Trimethylsilanol

Trimethylsilanol (TMS) is an organosilicon compound with the formula (CH₃)₃SiOH. The Si centre bears three methyl groups and one hydroxyl group. It is a colourless volatile liquid.Paul D. Lickiss: ''The Synthesis and Structure of Organosilanols'', Advances in Inorganic Chemistry 1995, Volume 42, Pages 147–262, .

Occurrence

TMS is a contaminant in the atmospheres of spacecraft, where it arises from the degradation of silicone-based materials. Specifically, it is the volatile product from the hydrolysis of polydimethylsiloxane, which are generally terminated with trimethylsilyl groups:
:(CH₃)₃SiO i(CH₃)₂Osub>nR + H₂O → (CH₃)₃SiOH + HO i(CH₃)₂Osub>nR

TMS and related volatile siloxanes are formed by hydrolysis of silicones-based containing materials, which are found in detergents and cosmetic products.

Traces of trimethylsilanol, together with other volatile siloxanes, are present in biogas and landfill gas, again resulting from the degradation of silicones. As their combustion forms particles of silicates and microcrystalline quartz, which cause abrasion of combustion engine parts, they pose problems for the use of such gases in combustion engines.

Production

Trimethylsilanol is accessible by weakly basic hydrolysis of chlorotrimethylsilane, since the dimerization can thus be avoided. Trimethylsilanol can also be obtained by the basic hydrolysis of hexamethyldisiloxane.

Trimethylsilanol cannot be produced by simple hydrolysis of chlorotrimethylsilane as this reaction leads to the etherification product hexamethyldisiloxane, because of the by-product hydrochloric acid.Didier Astruc: ''Organometallic Chemistry and Catalysis.'' Springer Science & Business Media, 2007, , S. 331 ().
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Reactions

Trimethylsilanol is a weak acid with a pK_a value of 11.T. Kagiya, Y. Sumida, T. Tachi: ''An Infrared Spectroscopic Study of hydrogen Bonding Interaction. Structural Studies of Proton-donating and -accepting Powers'' in Bull. Chem. Soc. Jpn. 43 (1970), 3716–3722. The acidity is comparable to that of orthosilicic acid, but much higher than the one of alcohols like ''tert''-butanol (pK_a 19). Deprotonation with sodium hydroxide gives sodium trimethylsiloxide.

TMS reacts with the silanol groups (R³SiOH) giving silyl ethers.

Structure

In terms of its structure, the molecule is tetrahedral. The compound forms monoclinic crystals.R. Minkwitz, S. Schneider: ''Die Tieftemperaturkristallstruktur von Trimethylsilanol.'' In: ''Zeitschrift für Naturforschung B.'' 53, 1998, S. 426–429
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Additional properties

The heat of evaporation is 45.64 kJ·mol⁻¹, the evaporation entropy 123 J·K⁻¹·mol⁻¹. The vapor pressure function according to Antoine is obtained as log₁₀(P/1bar) = A − B/(T + C) (P in bar, T in K) with A = 5.44591, B = 1767.766K and C = −44.888K in a temperature range from 291K to 358K. Below the melting point at −4.5 °C,Batuew et al. in Doklady Akademii Nauk SSSR 95 (1954) 531. The ¹H NMR in CDCl₃ shows a singlet at δ=0.14 ppm.

Bioactivity

Like other silanols, trimethylsilanol exhibits antimicrobial properties.{{cite journal, journal=Electronic Journal of Biotechnology , volume= 9 , issue= 2 , year=2006 , title=Silanol – A novel class of antimicrobial agent , author=Yun-mi Kim, Samuel Farrah, Ronald H. Baney , url=http://www.ejbiotechnology.info/content/vol9/issue2/full/4/index.html , doi=10.2225/vol9-issue2-fulltext-4 , pages=176–180

References

Silanols
Trimethylsilyl compounds