Chlorodimethylvinylsilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.12 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

1.2 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.012 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

7.4 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

1.2 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.12 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.17 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Chloro(dimethyl)vinylsilane hydrolyses rapidly in contact with water or atmospheric moisture (estimated <1 min. at pH and 25°C), to produce dimethyl(vinyl)silanol and hydrochloric acid. The effects of hydrogen chloride on aquatic organisms are limited to those that result from changes to pH in unbuffered media. REACH guidance (ECHA 2012, 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 REACH guidance (EC 2003) and R.16 (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 (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product,dimethyl(vinyl)silanol, in accordance with REACH guidance. As described in Section 1.3 of the CSR and Section 4 of the technical dossier, condensation reactions of the silanol are possible.

 

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, chloro(dimethy)lvinylsilane hydrolyses very rapidly (half-life < 1 min at 25°C and pH 4, 7 and 9) to dimethyl(vinyl)silanol and hydrochloric acid. Dimethyl(vinyl)silanol is a silanol with a vinyl side chain and a log K_owof 1.5.The effects of hydrochloric acid are related to effects in an unbuffered media and are assessed below.Therefore, the assessment and the read-across are based on the silanol hydrolysis product, dimethyl(vinyl)silanol.

Chloro(dimethyl)vinylsilane, its silanol hydrolysis product and the substances used as surrogates for read-across are part of a class of non-functional compounds which act via a non-polar narcosis mechanism of toxicity. Substances in this group include alkoxy- and chloro- silanes having secondary features that do not affect the toxicity of the substances. The registered substance hydrolyses very rapidly in water and therefore the selection of the surrogate substance is based on log K_ow of the silanol hydrolysis product and the vinyl groups present on the side chains. Additional information is given in a supporting report (PFA (2013y) attached in Section 13 of the IUCLID 5 dossier.

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

·        Read-across from trimethylsilanol to chloro(dimethyl)vinylsilane:

The registered substance, chloro(dimethyl)vinylsilane (CAS 1719-58-0) is a monochlorosilane with one vinyl and two methyl groups. Hydroxytrimethylsilane, or trimethylsilanol is a monosilanol containing three methyl groups. Dimethyl(vinyl)silanol, the hydrolysis product of the registered substance on which the hazard assessment is based, and trimethylsilanol are monosilanols, with low molecular weights (102 and 90 respectively) and similar log K_ow(1.5 and 1.2). The difference in log K_ow is considered to be within normal experimental error and therefore read-across from trimethylsilanol to chloro(dimethyl)vinylsilane is considered valid.

Short-term data available with trimethylsilanol indicate that this substance is of low toxicity to aquatic organisms. The read-across of aquatic effects data from trimethylsilanol to dimethyl(vinyl)silanol is made in terms of the mg/l concentration of silanol without further molecular weight correction.

Read-across from dichloro(methyl)(vinyl)silane to chloro(dimethyl)vinylsilane:

The registered substance, chloro(dimethyl)vinylsilane (CAS 1719-58-0) is a monochlorosilane with one vinyl and two methyl groups. Dichloro(methyl)(vinyl)silane (CAS 124 -70 -9) is a dichlorosilane with one methyl and one vinyl group. Both substances hydrolyse very rapidly and have structurally similar silanol hydrolysis products, dimethyl(vinyl)silanol and methyl(vinyl)silanediol, respectively. In view of the very rapid hydrolysis, it is the silanol hydrolysis product that is relevant for environmental risk assessment. Dimethyl(vinyl)silanol and methyl(vinyl)silanediol are silanols with low MW (102 and 104 respectively) and low log K_ow(1.5 and -0.05).

This read-across is used for toxicity to microorganisms endpoint.

Chloro(dimethyl)vinylsilane(CAS 1719-58-0) and dichloro(methyl)(vinyl)silane (CAS 124-70-9) are within an analogue group of substances within which, in general, there is no evidence of significant toxicity to microorganisms. The test result is converted to concentration in terms of the silanol hydrolysis product of the tested substance. The read-across from methyl(vinyl)silanediol to dimethyl(vinyl)silanol is made in terms of the mg/l concentration of silanol without further molecular weight correction.

