in the presence of catalysts such as H2PtCl6, hexachloroplatinic acid. Other catalysts are also available.71 Halosilanes are more reactive than trialkylsilanes.72... 

While trifluoro and other halosilanes function by increased electrophilicity at silicon, nucleophilic reactivity of allylic silanes can be enhanced by formation of anionic adducts (silicates). Reaction of allylic silanes with aldehydes and ketones can... 

An important reaction of silicon compounds containing electronegative substituents, such as the halogens or a Group 15 or 16 element, is hydrolysis, which often occurs very readily even with atmospheric moisture. The primary reaction in silicone production is the hydrolysis of halosilanes, usually the readily available chlorosilanes, to give silanols, which then undergo acid-catalyzed condensation with loss of water, as shown in Scheme 1. 

The presence of bulky substituents such as Bul on silicon greatly reduces the tendency towards siloxane formation, and with two or more such groups present the precautions suggested in points 1, 2, and 3 can often be relaxed. For example, the hydrolysis of highly sterically hindered halosilanes may require rather severe conditions. 

It was the action of reactive metals on (halomethyl)halosilanes (eq. 3) which provided a general route to such polymers. Thus, the action of sodium on ClCH2Si(CH3)2Cl gave [(CH3)2SiCH2]n [8]. Removal... 

---