Chlorine (trimethyl) silane
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| General | |||||||||||||||||||
| Surname | Chlorine (trimethyl) silane | ||||||||||||||||||
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| Molecular formula | C 3 H 9 ClSi | ||||||||||||||||||
| Brief description |
colorless liquid with a pungent odor |
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| properties | |||||||||||||||||||
| Molar mass | 108.64 g mol −1 | ||||||||||||||||||
| Physical state |
liquid |
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| density |
0.854 g cm −3 |
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| Melting point |
−40 ° C |
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| boiling point |
57 ° C |
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| Vapor pressure |
253 h Pa (20 ° C) |
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| solubility |
violent decomposition in water |
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| Thermodynamic properties | |||||||||||||||||||
| ΔH f 0 |
−382.8 kJ / mol |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | |||||||||||||||||||
Chlorine (trimethyl) silane is a colorless, water-clear liquid that smokes in moist air and has a suffocating, pungent odor. It belongs to the group of halogenated silanes and is an important basic chemical in organic chemical synthesis, especially in protecting group chemistry .
Manufacturing
Chlor (trimethyl) silane is produced together with dichloro (dimethyl) silane and trichloro (methyl) silane via the Müller-Rochow synthesis . Powdered silicon reacts with chloromethane at 350 ° C in the presence of powdered copper and copper oxide as a catalyst to initially form dichloro (dimethyl) silane, which disproportionates to chloro (trimethyl) silane and trichloro (methyl) silane under the reaction conditions :
The product mixture can be separated by distillation due to the different boiling points of its components. Traces of hydrogen chloride , which may be present in the chloro (trimethyl) silane due to hydrolysis , can be removed by distillation with the addition of a little quinoline .
properties
Physical Properties
Chlorine (trimethyl) silane is a colorless, easily mobile liquid. At 0.85 g / cm 3, it has a lower density than water. It is very volatile and boils at 57 ° C. According to Antoine, the vapor pressure function results accordingly:
in the temperature range from 274.3 to 326.0 K. It can be mixed in any ratio with most non-protic solvents (e.g. hexane, benzene, toluene, diethyl ether, tetrahydrofuran, chloroform, dichloromethane, ethyl acetate etc.).
Chemical properties
Chlor (trimethyl) silane is a typical electrophilic substance that is very easily attacked by nucleophiles on the silicon atom . This is already evident in its sensitivity to hydrolysis. Upon contact with water, the chloro (trimethyl) silane is first hydrolyzed in a violent reaction with evolution of heat and formation of hydrogen chloride to trimethylsilanol, which is unstable and condenses to the water-insoluble bis (trimethylsilyl) ether ( hexamethyldisiloxane ) with elimination of water . The heat of hydrolysis is −46.8 kJ mol −1 .
On a laboratory scale, chlorine (trimethyl) silane can be purified by distillation over calcium hydride .
As a typical Lewis acid , chlorine (trimethyl) silane coordinates to compounds that have lone pairs of electrons . It reacts with numerous protic compounds (e.g. alcohols, peroxides), their salts and some special ethers to form the corresponding silyl ether (Me 3 SiOR). In the case of protic compounds, it is necessary to add a base which scavenges the hydrogen chloride formed. Because of this property, the trimethylsilyl group is also known as the noble proton . It is largely inert towards non-protic substances.
With suitable reducing agents (e.g. lithium aluminum hydride ), chlorotrimethylsilane reacts to trimethylsilane , (CH 3 ) 3 SiH.
use
Chlor (trimethyl) silane is mainly used in organic chemical synthesis to introduce trimethylsilyl groups into molecules. By deprotonation at a correspondingly acidic position, an anion is formed which nucleophilically attacks the silicon atom and releases a chloride ion in the sense of an S N 2 reaction . In this way, chlorine (trimethyl) silane is produced from alcohols. However, non-ionic nucleophiles can also attack silicon and be silylated. The resulting hydrogen chloride is neutralized by a base added to the reaction medium.
