Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.32 mg/L
Assessment factor:
1 000
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
3.2 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.032 mg/L
Assessment factor:
10 000
Extrapolation method:
assessment factor

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
100 mg/L
Assessment factor:
10
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
2 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.2 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:
0.22 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, methylphenylsilanediol, 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 methylphenylsilanediol 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 1.3, condensation reactions of the silanediol are possible.

 

Methylphenylsilanediol is highly water soluble (34000 mg/l at 20 °C) and has low log Kow (0.8 at 20 °C) and low vapour pressure (3.63E-03 Pa at 25 °C).

 

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.

 

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 Kow 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(methyl)(phenyl)silane (CAS 149-74-6), and the following substances used as surrogate for read-across (source substances), dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1), (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) and trichloro(phenyl)silane (CAS 98-13-5), are part of a class of chlorosilane compounds which hydrolyse rapidly or moderately rapidly to produce relevant analogous silanol hydrolysis products with similar physicochemical properties, and another non-Si hydrolysis product.

The source substances dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) and (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) were selected on the basis of similar log Kow for the silanols, whereas trichloro(phenyl)silane (CAS 98-13-5) is included to account for any possible effect from the phenyl group.

In the context of the RAAF, the basis of the read-across hypothesis for this substance is “Different compounds have the same type of effect(s)”; Scenario 2 applies. Both the source and target substance hydrolyse to compounds with qualitatively similar properties. The non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological property. 

 

 

Read-across from dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) to dichloro(methyl)(phenyl)silane (CAS 149-74-6):

This scenario covers the analogue approach for which the hypothesis is based on different compounds with the same type of effect(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(methyl)(phenyl)silane (CAS 149-74-6) (target), and the substance used as surrogate for read-across, dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) (source), are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism of toxicity. The substances are susceptible to hydrolysis; dichloro(methyl)(phenyl)silane (CAS 149-74-6) undergoes a very rapid hydrolysis reaction to produce methylphenylsilanediol and hydrochloric acid. Dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) reacts very rapidly in water to produce 3-chloropropyl(methyl)silanediol and hydrochloric acid. For both the test undertaken with the source substance, and the chemical safety assessment of the target substance, the silanol hydrolysis product is the assessment entity of primary interest, as discussed below.

The hydrolysis half-life of dichloro(methyl)(phenyl)silane (CAS 149-74-6) is 5 seconds at pH7 and 25°C, producing methylphenylsilanediol and hydrochloric acid; the substance will therefore undergo very rapid hydrolysis in contact with water to form methylphenylsilanediol and hydrochloric acid.

The hydrolysis half-life of the source substance dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) at pH7 and 25 °C is 5 seconds; the substance therefore reacts very rapidly in water to produce 3-chloropropyl(methyl)silanediol and hydrochloric acid.

The environmental chemical safety assessment of the registered (target) substance (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product, methylphenylsilanediol, as the primary assessment entity. The considerations for the hydrochloric acid hydrolysis product are discussed below (see AE A.2). The characterisation therefore focuses on the silicon containing hydrolysis products of the target substance, methylphenylsilanediol, and of the source substance, 3-chloropropyl(methyl)silanediol. 

 

Methylphenylsilanediol and 3-chloropropyl(methyl)silanediol share similar physicochemical properties: low log Kow (both 0.8), high water solubility (29000 mg/l (QSAR), limited to approximately 30 mg/l by condensation reactions, and 60000 mg/l at 20°C (QSAR), respectively) and moderate molecular weight (MW 154.24 and 154.67 g mol-1, respectively).

During the tests with the source substance, test organisms will have been exposed to the hydrolysis products of the test substance.

The non-Si hydrolysis product for both substances is hydrochloric acid.

 

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

As discussed above, the characterisation focuses on the silicon containing hydrolysis products of the target substance, methylphenylsilanediol, and of the source substance, 3-chloropropyl(methyl)silanediol.

 

Due to the rapid hydrolysis of the substance, during the short-term toxicity to fish study conducted with the source substance dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1), the organisms would have been exposed to the hydrolysis products 3-chloropropyl(methyl)silanediol and hydrochloric acid.

 

The silanol hydrolysis products are considered relevant analogues because they are both small molecule disilanols with a methyl side-chain. Although the second side-chain is different for the two substances, the predicted log Kow is very similar (0.8 in each case). Both substances are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism of toxicity. Neither substance is expected to show significant biodegradation in the environment. However, bioaccumulation potential is low due to the low log Kow values.

 

Consideration of the non-Si hydrolysis product hydrochloric acid:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid 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. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms 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 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 management measures (e.g. neutralisation) for effluents/aqueous waste. 

