Dimethoxymethylvinylsilane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of dimethoxy(methyl)vinylsilane is 18.3 minutes at pH 7 and 25⁰C. The registered substance will hydrolyse in contact with water and atmospheric moisture to methylvinylsilanediol (CAS 3959-12-4) and methanol. REACH guidance (ECHA 2010A, R.16) states that “for substances where hydrolytic DT₅₀ is 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 2010A) 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 Section 4.8 of IUCLID, the silanol hydrolysis product may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex X 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, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour.

 

The registered substance and the substances used as surrogate for read-across are part of a class of low-functionality compounds acting via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the silicon atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. The silanol hydrolysis products may be susceptible to condensation reactions, see Section 4.8 of IUCLID.

 

The registration substance hydrolyses rapidly in water and therefore the selection of surrogate substances is based on log K_ow of the resulting silanol and the chemical groups present in the silanol hydrolysis product. The first surrogate substance, dimethylsilanediol (1066-42-8), is an analogue of the hydrolysis product of the registration substance, methylvinylsilanediol (CAS 3959-12-4). The second surrogate substance, dimethoxydimethylsilane (CAS 1112-39-6), is an analogue of the registration substance and rapidly hydrolyses to dimethylsilanediol, which is analogous to the hydrolysis product of the registration substance. The third surrogate substance, dichloro(methyl)(vinyl)silane (124-70-9) rapidly hydrolyses to the same hydrolysis product as the registration substance, methylvinylsilanediol. Read-across data have also been provided for trimethoxyvinylsilane (CAS 2768-02-7) to show that the vinyl side chain present in the registration substance is not expected to contribute to toxicity beyond narcosis (e.g. by any specific mode of action associated with this group).

Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID 5 dossier.

 

In the following paragraphs the read-across approach for dimethoxymethylvinylsilane is assessed for the surrogate substances taking into account structure, hydrolysis rate and physicochemical properties.

Table 1. Read-across substances and their end-points.

End point	Purpose flag	Substance name	CAS Number
Short-term Aquatic Invertebrates	Key	Dimethoxydimethylsilane	1112-39-6
	Supporting	Trimethoxyvinylsilane	2768-02-7
Toxicity to Aquatic algae and cyanobacteria	WoE	Dimethylsilanediol	1066-42-8
	WoE	Dichloro(methyl)(vinyl)silane	124-73-9
	Supporting	Trimethoxyvinylsilane	2768-02-7

Read-across from dimethylsilanediol to dimethoxymethylvinylsilane

The registered substance, dimethoxy(methy)lvinylsilane (CAS 16753-62-1) is an alkoxysilane with vinyl and methyl groups bound to the silicon. In moist medium, dimethoxymethylvinylsilane hydrolyses rapidly (half-life 18.3 minutes at pH 7, 25°C) to methylvinylsilanediol (CAS 3959-12-4) and methanol. Methylvinylsilanediol and the read-across substance dimethylsilanediol (CAS 1066-42-8), are analogues; both are silanediols with small hydrocarbon side-chains. Dimethylsilanediol has two methyl groups bound to silicon; methylvinylsilanediol has one methyl and one vinyl group bound to silicon. The replacement of one methyl with vinyl should not significantly affect the toxicity of the substance (see discussion below).

Neither substance is subject to further hydrolysis, both dimethylsilanediol and methylvinylsilanediol have low molecular weight (92.17 and 104.18 respectively), low log K_ow (-0.38 and -0.1 respectively) and high water solubility (1000000 mg/l at 20 -25°C and 5.0E+05 at 20°C respectively). The aquatic data for the surrogate substance indicates that it is not acutely toxic to aquatic organisms. This conclusion is read-across to the organosilicon hydrolysis product of the registration substance. The other hydrolysis product, methanol, is not toxic to aquatic organisms at relevant concentrations (see discussion below).

