Dimethoxymethylsilane

Endpoint summary

Additional information

Dimethoxy(methyl)silane (CAS No. 16881-77-9) is characterized by a high water solubility of 1.7E+04 mg/L (QSAR), a vapour pressure of 7.1E03 Pa (QSAR) and a low log Kow of 1.4 (QSAR).

It is expected to undergo a rapid two-step hydrolysis under environmental conditions. In a first step, the parent substance hydrolyses to the silanol hydrolysis product methylsilanediol, which further hydrolyses to methylsilanetriol (CAS No. 2445-53-6) in a second step. The non-silanol hydrolysis product is methanol. The predicted half-life is 0.3 h at pH 7 (QSAR). However, a hydrolysis study according to OECD Guideline 111 is currently being conducted and the dossier will be updated once the final study report is available.

The ECHA guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). The TGD (EC2003) and ECHA guidance R.16 (ECHA 2010) also suggests that in case the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity.

It is anticipated that the second silanol hydrolysis product methylsilanetriol is the final product of the hydrolysis reactions that rapidly occur under environmentally relevant conditions and is thus considered the relevant assessment entity for the present Chemical Safety Assessment according to REACH. Thus, the environmental fate assessment as well as the hazard and risk assessment are based on the hydrolysis products methylsilanetriol and methanol rather than the parent substance.

 

Methylsilanetriol (CAS No. 2445-53-6)

Methylsilanetriol is characterized by high water solubility (1.0E+06 mg/L, QSAR), low vapour pressure (0.05 Pa, QSAR) and a low partition coefficient (log Kow = -2.4). It has an estimated dissociation constant of about 10 (pKa), indicating that it will not undergo significant dissociation in the environmentally relevant pH range of 4 to 9. No significant biodegradation is expected but its potential for bioaccumulation is low.

Based on its physico-chemical properties, methylsilanetriol is expected to preferentially distribute to the aquatic compartment. Exposure of the sediment and soil compartments are expected to be low.

 

 

Methanol (CAS No. 67-57-1)

Methanol is well characterised in the public domain and has a low environmental hazard profile (OECD SIDS, 2004).

It is fully water miscible at ambient temperatures and does not undergo hydrolysis. Volatilization is not a significant removal process from the aquatic phase. It is readily biodegradable and is characterized by a low log Kow of -0.82 to -0.64 (measured). Therefore, bioaccumulation in fish is expected to be low.

The short- and long-term aquatic toxicity of methanol has been extensively studied for a broad spectrum of aquatic species and its toxicity to aquatic organisms is low (effect values > 1000 mg/L; OECD SIDS, 2004). Therefore, it is not hazardous at the concentrations relevant to the studies in this dossier and is not considered contributory to the overall aquatic toxicity profile of the registered substance.

 

Reference
OECD, 2004: SIDS Initial Assessment Report for Methanol, SIAM 19, Berlin, Germany, 19-22 October 2004, Methanol, CAS 67-56-1