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The '''Direct Process''', also called the '''Direct Synthesis''', '''Rochow Process''', and '''Müller-Rochow Process''' is the most common technology for preparing [[organosilicon]] compounds on an industrial scale. It was first reported independently by [[Eugene G. Rochow]] and [[Richard Müller (chemist)|Richard Müller]]  in the 1940s.<ref name=Roeshe>Röshe, L.; John, P.; Reitmeier, R.  “Organic Silicon Compounds”  Ullmann’s Encyclopedia of Industrial Chemistry.  John Wiley and Sons: San Francisco, 2003.  {{DOI|10.1002/14356007.a24_021}}.</ref>  The reaction involves a copper-catalyzed reaction of alkyl halides with silicon metal, which takes place in a fluidized bed reactor.  Although theoretically possible with any type of alkyl halide, the best results in terms of selectivity and yield occur with [[methyl chloride]].  Typical conditions are 300 °C and 2-5 bar.  These conditions allow for 90-98% conversion for silicon and 30-90% for [[chloromethane]].  Approximately 1.4 Mton of [[dimethyldichlorosilane]] (Me<sub>2</sub>SiCl<sub>2</sub>) is produced annually using this process.<ref name=Elschenbroich>Elschenbroich, Christoph Organometallics VCH, Weinheim, Germany: 1992.  ISBN 978-3527293902.</ref>

Few companies actually carry out the Rochow process, because of the complicated process technology and high capital requirements. Since the [[silicon]] is [[crushed]] prior to reaction in a [[fluidized bed]], the companies practicing this technology are referred to as “silicon crushers”.<ref>[http://www.siliconespectator.com/articles/Silicone_Spectator_January_2009.pdf Basic Silicone Chemistry – A Review]</ref>

==Reaction and mechanism==
The relevant reactions are (Me = CH<sub>3</sub>):
:x MeCl + Si → Me<sub>3</sub>SiCl, Me<sub>2</sub>SiCl<sub>2</sub>, MeSiCl<sub>3</sub>, other products

The mechanism of the direct process is still not well understood, despite much research.  [[Copper]] plays an important role.  The copper and silicon form an [[intermetallic]]s with the approximate composition Cu<sub>3</sub>Si.  This intermediate facilitates the formation of the Si-Cl and Si-Me bonds.  It is proposed that close proximity of the Si-Cl to a copper-chloromethane “adduct” allows for formation of the Me-SiCl units.  Transfer of a second chloromethane allows for the release of the Me<sub>2</sub>SiCl<sub>2</sub>.  Thus, copper is oxidized from the zero oxidation state and then reduced to regenerate the catalyst.<ref name=Roeshe/>

The chain reaction can be terminated in many ways.  These termination processes give rise to the other products that are seen in the reaction.  For example, combining two Si-Cl groups gives the SiCl<sub>2</sub> group, which undergoes Cu-catalyzed reaction with MeCl to give MeSiCl<sub>3</sub>.<ref name=Roeshe/>

In addition to copper, the catalyst optimally contains promoter metals that facilitate the reaction.  Among the many promoter metals, zinc, tin, antimony, magnesium, calcium, bismuth, arsenic and cadmium have been mentioned.<ref name=Roeshe/>
,<ref name=Elschenbroich/>

==Product distribution and isolation==
The major product for the direct process should be dichlorodimethylsilane, Me<sub>2</sub>SiCl<sub>2</sub>.  However, many other products are formed.  Unlike most reactions, this distribution is actually desirable because the product isolation is very efficient.<ref name=Roeshe/>  Each methylchlorosilane has specific and often substantial applications.  Me<sub>2</sub>SiCl<sub>2</sub> is the most useful.  It is the precursor for the majority of silicon products produced on an industrial scale.  The other products are used in the preparation of siloxane polymers as well as specialized applications.<ref name=Roeshe/>

Dichlorodimethylsilane is the major product of the reaction, as is expected, being obtained in about 70-90% yield.  The next most abundant product is MeSiCl<sub>3</sub>, at 5-15% of the total.  Other products include Me<sub>3</sub>SiCl (2-4%), MeHSiCl<sub>2</sub>  (1-4%), and Me<sub>2</sub>HSiCl (0.1-0.5%).<ref name=Roeshe/>

The Me<sub>2</sub>SiCl<sub>2</sub> is purified by [[fractional distillation]].  Although the boiling points of the various chloro methyl silanes are similar (Me<sub>2</sub>SiCl<sub>2</sub>:70 C, MeSiCl<sub>3</sub>: 66 C, Me<sub>3</sub>SiCl: 57 C, MeHSiCl<sub>2</sub>: 41 C, Me<sub>2</sub>HSiCl: 35 C), the distillation utilizes columns with high separating capacities, connected in series.  The purity of the products crucially affects the production of siloxane polymers, otherwise chain branching arises.<ref name=Roeshe/>

==References==
<references/>

[[Category:Chemical synthesis]]
[[Category:Chemical reactions]]
[[Category:Silicon]]

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