 

Read-across from dimethoxydimethylsilane to dimethoxymethylvinylsilane

Dimethoxydimethylsilane (CAS 1112-39-6) and dimethoxymethylvinylsilane (CAS 16753-62-1) both hydrolyse rapidly in contact with water (half-life <0.6 minutes at pH 7, 25⁰C, and 18.3 minutes at pH 7, 25⁰C respectively). The organosilicon hydrolysis products are dimethylsilanediol and methylvinylsilanediol respectively; the other hydrolysis product is methanol in both cases. Dimethylsilanediol and methylvinylsilanediol are analogues; they are both silanediols with small hydrocarbon side-chains. Dimethylsilanediol has two methyl groups bound to silicon; methylvinylsilanediol has one methyl and one vinyl group bound to silicon. The replacement of one methyl with vinyl should not significantly affect the toxicity of the substance (see discussion below).Dimethylsilanediol and methylvinylsilanediol also share similar physicochemical properties: low molecular weight (92.17 and 104.18 respectively), low log K_ow (-0.38 and -0.1 respectively) and high water solubility (1000000 mg/l at  20-25°C and 5.0E+05 mg/L at 20°C respectively).

The aquatic toxicity data for the surrogate substance indicates that it is not acutely toxic to aquatic organisms, with L(E)C₅₀ values all >100 mg/l.

 

Read-across from dichloro(methyl)(vinyl)silane to dimethoxymethylvinylsilane

Dimethoxymethylvinylsilane (CAS 16753-62-1) hydrolyses rapidly in contact with water (half-life 18.3 minutes pH 7, 25 °C) to form methylvinylsilanediol (CAS 3959-12-4) and methanol. Methanol is not toxic to aquatic organisms at relevant concentrations (see discussion below). Dichloro(methyl)(vinyl)silane (CAS 124 -70 -9) also rapidly hydrolyses to form methylvinylsilanediol, as well as hydrochloric acid (half-life <1 min at pH 7, 25°C).

Effects on aquatic organisms arising from exposure to hydrogen chloride result from a reduction in the pH of the ambient environment (arising from an increase in the H⁺concentration) to a level below their tolerable range. Effects will not be a consequence of true chemical toxicity but will be a function of, and dependent on, the buffering capacity of the environment.

Therefore, read-across is appropriate because both substances hydrolyse rapidly to give the same organosilicon hydrolysis product.

Consideration of low functionality side chains and reactive groups

·        Read-across from trimethoxyvinylsilane to dimethoxymethylvinylsilane:

From the available dataset in the whole of the silanes and siloxanes database, vinyl fragments are understood to not have any specific mode of action with respect to ecotoxicity.

To provide evidence that the vinyl side chain does not contribute to toxicity of the registered substance read-across from trimethoxyvinylsilane (CAS 2768-02-7) has been used.

Trimethoxyvinylsilane is an alkoxysilane with a vinyl group and a log K_ow 1.1. It rapidly hydrolyses to vinylsilanetriol (half-life 0.1 hour at pH 7 and 20-25°C) which has a log Kow of -2.0.

Short-term aquatic toxicity data for the substance report L(E)C₅₀ values in the range 169-210 mg/l which are consistent with non-polar narcosis as seen with other alkoxysilanes.

 

 

Consideration of the non-silanol hydrolysis product methanol

The non-silicon-containing hydrolysis product of dimethoxy(methyl)vinylsilane is methanol. Methanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for this substance are in excess of 1000 mg/l (OECD, 2004a - SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18 -20 October 2004, Methanol, CAS 67-56-1).

Table 2 Summary of physicochemical and ecotoxicological properties of the registered and surrogate substances.

