Registration Dossier
Registration Dossier
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EC number: 213-914-1 | CAS number: 1066-40-6
Hydrolysis
Administrative data
Link to relevant study record(s)
Description of key information
Key value for chemical safety assessment
Additional information
The study does not need to be conducted because trimethylsilanol (TMS, CAS 001066-40-6), (CH3)3SiOH, is an organosilicon compound with no hydrolysable groups. Organosilanol compounds, RnSi(OH)4-n, are the endpoint for hydrolysis of all organosilicon substances, with Si-X (where X can be OR, Cl, etc) hydrolysing to give Si-OH and XH. For example, the linear methylsiloxanes, (CH3)3SiO[Si(CH3)2O]nSi(CH3)3, hydrolyse to give trimethylsilanol and dimethylsilanediol as the final products of hydrolysis. No hydrolysis of Si-C bond has been observed in any of these studies and there is no conceivable mechanism by which this reaction could occur under conditions relevant to the environment or in vivo. TMS in the aqueous phase can react with itself in a condensation reaction to form hexamethyldisiloxane (HMDS, CAS 000107-46-0) and water. In aqueous solution, an equilibrium is established between TMS (monomer) and HMDS (dimer). The equilibrium of the reaction is in favour of TMS, particularly at low concentrations. This is supported by the results of an OECD 111 study in which HMDS was found to hydrolyse in water with a half-life of approx. 5 days at pH 7 to TMS. This reaction is acid and base catalysed and is faster in acidic or basic environments. The condensation reactions of TMS are reversible unless the concentration of HMDS exceeds its solubility; in this case, HMDS forms a separate phase, driving the equilibrium towards HMDS. At a loading of 100 mg/l, >99.9% TMS is expected. At loadings above about 500-1000 mg/l the concentration of HMDS is predicted to exceed its solubility, resulting in formation of a separate phase. Therefore, condensation rather than hydrolysis is the important process for an understanding of the behaviour of TMS in water.
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