Dichlorocyclohexylmethylsilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.02 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.2 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.002 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.81 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.28 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.028 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:

0.043 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hazard assessment for the environment is based on the properties of the silanol hydrolysis product, cyclohexyl(methyl)silanediol, because the hydrolysis half-life of the registered substance is <1 minute at 20-25°C and pH 7. The substance will hydrolyse very rapidly in contact with water and atmospheric moisture to form cyclohexyl(methyl)silanediol 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”. REACH guidance, (ECHA 2017, R.7b) 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 4.8, condensation reactions of the silanediol are possible.

 

Cyclohexyl(methyl)silanediol is predicted to be highly water soluble (3.2E+04 mg/L at 20°C using a QSAR method) but may undergo condensation reactions in water; these become important above approximately 30 mg/L and can limit the concentration dissolved in water. Cyclohexyl(methyl)silanediol has a log K_ow of 1.9 at 20 °C and low vapour pressure (8.7E-03 Pa at 25°C).

 

In order to reduce 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.

 

The registered substance and the substance 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 Si 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 registered substance hydrolyses rapidly in water and therefore the selection of surrogate substance is based on log K_ow of the resulting silanols and the chemical groups present in them

 

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

 

The analogue approach for fulfilling the data requirement by read-across is discussed below, according to the Read-across Assessment Framework (RAAF).

The registered substance (target substance), dichloro(cyclohexyl)methylsilane (CAS 5578-42-7), and the following substance used as surrogate for read-across (source substance), cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), are part of a class of chlorosilane compounds which hydrolyse rapidly or moderately rapidly to produce the same silanol hydrolysis product, cyclohexyl(methyl)silanediol, and another non-Si hydrolysis product.

 

Read-across from cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) (source) to dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) (target):

 

In the context of the RAAF, the basis of the read-across hypothesis is “(Bio)transformation to common compound(s)”; Scenario 1 applies. The source substance hydrolyses to the same silicon containing hydrolysis product as the target substance, and the non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological property.

 

This scenario covers the analogue approach for which the read-across hypothesis is based on (bio) transformation to common compound(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst-case approach.

 

AE A.1 Characterisation of source and target substances

The registered substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) (target) and the substance used as surrogate for read-across cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) (source) are part of a class of compounds which hydrolyse rapidly or moderately rapidly to produce cyclohexyl(methyl)silanediol and another non-Si hydrolysis product.

 

The target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) and the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) are closely related substances, each consisting of one Si atom linked to a cyclic hydrocarbon and one methyl group. The main difference is that the Si atom of the target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) is linked to two chloride groups while in the source substancecyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6)these groups are substituted for two methoxy groups.

 

Dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) hydrolyses very rapidly in contact with water to produce cyclohexyl(methyl)silanediol and hydrochloric acid (half-life <1 min at pH 7 and 20-25°C). Cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) hydrolyses moderately in contact with water to produce cyclohexyl(methyl)silanediol and methanol (half-life 19 h at pH 7 and 20 -25°C).

 

The target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) and the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) are closely related substances, each consisting of one Si atom linked to a cyclic hydrocarbon and one methyl group. The main difference is that the Si atom of the target substance dichloro(cyclohexyl)methylsilane(CAS 5578-42-7) is linked to two chloride groups while in the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) these groups are substituted for two methoxy groups.

 

Both target and source substances share the same silicon containing hydrolysis product cyclohexyl(methyl)silanediol, which is the relevant silicon containing substance for environmental assessment.

 

The source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) and the target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) are structurally similar substances. Both substances are transformed to the common compound cyclohexyl(methyl)silanediol and to the non-common compounds methanol and hydrochloric acid, respectively. The transformation of the parent substance is rapid for the target substance but much slower for the source substance. The test organisms are likely to have been exposed to a mixture of the parent substance and its hydrolysis products during the tests with the source substance. It is therefore considered conservative to read-across from cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6). However, the parent substances are structural analogues therefore read-across is considered valid. Exposure to the non-common compounds methanol and hydrochloric acid does not influence the prediction of the property under consideration.