The read-across and relevant analogue group are discussed further in the endpoint summary.

 

·        Read-across from trimethoxy(vinyl)silane to chloro(dimethyl)vinylsilane:

To provide evidence that the vinyl side chain does not contribute to toxicity of the registered substance read-across from trimethoxy(vinyl)silane has been used. From the available dataset, the vinyl fragment of chloro(dimethyl)vinylsilane is understood to not have any specific mode of action with respect to ecotoxicity; trimethoxy(vinyl)silane (CAS 2768-02-7) has E(L)C₅₀ values in the range 169-210 mg/l, which are consistent with non-polar narcosis as seen with other alkoxysilanes.

 

·        Considerations on the non-silanol hydrolysis products:

Effects on aquatic organisms arising from exposure to the other hydrolysis product of chloro(dimethyl)vinylsilane, hydrochloric acid, are thought to result from a reduction in the pH of the ambient environment to a level below their tolerable range. Aquatic ecosystems are characterized by, among other factors, their pH; the organisms of the ecosystem are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e.g. OECD SIDS for CAS No. 7647-01-0, hydrogen chloride). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Physical hazards related to pH effects are consideredto be negligible in the environment because the substance and its hydrolysis products will enter the environment through a buffered WWTP before reaching the effluents or will be buffered by the surrounding environment.

Table 1 The key physicochemical parameters and ectoxicological data for the registered and surrogate substances.

  

CAS Number	1719-58-0	1066-40-6	124-70 -9	2768-02-7
Chemical Name	Chloro(dimethyl)vinysilane	Trimethylsilanol	Dichloro(methyl)(vinyl)silane	Trimethoxy(vinyl)silane
Si hydrolysis product	Dimethyl(vinyl)silanol	Trimethylsilanol	Methyl(vinyl)silanediol	Vinylsilanetriol
Molecular weight (parent)	120.7	90.2	141.1	148.2
Molecular weight (silanol hydrolysis product)	102.2	90.2	104.2	106.2
log Kow(parent)	n/a	1.2	n/a	1.1
log Kow(silanol hydrolysis product)	1.5	1.2	-0.05	-2.0
Water sol (parent)	n/a	995 mg/l	n/a	9400 mg/l
Water sol (silanol hydrolysis product))	3700 mg/l (Note: condensation reactions can occur over time, limiting the concentration dissolved in water).	995 mg/l (Note: condensation reactions can occur over time, limiting the concentration dissolved in water).	146000 mg/l (Note: condensation reactions can occur over time, limiting the concentration dissolved in water).	1.0E+06 mg/l (Note: condensation reactions can occur over time, limiting the concentration dissolved in water).
Vapour pressure (parent)	9500 Pa	1900 Pa	5880 Pa	1190 Pa
Vapour pressure (silanol hydrolysis product)	250 Pa	1900 Pa	2.11 Pa	0.02 Pa
Hydrolysis t1/2at pH 7 and 25°C	<1 min	n/a	<1 min	0.2 h (at 20°C)
Hydrolysis t1/2at pH 4 and 25°C	<1 min	n/a	<1 min	0.04 h
Hydrolysis t1/2at pH 9 and 25°C	<1 min	n/a	<1 min	0.004 h
Short-term toxicity to fish LC50	No data	271 mg/l	No data	191 mg/l
Short-term toxicity to aquatic invertebrates EC50	No reliable data	124 mg/l	No data	169 mg/l
Algal inhibition EC50	No data	>684 mg/l (geometric mean); >750 mg/l (arithmethic mean)	No data	210 mg/l

Reference:

ECHA (2012b): European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1, October 2012 (Section R.16.5.4.1. Hydrolysis)

Conclusion on classification

Chloro(dimethyl)vinylsilane hydrolyses very rapidly; the silanol hydrolysis product has a log Kow of <<3 and is not readily biodegradable. The silanol hydrolysis product has reliable short-term E(L)C50 values of 271 mg/l in fish, 124 mg/l in invertebrates and >684 mg/l in algae based on read-across from a structural analogue. It has a reliable NOEC of 44 mg/l in algae. 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. These data are consistent with the following classification under Directive 67/548/EEC (as amended) (DSD): Not classified.