Organosilicon chemistry

Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable to air. However, silicon carbide is considered an inorganic compound.

History

In 1863, Charles Friedel and James Crafts made the first organochlorosilane compound. The same year, they also described a "polysilicic acid ether" in the preparation of ethyl- and methyl-o-silicic acid. Extensive research in the field of organosilicon compounds was pioneered in the beginning of 20th century by Frederic S. Kipping. He also had coined the term "silicone" (resembling ketones, though this is erroneous) in relation to these materials in 1904. In recognition of Kipping's achievements, the Dow Chemical Company had established an award in the 1960s that is given for significant contributions to the field of silicon chemistry. In his works, Kipping was noted for using Grignard reagents to make alkylsilanes and arylsilanes and preparing silicone oligomers and polymers for the first time. In 1945, Eugene G. Rochow also made a significant contribution to the field of organosilicon chemistry by first describing the Müller-Rochow process.

Occurrence and applications

Organosilicon compounds are widely encountered in commercial products. Most common are antifoamers, caulks (sealant), adhesives, and coatings made from silicones. Other important uses include agricultural and plant control adjuvants commonly used in conjunction with herbicides and fungicides.

Biology and medicine Carbon–silicon bonds are absent in biology, however enzymes have been used to artificially create carbon-silicon bonds in living microbes. Silicates, on the other hand, have known existence in diatoms. Silafluofen is an organosilicon compound that functions as a pyrethroid insecticide. Several organosilicon compounds have been investigated as pharmaceuticals.

Bonding

In the great majority of organosilicon compounds, Si is tetravalent with tetrahedral molecular geometry. Compared to carbon–carbon bonds, carbon–silicon bonds are longer and weaker. The C–Si bond is somewhat polarised towards carbon due to carbon's greater electronegativity (C 2.55 vs Si 1.90), and single bonds from Si to electronegative elements are very strong. Silicon is thus susceptible to nucleophilic attack by O−, Cl−, or F−; the energy of an Si–O bond in particular is strikingly high. This feature is exploited in many reactions such as the Sakurai reaction, the Brook rearrangement, the Fleming–Tamao oxidation, and the Peterson olefination. The Si–C bond (1.89 Å) is significantly longer than a typical C–C bond (1.54 Å), suggesting that silyl substitutents have less steric demand than their organyl analogues. When geometry allows, silicon exhibits negative hyperconjugation, reversing the usual polarization on neighboring atoms.

Preparation The first organosilicon compound, tetraethylsilane, was prepared by Charles Friedel and James Crafts in 1863 by reaction of tetrachlorosilane with diethylzinc. Most organosilicon compounds derive from organosilicon chlorides (CH3)4−xSiClx. These methyl chlorides are produced by the "Direct process", which entails the reaction of methyl chloride with a silicon-copper alloy. The main and most sought-after product is dimethyldichlorosilane:

2 CH3Cl + Si → (CH3)2SiCl2 A variety of other products are obtained, including trimethylsilyl chloride and methyltrichlorosilane. About 1 million tons of organosilicon compounds are prepared annually by this route. The method can also be used for phenyl chlorosilanes.

Hydrosilylation

Another major method for the formation of Si-C bonds is hydrosilylation (also called hydrosilation). In this process, compounds with Si–H bonds (hydrosilanes) are added to unsaturated substrates. Commercially, the main substrates are alkenes. Other unsaturated functional groups — alkynes, imines, ketones, and aldehydes — also participate, but these reactions are of little economic value.

Hydrosilylation requires metal catalysts, especially those based on platinum group metals. In the related silylmetalation, a metal replaces the hydrogen atom.

Via cleavage of Si–Si bonds Hexamethyldisilane reacts with methyllithium to give trimethylsilyl lithium:

(CH3)6Si2 + CH3Li → (CH3)3SiLi + (CH3)4Si Similarly, tris(trimethylsilyl)silyl lithium is derived from tetrakis(trimethylsilyl)silane:

((CH3)3Si)4Si + CH3Li → ((CH3)3Si)3SiLi + (CH3)4Si

Functional groups Silicon is a component of many functional groups. Most of these are analogous to organic compounds. The overarching exception is the rarity of multiple bonds to silicon, as reflected in the double bond rule.

Silanols, siloxides, siloxanes, and silazanes Silanols are analogues of alcohols. They are generally prepared by hydrolysis of silyl chlorides:

R3SiCl + H2O → R3SiOH + HCl Less frequently silanols are prepared by oxidation of silyl hydrides, a reaction that uses a metal catalyst:

2 R3SiH + O2 → 2 R3SiOH Many silanols have been isolated including (CH3)3SiOH and (C6H5)3SiOH. They are about 500x more acidic than the corresponding alcohols. Siloxides are the deprotonated derivatives of silanols:

R3SiOH + NaOH → R3SiONa + H2O Silanols tend to dehydrate to give siloxanes:

2 R3SiOH → R3Si-O-SiR3 + H2O Polymers with repeating siloxane linkages are called silicones. Compounds with an Si=O double bond called silanones are extremely unstable. Analogous compounds with nitrogen instead of oxygen are the silazanes.

Silyl ethers Silyl ethers have the connectivity Si–O–C. They are typically prepared by the reaction of alcohols with silyl chlorides:

(CH3)3SiCl + ROH → (CH3)3Si−O−R + HCl Silyl ethers are extensively used as protective groups for alcohols. Exploiting the strength of the Si–F bond, fluoride sources such as tetra-n-butylammonium fluoride (TBAF) are used in deprotection of silyl ethers:

(CH3)3Si−O−R + F− + H2O → (CH3)3Si−F + H−O−R + OH−

Silyl chlorides

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