Hydrogen gas, is a ubiquitous element present in the atmosphere at 0.55 ppmV. Any anthropogenic contribution of hydrogen gas to the atmosphere from hydrolysis of dichlorosilane or trimethoxysilane is negligible, therefore the substance is not considered further. 

 

AE A.3 Impact of impurities on the prediction

The source substance has a reported purity of 99.1%. There are no impurities described in the studies that are read across. The Substance Identification Profile reports a purity of ≥98%. For confidential reasons no impurities are are described, however, none of the impurities impact the classification and labelling of the substance.

The Substance Identification Profile for the target substance reports a purity of ≥98%, with chlorosilane impurities. None of the impurities impact the classification and labelling of the substance.

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 data are read-across from dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) (data are presented here prior to molecular weight adjustment):

Short-term toxicity to fish: 96 h LC50 >107 mg/l (highest concentration tested), Cyprinus carpio.

There are no other reliable short-term toxicity data available with dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1).

This short-term toxicity to fish experimental result is consistent with the expected low toxicity of the silanol hydrolysis product, (3-chloropropyl)(methyl)silanediol, predicted using the ECOSAR toolbox for neutral organics, which gives the following short-term values:

Short-term toxicity to fish 96-hour LC50 1.43E+3 mg/l

Short-term toxicity to aquatic invertebrates 48-hour EC50 729 mg/l

Toxicity to aquatic algae 96-hour EC50 347 mg/l

 

AE A.5 Reliability and adequacy of the source data

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

 

AE A.6 Bias that influences the prediction

Data with the source substance, dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1), were selected because it hydrolyses to a relevant analogue of the hydrolysis product of the target substance.

Short-term toxicity to aquatic organisms data available with other analogous substances have also been included in the chemical safety assessment and are discussed below. 

 

Read-across from (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) to dichloro(methyl)(phenyl)silane (CAS 149-74-6):

This scenario covers the analogue approach for which the hypothesis is based on different compounds with the same type of effect(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(methyl)(phenyl)silane (CAS 149-74-6) (target), and the substance used as surrogate for read-across, (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) (source), are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism of toxicity. (3-Chloropropyl)diethoxymethylsilane is a dialkoxysilane with an alkyl and chloropropyl side chain. Dichloro(methyl)(phenyl)silane is a dichlorosilane with an alkyl and aryl side chain that will hydrolyse to methylphenylsilanediol. The substances are susceptible to hydrolysis; dichloro(methyl)(phenyl)silane (CAS 149-74-6) undergoes a very rapid hydrolysis reaction to produce methylphenylsilanediol and hydrochloric acid. (3-Chloropropyl)diethoxymethylsilane (CAS 13501-76-3) reacts moderately rapidly in water to produce 3-chloropropyl(methyl)silanediol and ethanol. For both the test undertaken with the source substance, and the chemical safety assessment of the target substance, the silanol hydrolysis product is the assessment entity of primary interest, as discussed below.

The hydrolysis half-life of dichloro(methyl)(phenyl)silane (CAS 149-74-6) is 5 seconds at pH7 and 25°C, producing methylphenylsilanediol and hydrochloric acid; the substance will therefore undergo very rapid hydrolysis in contact with water to form methylphenylsilanediol and hydrochloric acid.

The hydrolysis half-life of the source substance (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) at pH 7 and 25 °C is 9.5 hours; the substance therefore reacts moderately rapidly in water to produce 3-chloropropyl(methyl)silanediol and ethanol.

The environmental chemical safety assessment of the registered (target) substance (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product, methylphenylsilanediol, as the primary assessment entity. The considerations for the hydrochloric acid and ethanol hydrolysis products are discussed below (see AE A.2). The characterisation therefore focuses on the silicon containing hydrolysis products of the target substance, methylphenylsilanediol, and of the source substance, 3-chloropropyl(methyl)silanediol. 

 

Methylphenylsilanediol and 3-chloropropyl(methyl)silanediol share similar physicochemical properties: low log Kow (both 0.8), high water solubility (29000 mg/l (QSAR), limited to approximately 30 mg/l by condensation reactions, and 60000 mg/l at 20°C (QSAR), respectively) and moderate molecular weight (MW 154.24 and 154.67 g mol-1, respectively).

During the tests with the source substance, test organisms will have been exposed to the hydrolysis products of the test substance due to the 18 hour preparation of the test media, prior to introduction of test organisms.

The non-Si hydrolysis product is hydrochloric acid for the target substance and ethanol for the source substance.

