tert-butylchlorodimethylsilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.006 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.065 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.001 mg/L

Assessment factor:

10 000

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

3 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.16 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.016 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:

0.028 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hydrolysis half-life of tert-butyl(chloro)dimethylsilane (CAS 18162-48-6) is <1 min at pH 4, 7 and 9, 25⁰C. The registered substance will hydrolyse in contact with water and atmospheric moisture to tert-butyl(dimethyl)silanol (CAS 18173-64-3) and hydrochloric acid. REACH guidance (ECHA 2016, 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 2016) 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, the silanol hydrolysis products may be susceptible to condensation reactions.

Data are available with the silanol hydrolysis product of the registration substance, tert-butyl(dimethyl)silanol (CAS 18173-64-3), and a structural analogue of the registration substance, butyl(chloro)dimethylsilane (CAS 1000-50-6).

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex VIII 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 registration 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.

 

The registered substance hydrolyses very rapidly in water and therefore the selection of surrogate substances is based on log K_ow of the resulting silanols and the chemical groups present in them. The surrogate substance used for read-across, butyl(chloro)dimethylsilane (CAS 1000-50-6), very rapidly hydrolyses to a similar silanol to that of the registration substance.

Additional information is given in a supporting report (PFA 2016y) attached in Section 13.

 

In the following paragraphs the read-across approach for tert-butyl(chloro)dimethylsilane is assessed for the surrogate substance taking into account structure, hydrolysis rate and physicochemical properties, presented in the table below.

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

CAS Number	18162-48-6	1000-50-6
Chemical Name	tert-Butyl(chloro)dimethylsilane	Butyl(chloro)dimethylsilane
Si hydrolysis product	tert-Butyl(dimethyl)silanol	Butyl(dimethyl)silanol
Molecular weight (parent) (g/mol)	150.73	150.73
Molecular weight (silanol hydrolysis product) (g/mol)	132.28	132.28
log Kow(parent)	n/a	n/a
log Kow(silanol hydrolysis product)	2.5 (QSAR prediction)	2.6
Water sol (parent)	n/a	n/a
Water sol (silanol hydrolysis product)	970 mg/l (QSAR prediction) (limited by condensation reactions)	390 mg/l (QSAR prediction) (limited by condensation reactions)
Vapour pressure (parent)	710 Pa at 25°C (QSAR prediction)	704 Pa at 25°C (QSAR prediction)
Vapour pressure (hydrolysis product)	60 Pa at 25°C (QSAR prediction)	18 Pa at 25°C (QSAR prediction)
Hydrolysis t1/2 at pH 7 and 25°C	<1 min (read-across from chlorotrimethylsilane)	<1 min
Hydrolysis t1/2 at pH 4 and 25°C	<1 min (read-across from chlorotrimethylsilane)	<1 min
Hydrolysis t1/2 at pH 9 and 25°C	<1 min (read-across from chlorotrimethylsilane)	<1 min
Short-term toxicity to fish (LC50)	n/a	>30 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	>100 mg/l	>72 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	ErC50 10 mg/l NOEC 1.9 mg/l
Long-term toxicity to fish (NOEC)	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a
Long-term sediment toxicity (NOEC)	n/a	n/a
Short-term terrestrial toxicity (L(E)C50)	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a

Read-across from butyl(chloro)dimethylsilane (1000-50-6) to tert-butyl(chloro)dimethylsilane (CAS 18162-48-6)

The registration substance tert-butyl(chloro)dimethylsilane (CAS 18162-48-6) is a chlorosilane with two methyl groups bound to silicon. In aqueous media, tert-butyl(chloro)dimethylsilane hydrolyses very rapidly (half-life <1 minute at pH 7 and 25 °C) to tert-butyl(dimethyl)silanol and hydrochloric acid. Butyl(chloro)dimethylsilane (CAS 1000-50-6) is also a chlorosilane with two methyl groups bound to silicon. The fourth substituent groups are C4 alkyl groups in both cases, tert-butyl and n-butyl respectively. Butyl(chloro)dimethylsilane reacts very rapidly in water (half-life <1 minute at pH 7 and 25 °C) to produce butyl(dimethyl)silanol and hydrochloric acid. Any effects seen, determined using the available test data, are attributed to the hydrolysis products. tert-Butyl(dimethyl)silanol, the hydrolysis product of the registered substance on which the chemical safety assessment is based, and butyl(dimethyl)silanol are both silanes with alkyl groups bound to silicon, low MW (both 132.28) and similar log K_ow values (2.5 and 2.6 respectively). Therefore, read-across from butyl(chloro)dimethylsilane to tert-butyl(chloro)dimethylsilane is considered valid. The properties of the registered and surrogate substances are summarised in the table above.

The substance is used to read-across to short-term toxicity to fish and toxicity to aquatic algae endpoints. EC₅₀ values of >30 mg/l and 10 mg/l have been determined for fish and algae, respectively, and a NOEC value of 1.9 mg/l has been determined for algae.

Consideration of the non-silanol hydrolysis product hydrochloric acid

Aquatic ecosystems are characterised 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). Effects on aquatic organisms arising from exposure to hydrogen chloride are thought to 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. Therefore, 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. In addition, neutralisation of waste water is typically part of waste water treatment of effluents containing acids. This significantly removes or reduces the pH effects of HCl released to the environment.

ECHA 2016: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 3.0, February 2016

PFA 2016y: Peter Fisk Associates, Analogue report - ecotoxicity of low functionality groups. PFA.404.003.004.

 

Conclusion on classification

The registration substance, tert-butyl(chloro)dimethylsilane (CAS 18162-48-6) hydrolyses very rapidly in contact with water to form tert-butyldimethylsilanol (CAS 18173-64-3) and hydrochloric acid.

REACH guidance (ECHA 2016, R.7b and R.16) states 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 is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance.

Data are available with the hydrolysis product, tert-butyldimethylsilanol (CAS 18173-64-3), which has reliable short-term E(L)C₅₀ values of >100 mg/l for fish, of 6.49 mg/l for Daphnia magna and of 84.0 mg/l for algae. A NOEC value of 12.5 mg/l has also been determined in the algal test. Data have been read-across from the analogous substance butyl(chloro)dimethylsilane (CAS 1000-50-6), where an EC₅₀ of 10 mg/l and an EC₁₀ of 3.8 mg/l were determined for the effects of the test substance on growth rate of Desmodesmus subspicatus. However, acidification of the test media may have contributed to the effects observed, therefore the data with tert-butyldimethylsilanol (CAS 18173-64-3) have been selected as key.

The silanol hydrolysis product is not expected to be significantly degradable.

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

Acute toxicity: Not classified.

Chronic toxicity: Category Chronic 2.

Hydrochloric acid is listed on Annex VI of Regulation (EC) No 1272/2008 (CLP) and does not require classification for the environment.