Organosilicon chemistry

Organosilicon chemistry is the scientific study of organometallic compounds that contain carbon–silicon bonds. These specific connections define the class of substances known as organosilicon compounds. Most of these materials share physical traits with common organic compounds. They are typically colorless, flammable, and stable when exposed to air. They are also hydrophobic, meaning they repel water.

At the atomic level, the behavior of these compounds is driven by how silicon and carbon interact. In most organosilicon molecules, the silicon atom is tetravalent. This means it forms four bonds and adopts a tetrahedral molecular geometry. When comparing these to standard organic molecules, the carbon–silicon bond is notably different. The C–Si bond length is approximately 1.89 Å, which is significantly longer than the 1.54 Å found in a typical C–C bond. This increased length suggests that silyl substituents have less steric demand than their organic counterparts. Furthermore, the C–Si bond is somewhat polarized toward the carbon atom. This happens because carbon has a higher electronegativity of 2.55 compared to silicon's 1.90.

Because of this polarization, silicon is susceptible to nucleophilic attack by elements like oxygen, chlorine, or fluorine. The bond between silicon and oxygen is particularly noteworthy due to its strikingly high energy. This chemical reactivity is exploited in several specialized laboratory reactions. These include the Sakurai reaction, the Brook rearrangement, the Fleming–Tamao oxidation, and the Peterson olefination. Silicon also possesses a unique ability called negative hyperconjugation. When the molecular geometry allows, this effect can reverse the usual polarization on neighboring atoms.

Chemists use several distinct methods to synthesize these compounds. One primary method is the Direct process. In this reaction, methyl chloride reacts with a silicon-copper alloy. This method is highly productive, yielding about 1 million tons of organosilicon compounds annually. The most sought-after product from this process is dimethyldichlorosilane. Other useful products include trimethylsilyl chloride and methyltrichlorosilane. Another major pathway for forming carbon–silicon bonds is hydrosilylation. In this process, compounds with Si–H bonds, known as hydrosilanes, are added to unsaturated substrates like alkenes. This reaction usually requires metal catalysts, particularly those from the platinum group metals.

The history of this field is marked by several key discoveries. In 1863, Charles Friedel and James Crafts produced the first organochlorosilane compound. That same year, they also described a polysilicic acid ether. At the start of the 20th century, Frederic S. Kipping pioneered extensive research in the field. In 1904, Kipping coined the term "silicone," though the name was actually based on a mistaken resemblance to ketones. Kipping was famous for using Grignard reagents to create alkylsilanes and arylsilanes. He also succeeded in preparing the first silicone oligomers and polymers. To honor his work, the Dow Chemical Company established a silicon chemistry award in the 1960s.

Organosilicon compounds are categorized into various functional groups that behave similarly to organic groups. Silanols are analogues of alcohols and are often prepared by the hydrolysis of silyl chlorides. These silanols are quite acidic, roughly 500 times more acidic than their alcohol counterparts. When silanols undergo dehydration, they form siloxanes. Polymers made of repeating siloxane linkages are known as silicones.

Other groups include siloxides, which are deprotonated silanols, and silazanes, which use nitrogen instead of oxygen. Scientists also use silyl ethers, which have a Si–O–C connectivity, as protective groups for alcohols in chemical synthesis.

These substances have widespread applications in industry and biology. Silicone caulk and commercial sealants are common products made from organosilicon compounds mixed with a hardener. They are also used in adhesives, coatings, and antifoamers. In agriculture, they serve as adjuvants for herbicides and fungicides. One specific compound, silafluofen, acts as a pyrethroid insecticide. However, research shows these compounds can affect the immune expression of bees, making them more vulnerable to viruses. While carbon–silicon bonds do not occur naturally in biology, scientists have used enzymes to create them artificially in living microbes.