Dibutoxy(dimethyl)silane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of dibutoxy(dimethyl)silane is <2 h at pH 7, 20-25°C. The registered substance will hydrolyse in contact with water and atmospheric moisture to dimethylsilanediol and n-butanol. REACH guidance (ECHA 2012, 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 and ECHA guidance, (EC 2003, ECHA 2012) also suggest 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 Sections 4.8 and 5.1.2, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

Read-across is proposed to fulfil REACH Annex VII 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 reactivity of the substance in water is particularly important, because after reaction in water occurs the resulting product has different structural features, physicochemical properties and structure.

Dibutoxy(dimethyl)silane (CAS 1591-02-2) undergoes a rapid hydrolysis reaction (half-life <2 hours at pH 7) toform the hydrolysis products dimethylsilanediol and n-butanol. Therefore, it is more relevant to assess the ecotoxicity of the dibutoxy(dimethyl)silane by reference to the properties of dimethylsilanediol (CAS 1066-42-8).Table 1 presents relevant physicochemical properties and the available ecotoxicological data.

Table 1: Summary of physicochemical and ecotoxicological properties of the registered and surrogate substances

CAS Number	1591-02-2	1066-42-8
Chemical Name	Dibutoxy(dimethyl)silane	Dimethylsilanediol
Si reaction product	Dimethylsilanediol	n/a (hydrolytically stable)
Molecular weight (parent) g/mol	204.39	92.17
Molecular weight (reaction product) g/mol	92.17	n/a
log Kow(parent)	4.9	-0.38
log Kow(silanol reaction product)	-0.38	n/a
Water sol (parent)	9.4 mg/l (QSAR)	>1000 mg/l (QSAR), limited by condensation reactions
Water sol (silanol reaction product)	>1000 mg/l (QSAR), limited by condensation reactions	n/a
Vapour pressure (parent)	30 Pa at 25°C (QSAR)	7 Pa (QSAR)
Vapour pressure (reaction product)	7 Pa (QSAR)	n/a
Reaction t1/2at pH 7 and 25°C	1.4 h	n/a
Reaction t1/2at pH 4 and 25°C	<8 minutes (at 19.6°C)	n/a
Reaction t1/2at pH 9 and 25°C	0.59 h	n/a
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	EC50: >118 and NOEC:≥118 mg/l
Long-term toxicity to fish (NOEC)	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a

Read-across from dimethylsilanediol to dibutoxy(dimethyl)silane

Dibutoxy(dimethyl)silane (CAS 1591-02-2) will react in water to form dimethylsilanediol (CAS 1066-42-8) and n-butanol. The reaction rate half-life is <2 hours. The environmental hazard assessment is based on the registered substance silanol reaction product dimethysilanediol for which there are data. The E(L)C50values for dimethylsilanediol, presented in Table 1, indicate that it is not toxic to freshwater fish, invertebrates and algae at the highest concentrations tested (in excess of 100 mg/l). Dimethylsilanediol is unlikely to biodegrade.

Consideration of the non-silanol hydrolysis product n-butanol

The other hydrolysis product, n-butanol, is of low toxicity to aquatic organisms, with EC or LC50values in excess of 100 mg/l for fish, invertebrates and algae (OECD SIDS 2001 for n-Butyl Alcohol). Therefore it is not necessary to consider n-butanol further in the chemical safety assessment.

Reference:

OECD (2001) SIDS Initial Assessment Report for SIAM 13, n-Butyl Alcohol, CAS 71-36-3

Conclusion on classification

There are no data for the registered substance, however the substance hydrolyses rapidly to dimethylsilanediol and the hazard assessment is based on this substance. Dimethylsilanediol has reliable short-term E(L)C₅₀ values of >117 mg/l for invertebrates and >118 mg/l for algae as well as a NOEC value of ≥118 mg/l for algae.

Dimethylsilanediol is not readily biodegradable.  

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified  

In addition the registered substance produces n-butanol as a hydrolysis product. Ecotoxicity data indicate that n-butanol is not classified for the environment since the short-term E(L)C₅₀ values are in excess of 100 mg/L.