 

AE 1.2 Degradation of non-common compounds

The target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) and source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) hydrolyse in water to produce the common compound cyclohexyl(methyl)silanediol, and hydrochloric acid and methanol, respectively, as the non-common compounds.

According to the SIDS INITIAL ASSESSMENT PROFILE for Hydrogen chloride (OECD SIDS 2002), hydrogen chloride is a colourless gas which has a pungent odour, and has a vapour pressure of 42,200 hPa at 20°C and a water solubility of 823 g/L at 0°C, 673 g/L at 30°C. Its aqueous solution (called hydrochloric acid) possesses strong acidity, and reacts with most metals producing explosive hydrogen gas. Hydrogen chloride is readily dissociated in water into hydrated protons and chloride ion.

The physico-chemical properties indicate that hydrogen chloride released into the environment is distributed into the air and water. Hydrogen chloride can react with hydroxyl radicals to form chloride free radicals and water and its half-life time is calculated as 11 days. No accumulation of hydrogen chloride per se in living organisms is expected due to its high solubility and dissociation properties.

Effects on aquatic organisms arising from exposure to 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 considered in the risk characterisation and risk management measures.

 

According to the SIDS INITIAL ASSESSMENT PROFILE for Methanol (OECD 2004a), methanol is degraded in the atmosphere by photochemical, hydroxyl-radical dependent reactions. The estimated elimination half-life is calculated to be about 17-18 days with a rate constant of 0.93 x 10-2 cm3 /molecule-sec. Methanol is readily biodegradable. Bioaccumulation in fish is expected to be low.

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 are in excess of 10 000 mg/l (OECD 2004 - SIDS for methanol, CAS 67-56-1). Therefore, it is considered unlikely that it’s presence significantly affected the results of the test performed with trimethoxy(propyl)silane.

 

AE 1.3 Bioaccumulation potential of non-common compounds

The target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) and source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) hydrolyse in water to produce the common compound cyclohexyl(methyl)silanediol.

The non-common compounds, hydrochloric acid and methanol, are discussed in AE 1.2. They are not subject to further transformation to different compounds, they do not have the potential to bioaccumulate and neither are toxic to aquatic organisms.

 

AE 1.4 Impact of non-common compounds

As discussed in AE 1.2, hydrochloric acid and methanol are not hazardous at the concentrations relevant to the studies.

 

AE A.2 Link of structural similarities and structural differences with the proposed prediction (presence of hypothesis)

During the ecotoxicity studies conducted with the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), the hydrolysis half-life of 19 h at pH 7 and 25°C, the static or semi-static design of the tests and the measured parent substance concentrations indicate that the testorganisms would have been exposed to a mixture of the parent substance and its hydrolysis products.

Due to the rapid hydrolysis of the target registration dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) (half-life <1 min at pH 7 and 25°C), the Si hydrolysis product cyclohexyl(methyl)silanediol is the relevant silicon containing substance for environmental assessment. Both source and target substances produce the same hydrolysis product upon which the chemical safety assessment is based.

This read-across is considered to be conservative because the parent test substance, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), hydrolyses much more slowly than the registration substance (half-life 19 hours at pH 7 and 25°C), therefore organisms will have been exposed to the parent substance during the read-across tests, but would be unlikely to be exposed to parent substance if the registration substance were tested. Due to the log K_ow of the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) being 4.3, it is likely that the parent substance is more toxic than the silanol hydrolysis product.

However, as the parent substances are structural analogues, read-across is considered justified.

The target and source substance are structurally similar. The target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) and the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) are closely related substances, each consisting of one Si atom linked to a cyclic hydrocarbon and one methyl group. The main difference is that the Si atom of the target substance dichloro(cyclohexyl)methylsilane (CAS 5578-42-7) is linked to two chloride groups while in the source substance cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) these groups are substituted for two methoxy groups.

