organosilicon compounds derive from organosilicon chlorides . These 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 + Si → 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) add to unsaturated substrates. Commercially, the main substrates are alkenes. Other unsaturated functional groups—alkynes, imines, ketones, and aldehydes, but these reactions are of little economic value. An example is the hydrosilation of phenylacetylene: Hydrosilylation requires metal catalysts, especially those based on platinum group metals. In the related silylmetalation, a metal replaces the hydrogen atom. Cleavage or Si-Si bonds Hexamethyldisilane reacts with methyl lithium to give trimethylsilyl lithium: Similarly, tris(trimethylsilyl)silyl lithium is derived from tetrakis(trimethylsilyl)silane: 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, and siloxanes Silanols are analogues of alcohols. They are generally prepared by hydrolysis of silyl chlorides: + → + HCl Less frequently silanols are prepared by oxidation of silyl hydrides, a reaction that uses a metal catalyst: 2 + → 2 Many silanols have been isolated including and . They are about 500x more acidic than the corresponding alcohols. Siloxides are the deprotonated derivatives of silanols: + NaOH → + Silanols tend to dehydrate to give siloxanes: 2 → + Polymers with repeating siloxane linkages are called silicones. Compounds with an Si=O double bond called silanones are extremely unstable. Silyl ethers Silyl ethers have the connectivity Si-O-C. They are typically prepared by the reaction of alcohols with silyl chlorides: + ROH → + 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: + + → + H-O-R + Silyl chlorides Organosilyl chlorides are important commodity chemicals. They are mainly used to produce silicone polymers as described above. Especially important silyl chlorides are dimethyldichlorosilane (), methyltrichlorosilane (), and trimethylsilyl chloride ()