It is often not necessary to generate an anion by deprotonation. The electrophilicity of the trimethylsilyl group can also be increased to improve reactivity. For this purpose, an organic nitrogen base is used and is conveniently selected so that it not only reacts with the hydrogen chloride produced, but also with the chlorotrimethylsilane. In the process, an N -silylated, quaternary ammonium salt is initially formed , which then transfers the trimethylsilyl group to the molecule to be silylated. In the example shown, pyridine is used as the base. The intermediate N -trimethylsilylpyridinium chloride is a very good silylating agent, since the electrophilicity of the Si atom is increased by the neighboring positive charge on nitrogen. Other suitable bases include: a. Imidazole , triethylamine and Hünig base . The two last-mentioned bases are sterically hindered too much at the nitrogen atom to react with the trimethylsilyl group, which is also very bulky. Therefore, when using such bases for the intermediate formation of an N -trimethylsilylated ammonium salt, a catalytic amount of DMAP (4- (dimethylamino) pyridine) is also added.
Such trimethylsilyl groups are mostly used as protective groups on heteroatoms . The cleavage can, for. B. done with fluoride ions , since silicon has a very high affinity for fluorine.
However, trimethylsilyl groups attached to carbon atoms also have an activating effect on the corresponding carbon atom. One makes u. a. in the Peterson olefination (an alternative to the Wittig reaction or the Horner-Wadsworth-Emmons reaction ).
Due to the high stability of Si – O bonds, chlorotrimethylsilane reacts with ambident nucleophiles, for example enolates , preferentially on oxygen. In this way, so-called silyl enol ethers are formed from α-deprotonatable aldehydes or ketones. The isomeric, C-silylated products are usually only formed in deficit.
Analogously to this, the silyl ketene acetals or the bis-silyl ketene acetals are obtained from α-deprotonatable esters and carboxylic acids. Silyl enol ethers and silyl ketene acetals play an important role as latent nucleophiles in synthetic chemistry, especially in stereoselective conversions.
In the Rühlmann variant of the acyloin condensation , TMS-Cl is used as a capture reagent . It is practically inert towards elemental sodium .
Another important area of application for chlor (trimethyl) silane is the production of hydrophobized silica gels (so-called reverse-phase silica gels) for column chromatography , especially for HPLC . One of the most frequently used hydrophobic silica gels is modified (“silanized”) on the surface with octadecyldimethylsilyl groups (ODS groups). Since these bulky groups cannot occupy all polar OH groups on the silica gel surface for reasons of space , the silica gel is allowed to react with chlorotrimethylsilane in a second reaction step in order to convert the remaining polar groups into non-polar silyl ethers. This process is known as endcapping .
Glass surfaces can also be silanized through contact with chlorine (trimethyl) silane. Chemically, it is the same process as the endcapping of silica gels. After this treatment, the glass surface is temporarily no longer wettable by water, so that the water rolls off almost without residue (as if the surface were greasy). Probably due to mechanical abrasion, the effect only lasts for a certain time, then the glass has to be treated again.
safety instructions
Chlorine (trimethyl) silane hydrolyzes in moist air with the formation of hydrogen chloride, which can cause severe burns to skin, eyes, mucous membranes and respiratory tract.
Individual evidence
- ↑ a b c d e f Entry on trimethylchlorosilane in the GESTIS substance database of the IFA , accessed on January 8, 2018(JavaScript required) .
- ↑ a b c Entry on methylchlorosilanes. In: Römpp Online . Georg Thieme Verlag, accessed on June 19, 2014.
- ↑ T. Rugina, M. Gaspar, L. Sacarescu: Liquid-vapor equilibrium for a binary system of dichlorodimethyl-silane with trichloromethylsilane, chloromethylsilane and silicon tetrachloride. In: Rev. Chim. (Bucharest). 38, 1987, p. 680.
- ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-25.
- ↑ A. Capkova, V. Fried: Equilibrium liquid-vapor in the system tetrachlorosilane-trimethylchlorosilane. In: Collect Czech Chem Commun . 29, 1964, pp. 336-340, doi: 10.1135 / cccc19640336 .
- ^ AE Beezer, CT Mortimer: Heat of formation and bond energies. Part XV. Chlorotrimethylsilane and Hexamethyldisilazane. In: J. Chem. Soc. A. 1966, pp. 514-516. doi: 10.1039 / J19660000514