Octamethylcyclotetrasiloxane

Not to be confused with 22-dihydroergocalciferol.

Octamethylcyclotetrasiloxane, also called D₄, is an organosilicon compound and one of a number of cyclic siloxanes (cyclomethicones). It is a colorless viscous liquid with a high boiling point. It has historically been used on a large scale in personal care products including cosmetics, hair conditioners and emollients (moisturising creams),^[5] Global production in 1993 was 136,000 tons,^[6] however it is now facing significant pressure from regulators. It is a substance of very high concern In the EU, where it was classified as a PBT and effectively banned in personal care products in 2018. The US EPA began reevaluating its risks in 2020.^[7] Replacements include branched-chain alkanes (isoparaffins).

Octamethylcyclotetrasiloxane

Names
Preferred IUPAC name Octamethyl-1,3,5,7,2,4,6,8-tetroxatetrasilocane
Other names D4 D4 OMCTS
Identifiers
CAS Number	556-67-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:25640
ChemSpider	10696
ECHA InfoCard	100.008.307
EC Number	209-136-7
PubChem CID	11169
UNII	CZ227117JE Y
CompTox Dashboard (EPA)	DTXSID7027205
InChI InChI=1S/C8H24O4Si4/c1-13(2)9-14(3,4)11-16(7,8)12-15(5,6)10-13/h1-8H3 Key: HMMGMWAXVFQUOA-UHFFFAOYSA-N
SMILES C[Si]1(O[Si](O[Si](O[Si](O1)(C)C)(C)C)(C)C)C
Properties
Chemical formula	[(CH3)2SiO]4
Molar mass	296.616 g·mol−1
Density	0.956 g/mL
Melting point	17–18 °C (63–64 °F; 290–291 K)
Boiling point	175–176 °C (347–349 °F; 448–449 K)
Solubility in water	56.2±2.5 ppb (23 °C)[1]
log P	6.98±0.13[2]
Vapor pressure	124.5±6.2 Pa (25 °C)[3]
Hazards
GHS labelling:
Pictograms	[4]
Signal word	Warning
Hazard statements	H361f, H410 M=10[4]
Related compounds
Related compounds	Disiloxane Tetramethylsilane Dimethyl ether Bis(trimethylsilyl)amine Tetrakis(trimethylsilyloxy)silane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Production and polymerization

Commercially D₄ is produced from dimethyldichlorosilane. Hydrolysis of the dichloride produces a mixture of cyclic dimethylsiloxanes and polydimethylsiloxane. From this mixture, the cyclic siloxanes including D₄ can be removed by distillation. In the presence of a strong base such as KOH, the polymer/ring mixture is equilibrated, allowing complete conversion to the more volatile cyclic siloxanes:^[5]

[(CH₃)₂SiO]_4n → n [(CH₃)₂SiO]₄

D₄ and D₅ are also precursors to the polymer. The catalyst is again KOH.

Safety and environmental considerations

D₄ is of low acute toxicity. The LC₅₀ for a single four hour inhalation exposure in rats is 36 mg/L. The oral LD₅₀ in rats is above 4800 mg/kg and the dermal LD₅₀in rats is above 2400 mg/kg.^[8]

As the smallest cyclic dimethylsiloxane that does not experience considerable ring strain,^[9] D₄ is one of the most abundant siloxanes in the environment, e.g. in landfill gases.^[10] D₄ and D₅ have attracted attention because they are pervasive. Cyclic siloxanes can be detected in some species of aquatic life.^[11] An independent, peer-reviewed study in the US found "negligible risk from D4 to organisms"^[12] while a scientific assessment by the Australian government stated, "the direct risks to aquatic life from exposure to these chemicals at expected surface water concentrations are not likely to be significant."^[13]

In the European Union, D₄ was characterized as a substance of very high concern (SVHC) due to its PBT and vPvB properties and was thus included in the candidate list for authorisation.^{[14][clarification needed]} D₄ shall not be placed on the market in wash-off cosmetic products in a concentration equal to or greater than 0.1% by weight of either substance, after 31 January 2020.^[15] Conversely, a detailed review and analysis of the science by the State of Washington in 2017 led to the removal of D₄ from their CHCC listing.^{[16][clarification needed]} That decision prompted the State of Oregon to follow suit in 2018.^[17]

See also

• Hexamethylcyclotrisiloxane (D₃)
• Decamethylcyclopentasiloxane (D₅)