 

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

As discussed above, the characterisation focuses on the silicon containing hydrolysis products of the target substance, methylphenylsilanediol, and of the source substance, 3-chloropropyl(methyl)silanediol.

 

Due to the moderate hydrolysis rate of the substance and preparation time of the test, during the short-term aquatic toxicity studies conducted with the source substance (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3), the organisms would have been exposed to the hydrolysis products 3-chloropropyl(methyl)silanediol and ethanol.

 

The silanol hydrolysis products are considered relevant analogues because they are both small molecule disilanols with a methyl side-chain. Although the second side-chain is different for the two substances, the predicted log Kow is very similar (0.8 in each case). Both substances are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism of toxicity. Neither substance is expected to show significant biodegradation in the environment. However, bioaccumulation potential is low due to the low log Kow values.

 

Consideration of the non-Si hydrolysis product hydrochloric acid:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid 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. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms 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 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 management measures (e.g. neutralisation) for effluents/aqueous waste. 

Hydrogen gas, is a ubiquitous element present in the atmosphere at 0.55 ppmV. Any anthropogenic contribution of hydrogen gas to the atmosphere from hydrolysis of dichlorosilane or trimethoxysilane is negligible, therefore the substance is not considered further. 

 

Consideration of the non-Si hydrolysis product ethanol:

Ethanol, produced upon hydrolysis of the surrogate substance(3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3), 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 are in excess of 1000 mg/l (OECD 2004b - SIDS for ethanol, CAS 64-17-5).

 

AE A.3 Impact of impurities on the prediction

The source substance has a reported purity of 99.1%. There are no impurities described in the studies that are read across. The Substance Identification Profile reports a purity of ≥99%. No impurities are present at >1% and none of the impurities impact the classification and labelling of the substance.

The Substance Identification Profile for the target substance reports a purity of ≥98%, with chlorosilane impurities. None of the impurities impact the classification and labelling of the substance.

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 aquatic organisms data are read-across from (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) (data are presented here prior to any relevant molecular weight adjustment):

Short-term toxicity to fish: 96 h LC50 574 mg/l, Danio rerio.

Short-term toxicity to invertebrates: 48 h EC50 >100 mg/l (highest concentration tested), Daphnia magna.

Toxicity to aquatic algae: 72 h EC50 >437 mg/l and EC10 137 mg/l, Desmodesmus subspicatus, or ErC50 321 mg/l and ErC10 101 mg/l as (3-chloropropyl)(methyl)silanediol.

These short-term toxicity to fish, invertebrates and algae experimental results are consistent with the expected low toxicity of the silanol hydrolysis product, (3-chloropropyl)(methyl)silanediol, predicted using the ECOSAR toolbox for neutral organics, which gives the following short-term values:

Short-term toxicity to fish 96-hour LC50 1.43E+3 mg/l

Short-term toxicity to aquatic invertebrates 48-hour EC50 729 mg/l

Toxicity to aquatic algae 96-hour EC50 347 mg/l

 

AE A.5 Reliability and adequacy of the source data

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

 

AE A.6 Bias that influences the prediction

Data with the source substance, (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3), were selected because it hydrolyses to a relevant analogue of the hydrolysis product of the target substance.

Short-term toxicity to aquatic organisms data available with other structural analogues have also been included in the chemical safety assessment and are discussed above. 

Read-across from trichloro(phenyl)silane (CAS 98-13-5) to dichloro(methyl)(phenyl)silane (CAS 149-74-6):

This scenario covers the analogue approach for which the hypothesis is based on different compounds with the same type of effect(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(methyl)(phenyl)silane (CAS 149-74-6) (target), and the substance used as surrogate for read-across, trichloro(phenyl)silane (CAS 98-13-5) (source), are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism, therefore the toxicity is related to log Kow

Trichloro(phenyl)silane (CAS 98-13-5) and dichloro(methyl)(phenyl)silane (CAS 149-74-6) are low molecular weight chlorosilane substances with three and two chlorine groups, respectively, linked to one silicon (Si) atom and one aromatic ring. The target substance has the addition of a methyl side chain. 

The substances are susceptible to hydrolysis; dichloro(methyl)(phenyl)silane (CAS 149-74-6) undergoes a very rapid hydrolysis reaction to produce methylphenylsilanediol and hydrochloric acid. Trichloro(phenyl)silane (CAS 98-13-5) reacts very rapidly in water to produce phenylsilanetriol and hydrochloric acid. For both the test undertaken with the source substance, and the chemical safety assessment of the target substance, the silanol hydrolysis product is the assessment entity of primary interest, as discussed below.

