Dimethoxymethylsilane

Endpoint summary

Additional information

The registered substance is part of a group of organosilicon substances, the toxicity of which is determined by a non-polar mechanism of toxicity, and as such log K_owdrives toxicity. Substances in this group include alkoxy- and chloro-silanes having secondary features that do not affect the toxicity of the substances. Additional information is given in a supporting report (PFA, 2013y) attached in Section 13 of the IUCLID 5 dossier.

The hydrolysis half-life of dimethoxymethylsilane (CAS No.16881-77-9) is approximately 0.3 h at 20-25⁰C and pH 7; the substance will therefore undergo rapid hydrolysis in contact with water to form methylsilanetriol and methanol.

REACH guidance (ECHA 2010d, R.16) states that “for substances where hydrolytic DT₅₀is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. TGD guidance, (EC 2003) and R.16 (2010d) also suggests that when the hydrolysis half-life is less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product in accordance with REACH guidance. As described in Section 1, condensation reactions of the silanols are possible.

The silanol hydrolysis product, methylsilanetriol, is a weak, nonionic surface active substance (see Section 1 for further details). The mechanism of toxicity of nonionic surfactants is considered to be the same as that of non-surfactant unreactive organic substances, i. e. non polar narcosis. In addition, due to the silanol hydrolysis product only being a weak surfactant, micelles are thought to be formed only at high concentrations, which are not environmentally realistic. The synergistic effects of micelles and other toxic compounds in the environment are therefore not considered further.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

In moist medium, dimethoxymethylsilane hydrolyses very rapidly (half-life 0.3 h at 20-25°C and pH 7) to methylsilanetriol. Methylsilanetriol is a silanetriol with log K_owof -2.4. The non-silanol hydrolysis product methanol is not expected to contribute to any adverse effects at the relevant dose levels. This is discussed further below.

In the following paragraphs the read-across approach for dimethoxymethylsilane is assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

·        Read-across from dimethylsilanediol to dimethoxymethylsilane:

Dimethylsilanediol is hydrolytically stable at environmentally- relevant temperature and pH (12 ± 1°C, pH range 7.6 to 8.4) for 96 hours and is a reliable read across for the registered substance.Dimethoxymethylsilane hydrolyses very rapidly (half-life 0.3 h at 20-25°C and pH 7) to methylsilanetriol. Methylsilanetriol is a silanetriol with log K_owof -2.4. Methylsilanetriol and the read across substancedimethylsilanediol are structural related and with similarphysico-chemical properties. Both substances are part of theorganosiliconsubstances, the toxicity of which is determined by a non-polar mechanism of toxicity, and as such log K_owdrives toxicity.

Based on short-term tests from each trophic level, the substance can be regarded as acutely not harmful to aquatic organisms. Short term toxicity data of dimethylsilanediol towards fish, daphnia and algae were a LC₅₀of > 126 mg/l OECD Guideline 203 (Fish, Acute Toxicity Test) an EC₅₀of > 117 mg/l (OECD Guideline 202 Daphnia sp. Acute Immobilisation Test) and an EC₅₀of > 118 mg/l (OECD Guideline 201 Alga, Growth Inhibition Test), respectively.

·        Considerations on the non-silanol hydrolysis products:

Methanol and ethanol are well characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC₅₀and LC₅₀values for these substances are in excess of 1000 mg/l (OECD2004a - SIDS for methanol, CAS No. 67-56-1, OECD 2004b - SIDS for ethanol, CAS No. 64-17-5).

·        QSAR

QSAR data were calculated for each of the trophic levels based on the silanol hydrolysis product, and this data was used to derive the PNECs. A 96-h LC₅₀of 100000 mg/l was determined for fish, a 48-h LC₅₀of 29600 mg/l for Daphnia and a 96-h EC₅₀of 2115 mg/l for algae.

Table7.1.1Key physicochemical parameters and ectoxicological data for the registered and surrogate substances.

CAS Number	16881-77-9	1066-42-8
Chemical Name	dimethoxymethylsilane	Dimethylsilanediol
Si hydrolysis product	methylsilanediol and methysilanetriol	-
Molecular weight (parent)	106,2	92,17
Molecular weight (silanol hydrolysis product)	94,14	-
log Kow(parent)	1.4 (QSAR)	-0.4 (QSAR)
log Kow(silanol hydrolysis product)	-2.4 (QSAR)
Water sol (parent)	1.7E+04 mg/l at 20°C (QSAR)	Soluble
Water sol (silanol hydrolysis product))	E+06 mg/l at 20°C (very soluble) (QSAR)	-
Vapour pressure (parent)	7100 Pa at 25°C (QSAR)	9.78 Pa at 25°C (QSAR)
Vapour pressure (hydrolysis product)	(methylsilanetriol) 0.05 Pa at 25°C (QSAR)	-
Hydrolysis t1/2at pH 7 and 25°C	0.3 h	No hydrolysis
Hydrolysis t1/2at pH 4 and 25°C	0.1 h	No hydrolysis
Hydrolysis t1/2at pH 9 and 25°C	0.0 h	No hydrolysis
Short-term toxicity to fish (LC50)	n/a	> 126 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	> 117 mg/l
Algal inhibition (ErC50and NOEC)	n/a	> 118 mg/l
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a