Both substances hydrolyse in contact with water. The target and the source substances hydrolyse to the same silanol hydrolysis products cyclohexyl(methyl)silanediol and hydrochloric acid and methanol, respectively. Since the target substance hydrolyses rapidly in contact with water, log K_ow and water solubility values are not relevant for it. Therefore, these values are only available for the hydrolysis product cyclohexyl(methyl)silanediol.

AE A.3 Impact of impurities on the prediction

The source substance has a reported analytical purity of 99.8%. There are no impurities described in the studies that are read across. The boundary composition of the target substance reports a purity of >98 - ≤100%. The Substance Identification Profiles for both substances report that no impurities have been identified above 1.0 % and that none of the impurities contribute to the classification and labelling of the substances.

It is therefore concluded that the source and target substances do not contain impurities at concentrations that would influence the effects observed in the available tests with the source substance.

 

AE A.4 Consistency of properties in the data matrix

Short-term toxicity to fish, invertebrates and algae data are read-across fromcyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6):

Short-term fish: 96-hour LC₅₀ 50 mg/l, Oncorhynchus mykiss.

Short-term toxicity to invertebrates: 48-hour EC₅₀ 24 mg/l, Daphnia magna;

Algae: 72-hour ErC₅₀ 35 mg/l and NOEC of 12.5mg/l, Desmodesmus subspicatus.

No aquatic toxicity data are available for the target substance.

 

AE A.5 Reliability and adequacy of the source data

All key data included in the chemical safety assessment have been reviewed and assigned Klimish scores of 1 or 2.

 

AE A.6 Bias that influences the prediction

Data with the source substance, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), were selected because it hydrolyses to the same substance as the hydrolysis product of the target substance.

The selection of the source studies used as the basis for the prediction was based on the fact that they are the only reliable tests with a substance that hydrolyses to cyclohexyl(methyl)silanediol.

 

 

Table: Overview of physico-chemical and ecotoxicological properties of the registered and surrogate substances.

CAS Number	5578-42-7	17865-32-6
Chemical Name	Dichloro(cyclohexyl)methylsilane	Cyclohexyl(dimethoxy)methylsilane
Si hydrolysis product	Cyclohexyl(methyl)silanediol	Cyclohexyl(methyl)silanediol
Molecular weight (parent)	197.18 g/mol	188.34 g/mol
Molecular weight (hydrolysis product)	160.29 g/mol	160.29 g/mol
log Kow (parent)	n/a	4.3
log Kow (silanol hydrolysis product)	1.9	1.9
Water sol (parent)	n/a	151 mg/L
Water sol (silanol hydrolysis product))	32 000 mg/L	32 000 mg/L
Vapour pressure (parent)	6 Pa	63 Pa
Vapour pressure (hydrolysis product)	8.7E-03 Pa	8.7E-03 Pa
Hydrolysis t1/2 at pH 7 and 25°C	< 1 min	19 hours
Short-term toxicity to fish (LC50)	n/a	50 mg/L (43 mg/l in terms of concentration of cyclohexyl(methyl)silanediol)
Short-term toxicity to aquatic invertebrates (EC50)	n/a	24 mg/L (20 mg/l in terms of concentration of cyclohexyl(methyl)silanediol)
Algal inhibition (ErC50 and NOEC)	n/a	EC50: 35 mg/L; NOEC: 12.5 mg/L (EC50 30 mg/l and NOEC 11 mg/l in terms of concentration of cyclohexyl(methyl)silanediol)

Conclusion on classification

Data available for a structurally analogous read-across substance indicate that short-term aquatic toxicity data are in the range 10-100 mg/L (lowest EC₅₀ value of 13 mg/L measured parent substance, 24 mg/l nominal concentration and 20 mg/l nominal in terms of the silanol hydrolysis product, for invertebrates). The registered substance hydrolyses very rapidly and its hydrolysis product, cyclohexyl(methyl)silanediol, is not readily biodegradable. Therefore the substance is classified for the environment in the EU as Chronic Category 3 according to the current Regulation (EC) No. 1272/2008.