CAS Number	16753-62-1	1112-39-6	1066-42-8	124-70-9	2768 -02 -7
Chemical Name	Dimethoxy(methyl)vinylsilane	Dimethoxydimethylsilane	Dimethylsilanediol	Dichloro(methyl)(vinyl)silane	Trimethoxyvinylsilane
Si hydrolysis product	Methylvinylsilanediol	Dimethylsilanediol	Dimethylsilanediol	Methylvinylsilanediol	Vinylsilanetriol
Molecular weight (parent)	132.24	120.22	92.17	141.07	148.24
Molecular weight (hydrolysis product)	104.18	92.17	n/a	104.18	106.15
log Kow(parent)	2.3 at 20°C (QSAR prediction)	2.0 at 20°C (QSAR prediction)	-0.38 at 20°C	n/a	1.1 at  20°C (QSAR prediction)
log Kow(silanol hydrolysis product)	-0.1 at 20°C (QSAR prediction)	-0.38 at 20°C	n/a	-0.1 at 20°C (QSAR prediction)	-2.0 at 20°C (QSAR prediction)
Water sol (parent)	3000 mg/l at 20°C (QSAR prediction)	9200 mg/l at 20-25°C (QSAR prediction)	1000000 mg/L at 20-25°C (QSAR prediction). Limited to approximately 1000 mg/l due to condensation reactions	n/a	30000 mg/l (QSAR prediction)
Water sol (silanol hydrolysis product))	5.0E+05 mg/l at 20°C (QSAR prediction). Limited to approximately 1000 mg/l due to condensation reactions	1000000 mg/l at 20 -25°C (QSAR prediction). Limited to approximately 1000 mg/l due to condensation reactions	n/a	5.0E+05 mg/l (QSAR prediction). Limited to approximately 1000 mg/l due to condensation reactions	1E +06 mg/l (QSAR prediction)  Limited to approximately 1000 mg/l due to condensation reactions
Vapour pressure (parent)	3000 Pa at 20°C	7400 Pa at 25°C (QSAR prediction)	7 Pa at 25°C (QSAR prediction)	5880 Pa at 20°C	1190 Pa at 20°C
Vapour pressure (hydrolysis product)	2.1 Pa at 20°C (QSAR prediction)	7 Pa 25°C (QSAR prediction)	n/a	2.1 Pa at 20°C (QSAR prediction)	0.02 Pa at 25°C (QSAR prediction)
Hydrolysis t1/2at pH 7 and 25°C	18.3 min	<0.6 hour	n/a	<1 min (read-across from dichloro(dimethyl)silane)	0.1 hour (20-25°C) (QSAR prediction)
)Hydrolysis t1/2at pH 4 and 25°C	<<2 m	n/a	n/a	<1 min  (read-across from dichloro(dimethyl)silane)	0.04 hour (20-25°C) (QSAR prediction)
Hydrolysis t1/2at pH 9 and 25°C	0.7 min	<0.6 hour	n/a	<1 min  (read-across from dichloro(dimethyl)silane)	0.004 hour (20-25°C) (QSAR prediction)
Short-term toxicity to fish (LC50)	n/a	n/a	>126 mg/l	n/a	191 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>100 mg/l	>117 mg/l	n/a	169 mg/l
Algal inhibition (ErC50and NOEC)	n/a	n/a	ErC50 >118 mg/l NOEC ≥118 mg/l	ErC50 >100 mg/l NOEC ≥100 mg/l	7-day ErC50 210 mg/l 7 -day NOEC 25 mg/l
Long-term toxicity to fish (NOEC)	n/a	n/a	n/a	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a	n/a	n/a	28 mg/l
Long-term sediment toxicity (NOEC)	n/a	n/a	n/a	n/a	n/a
Short-term terrestrial toxicity (L(E)C50)	n/a	n/a	n/a	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a	n/a	n/a	n/a

Conclusion on classification

It is proposed that dimethoxymethylvinylsilane should not be classified in the EU for acute or chronic toxicity on the grounds that

it rapidly hydrolyses and the silanol hydrolysis product is not toxic at an initial loading rate of the substance of 100 mg/l. The silanol hydrolysis product has low log K_ow and high water solubility.

According to Regulation (EC) No 1272/2008:

Aquatic Acute Not classified.

Aquatic Chronic Not Classified

Methanol is included in the United Nations Environment Programme (UNEP) Chemicals Branch, OECD SIDS list of harmonised substances and does not require classification for the environment under the CLP Regulation (Regulation (EC) No. 1272/2008).