Trimethylsilyl chloride

• Trimethylchlorosilane ( TMCS )
• Trimethylsilyl chloride ( TMS -Cl)

Colorless liquid with a pungent odor

Liquid

0.854 g · cm -3

-40 ° C

57 ° C.

253 hPa ( 20 ° C)

Violent decomposition in water

Risk

-382.8 KJ / mol

Template: Infobox chemical / molecular formula search available

Chlorotrimethylsilane is a colorless, water-clear, fuming in moist air, liquid with suffocating, pungent odor. It belongs to the group of halogenated silanes and is an important base chemical in organic synthesis, especially in the protection group chemistry.

Production

Chlorotrimethylsilane is jointly manufactured with dichlorodimethyl and trichloromethylsilane about the Müller -Rochow synthesis. In this case, powdered silicon with chloromethane first reacted at 350 ° C in the presence of powdered copper and copper oxide as a catalyst to dichlorodimethylsilane, the disproportionation under the reaction conditions to chlorotrimethylsilane and trichloromethylsilane:

The product mixture can be separated by distillation due to the different boiling points of its components. Traces of hydrogen chloride, which is possibly present in the chlorotrimethylsilane by hydrolysis, can be removed by distillation with the addition of little quinoline.

Properties

Physical Properties

Chlorotrimethylsilane is a colorless, mobile liquid. It has 0.85 g/cm3 with a lower density than water. It is quite volatile and boils at 57 ° C. The vapor pressure function is given by Antoine corresponding log10 (P) = A- ( B / ( T C )) ( P in bar, T in K) with A = 3.9656, B = 1123.68 and C = -47.171 in temperature range from 274.3 to 326.0 C with most of the non - protic solvents (e.g., hexane, benzene, toluene, diethyl ether, tetrahydrofuran, chloroform, dichloromethane, ethyl acetate, etc. ), it is miscible in all proportions.

Chemical Properties

Chlorotrimethylsilane is a typical electrophilic substance that is very susceptible to attack on the Si atom by nucleophiles. This manifests itself already in its hydrolysis. Upon contact with water, the chlorotrimethylsilane is first hydrolyzed in a violent exothermic reaction and formation of hydrogen chloride to trimethylsilanol, which is unstable and condense with elimination of water to the water- bis ( trimethylsilyl ) ether ( hexamethyldisiloxane ). The heat of hydrolysis is -46.8 kJ · mol -1.

On a laboratory scale of chlorotrimethylsilane may be purified by distillation over calcium hydride.

As typical Lewis acid coordinated to trimethylsilyl chloride compounds having "free" electron pairs. Protic numerous compounds ( for example, alcohols, peroxides ), their salts and some special ethers are reacted to form the corresponding silyl ethers ( Me3SiOR ). With protic compounds, the addition of a base which absorbs the hydrogen chloride formed is necessary. The trimethylsilyl group is called due to this property as precious proton. Against non- protic materials, it is substantially inert.

With suitable reducing agents (eg, lithium aluminum hydride ) of chlorotrimethylsilane reacts to trimethylsilane, ( CH 3) 3SiH.

Use

Chlorotrimethylsilane is mainly used in organic chemical synthesis for the introduction of trimethylsilyl groups in molecules. By deprotonation of an appropriately position the acidic anion is formed, which acts nucleophilically on the silicon atom, and in the sense of a SN2 reaction to release a chloride ion. In this way one establishes trimethylsilyl ether from alcohols. However, it is also non- ionic nucleophiles attack at the silicon and are silylated. The resulting hydrogen chloride is neutralized by the reaction medium added base.

Often it is not necessary to generate an anion by deprotonation. To improve the reactivity can also increase the electrophilicity of the trimethylsilyl group. For this purpose, an organic nitrogen base is used and conveniently chosen so that they not only reacts with the hydrogen chloride formed, but already with chlorotrimethylsilane. It first forms an N- silylated quaternary ammonium salt, which now transfers the trimethylsilyl group to the silylating molecule. In the illustrated example, pyridine is used as base. The intermediate N- Trimethylsilylpyridiniumchlorid silylating agent is very good, since the electrophilicity of the Si atom is increased by the positive charge on the adjacent nitrogen. Other suitable bases include, inter alia, imidazole, triethylamine and Hunigs base. The latter two bases are too sterically hindered at the nitrogen atom to react with the also very bulky trimethylsilyl group. Therefore, it is the use of such bases for the intermediate formation of a N- trimethylsilylated ammonium salt in addition, a catalytic amount of DMAP - added (4 ( dimethylamino) pyridine ).

On heteroatoms such trimethylsilyl groups are mostly used as protecting groups. The cleavage can be effected, for example, fluoride ions, since silicon has a very high affinity to fluorine.

However, bonded to carbon atoms trimethylsilyl also exhibit activating effect on the relevant C atom. The one makes, among others, in the Peterson olefination (an alternative to the Wittig reaction or the Horner -Wadsworth -Emmons reaction) advantage.

Due to the high stability of Si -O bonds to react with chlorotrimethylsilane ambident nucleophiles, such as enolates, preferably in oxygen. In this way so-called silyl enol ethers are formed from α - deprotonated aldehydes or ketones. The isomeric C- silylated products are usually formed only in deficiency.

Analogously we obtain from α - deprotonated esters and carboxylic acids, silyl ketene acetals or the bis- silyl ketene acetals. Silyl enol ethers and silyl ketene acetals play as latent nucleophiles in synthetic chemistry, especially in stereoselective reactions, a significant role.

In the Rühlmann variant of acyloin condensation TMS -Cl is used as the capture reagent. Compared with elemental sodium it behaves practically inert.

Another important field of application for the production of chlorotrimethylsilane is hydrophobized silica gels ( so-called reverse-phase silica ) of the column, particularly for HPLC. One of the hydrophobic silica is most commonly used on the surface with octadecyldimethylsilyl groups (ODS ) groups modified ( " silanated "). Since these bulky groups for reasons of space but can not occupy all the polar OH groups on the silica gel, allowed to the silica gel in a second reaction step of reacting with chlorotrimethylsilane to convert the remaining polar groups in non-polar silyl ether. This process is referred to as end-capping.

Also, glass surfaces can be " silanized " by contact with chlorotrimethylsilane. Chemically, it is the same process as the end-capping of silica gels. The glass surface is after this treatment temporarily not wetted by water, so the water runs almost without residue ( as if the surface greasy ). Probably due to mechanical abrasion of the holding effect only for a certain time, the glass has to be treated again.

Safety

Chlorotrimethylsilane hydrolyzed in moist air to form hydrochloric acid, which can cause to the skin, eyes, mucous membranes and respiratory tract burns strong.