HMDS

Hexamethyldisiloxane

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Names
Preferred IUPAC name Hexamethyldisiloxane
Other names Bis(trimethylsilyl) ether Bis(trimethylsilyl) oxide
Identifiers
CAS Number	107-46-0
Abbreviations	HMDSO, (TMS)2O
Beilstein Reference	1736258
ChEBI	CHEBI:78002
ChemSpider	23150
EC Number	203-492-7
InChI[show]
Jmol interactive 3D	Image
MeSH	Hexamethyldisiloxane
PubChem	24764
RTECS number	JM9237000
SMILES[show]
UNII	D7M4659BPU
UN number	1993
Properties
Chemical formula	C6H18OSi2
Molar mass	162.38 g·mol−1
Appearance	Colourless liquid
Density	0.764 g cm−3
Melting point	−59 °C (−74 °F; 214 K)
Boiling point	100 to 101 °C (212 to 214 °F; 373 to 374 K)
Solubility in water	930.7±33.7 ppb (23 °C) [1]
Vapor pressure	43 hPa (20 °C) [2]
Refractive index (nD)	1.377
Hazards
Main hazards	Highly flammable liquid and vapour Causes serious eye irritation
EU classification (DSD)	F
R-phrases	R11
S-phrases	S16
NFPA 704	4 1 0
Flash point	-1(1) °C
Related compounds
Related compounds	Disiloxane Tetramethylsilane Dimethyl ether Bis(trimethylsilyl)amine
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Hexamethyldisiloxane

Hexamethyldisiloxane (HMDSO) is an organosilicon compound with the formula O[Si(CH₃)₃]₂. This volatile colourless liquid is used as a solvent and as a reagent in organic synthesis. It is prepared by the hydrolysis of trimethylsilyl chloride. The molecule is the protypical disiloxane and resembles a subunit of polydimethylsiloxane.

Synthesis and reactions[edit]

Hexamethyldisiloxane can be produced by hydrolysis of trimethylsilyl chloride:

2 Me₃SiCl + H₂O → 2 HCl + O[Si(CH₃)₃]₂

It also results from the hydrolysis of silylethers and other silyl-protected functional groups. HMDSO can be converted back to the chloride by reaction with Me₂SiCl₂.^[3]

Hexamethyldisiloxane is mainly used as source of the trimethylsilyl functional group (-Si(CH₃)₃) in organic synthesis. For example, in the presence of acid catalyst, it converts alcohols and carboxylic acids into the silyl ethers and silyl esters, respectively.^[4]

It reacts with rhenium(VII) oxide to give the silanoate complex:^[5]

Re₂O₇ + O[Si(CH₃)₃]₂ → 2 O₃ReOSi(CH₃)₃

Niche uses[edit]

HMDSO is used as an internal standard for calibrating chemical shift in¹H NMR spectroscopy. It is more easily handled since it is less volatile than the usual standard tetramethylsilane but still displays only a singlet near 0 ppm.

HMDSO has even poorer solvating power than alkanes. It is therefore sometimes employed to crystallize highly lipophilic compounds.

It is used in liquid bandages (spray on plasters) such as cavilon spray, to protect damaged skin from irritation from other bodily fluids. It is also used to soften and remove adhesive residues left by medical tape and bandages, without causing further skin irritation.

HMDSO is being studied for making low-k dielectric materials for the semi-conductor industries by PECVD (plasma-enhanced chemical vapor deposition).

HMDSO has been used as a reporter molecule to measure tissue oxygen tension (pO2). HMDSO is highly hydrophobic and exhibits high gas solubility, and hence strong nuclear magnetic resonance spin lattice relaxation rate (R1) response to changes in pO2. Molecular symmetry provides a single NMR signal. Following direct injection into tissues it has been used to generate maps of tumor and muscle oxygenation dynamics with respect to hyperoxic gas breathing challenge.^[6]

References[edit]

1. Jump up ^ Sudarsanan Varaprath, Cecil L. Frye, Jerry Hamelink (1996). "Aqueous solubility of permethylsiloxanes (silicones)". Environmental Toxicology and Chemistry 15 (8): 1263–1265. doi:10.1002/etc.5620150803.
2. Jump up ^ Record of CAS RN 107-46-0 in the GESTIS Substance Database of the IFA, accessed on March 11, 2015.
3. Jump up ^ 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.
4. Jump up ^ Pfeifer, J. "Hexamethyldisiloxane" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
5. Jump up ^ Schmidt, M.; Schmidbaur, H., "Trimethylsilyl perrhenate", Inorg. Synth. 1967, 9, 149-151. doi:10.1002/9780470132401.ch40
6. Jump up ^ Kodibagkar VD, Cui W, Merritt ME, Mason RP. A novel 1H NMR approach to quantitative tissue oximetry using hexamethyldisiloxane. Magn Reson Med 2006;55:743–748 and Kodibagkar VD, Wang X, Pacheco-Torres J, Gulaka P, Mason RP. Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL): a tool for quantitative tissue oximetry. NMRBiomed 2008;21:899-907.

   

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