Hexamethyldisiloxane (HMDSO or MM) is an organosilicon compound with the formula O i(CH₃)₃sub>2. This volatile colourless liquid is used as a

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

and as a reagent in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

. It is prepared by the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of trimethylsilyl chloride. The molecule is the protypical di

siloxane In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: . The parent siloxanes include the oligomeric and polymeric hydrides with the formulae and . Siloxanes ...

and resembles a subunit of

polydimethylsiloxane Polydimethylsiloxane (PDMS), also known as dimethylpolysiloxane or dimethicone, is a silicone polymer with a wide variety of uses, from cosmetics to industrial lubrication and passive daytime radiative cooling. PDMS is particularly known for its ...

Synthesis and reactions

Hexamethyldisiloxane can be produced by the addition of trimethylsilyl chloride to purified water: : 2 Me₃SiCl + H₂O → 2 HCl + O i(CH₃)₃sub>2 It also results from the hydrolysis of silyl ethers and other silyl-protected functional groups. HMDSO can be converted back to the chloride by reaction with Me₂SiCl₂.Röshe, L.; John, P.; Reitmeier, R. “Organic Silicon Compounds” ''Ullmann’s Encyclopedia of Industrial Chemistry''. John Wiley and Sons: San Francisco, 2003. . Hexamethyldisiloxane is mainly used as source of the

trimethylsilyl A trimethylsilyl group (abbreviated TMS) is a functional group in organic chemistry. This group consists of three methyl groups bonded to a silicon atom minus;Si(CH3)3 which is in turn bonded to the rest of a molecule. This structural group i ...

functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

(-Si(CH₃)₃) in

. For example, in the presence of acid

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

, it converts alcohols and carboxylic acids into the silyl ethers and silyl esters, respectively. It reacts with rhenium(VII) oxide to give a

siloxide Siloxides are chemical compounds with the formula R3SiOM, where R is usually an organic group and M is usually a metal cation. Also called silanolates, they are derived by deprotonation of Silanol, silanols. They also arise by the degradation of ...

: : Re₂O₇ + O i(CH₃)₃sub>2 → 2 O₃ReOSi(CH₃)₃

Niche uses

HMDSO is used as an internal standard for calibrating

chemical shift In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...

in¹ H NMR spectroscopy. It is more easily handled since it is less volatile than the usual standard tetramethylsilane but still displays only a singlet near 0ppm. HMDSO has even poorer solvating power than alkanes. It is therefore sometimes employed to crystallise 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 semiconductor industries by plasma-enhanced chemical vapour deposition (PECVD). HMDSO has been used as a reporter molecule to measure tissue oxygen tension (pO). HMDSO is highly hydrophobic and exhibits high gas solubility, and hence strong nuclear magnetic resonance spin lattice relaxation rate (R1) response to changes in pO{{sub, 2. Molecular symmetry provides a single NMR signal. Following direct injection into tissues it has been used to generate maps of tumour and muscle oxygenation dynamics with respect to hyperoxic gas breathing challenge.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.

References

Siloxanes Trimethylsilyl compounds