The hydrolysis half-life of dichloro(methyl)(phenyl)silane (CAS 149-74-6) is 5 seconds at pH 7 and 25°C, producing methylphenylsilanediol and hydrochloric acid; the substance will therefore undergo very rapid hydrolysis in contact with water to form methylphenylsilanediol and hydrochloric acid.

The hydrolysis half-life of the source substance Trichloro(phenyl)silane (CAS 98-13-5) at pH 7 and 25 °C is <1 minute; the substance therefore reacts very rapidly in water to produce phenylsilanetriol and hydrochloric acid.

The environmental chemical safety assessment of the registered (target) substance (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product, methylphenylsilanediol, as the primary assessment entity. The considerations for the hydrochloric acid hydrolysis product are discussed below (see AE A.2). The characterisation therefore focuses on the silicon containing hydrolysis products of the target substance, methylphenylsilanediol, and of the source substance, phenylsilanetriol. 

 

Methylphenylsilanediol and phenylsilanetriol share similar physicochemical properties: low log Kow (0.8 and -0.02 respectively), high water solubility (29000 mg/l (QSAR) and 1000000 mg/l at 20°C (QSAR), respectively (both are limited by condensation reactions)) and moderate molecular weight (MW 154.24 and 156.21 g mol-1, respectively).

During the tests with the source substance, test organisms will have been exposed to the hydrolysis products of the test substance.

The non-Si hydrolysis product for both substances is hydrochloric acid.

 

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

The target substance dichloro(methyl)(phenyl)silane (CAS 149-74-6) and the source substance trichloro(phenyl)silane (CAS 98-13-5) are structurally similar low molecular weight chlorosilane substances. Both substances contain a phenyl group. The main structural difference is that the Si atom in the target substance dichloro(methyl)(phenyl)silane (CAS 149-74-6) has two chloride and one methyl groups bonded to it while the Si atom in the source substance trichloro(phenyl)silane (CAS 98-13-5) has three chloride groups bonded to it. In the source substance trichloro(phenyl)silane (CAS 98-13-5) there is an additional reactive chlorosilane group in place of the unreactive methyl group in the target substance dichloro(methyl)(phenyl)silane (CAS 149-74-6).

Both substances hydrolyse rapidly in contact with water. The target substance has estimated hydrolysis half-lives of <1 min at pH 4, 7 and 9 and 25°C. The measured hydrolysis half-lives of the source substance are approximately 5 seconds at 25°C and pH 4, pH 7 and pH 9, based on read-across data. The target and the source substances hydrolyse rapidly to the closely related silanol hydrolysis products methylphenylsilanediol and phenylsilanetriol, respectively. They share a common non-Si hydrolysis product hydrochloric acid.

As discussed above, the risk characterisation focuses on the silicon containing hydrolysis products of the target substance, methylphenylsilanediol, and of the source substance, phenylsilanetriol.

Due to the rapid hydrolysis of the substance, during the short-term toxicity to fish, invertebrates and algal studies conducted with the source substance trichloro(phenyl)silane (CAS 98-13-5), the organisms would have been exposed to the hydrolysis products phenylsilanetriol and hydrochloric acid.

The silanol hydrolysis products are considered structural analogues because they are both small molecules with an aryl sidechain (phenyl). The hydrolysis product, methylphenylsilanediol, of the target substance dichloro(methyl)(phenyl)silane (CAS 149-74-6) is structurally similar to the hydrolysis product, phenylsilanetriol, of the source substance trichloro(phenyl)silane (CAS 98-13-5). Methylphenylsilanediol has one methyl group, one phenyl group and two hydroxyl groups bound to a Si atom, while phenylsilanetriol has one phenyl group and three hydroxyl groups bound to a Si atom. The main difference between methylphenylsilanediol and phenylsilanetriol is that the methyl group in the target substance hydrolysis product is replaced by a hydroxyl group in source substance hydrolysis product.

The log Kow of the source substance is lower than that of the target substance. However, the data with the source substance provides evidence that the presence of a phenyl group does not induce toxic effects in aquatic organisms.

Neither substance is expected to show significant biodegradation in the environment. However, bioaccumulation potential is low due to the low log Kow values.

 

Consideration of the non-Si hydrolysis product hydrochloric acid:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid 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. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms 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 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 management measures (e.g. neutralisation) for effluents/aqueous waste. 

Hydrogen gas, is a ubiquitous element present in the atmosphere at 0.55 ppmV. Any anthropogenic contribution of hydrogen gas to the atmosphere from hydrolysis of dichlorosilane or trimethoxysilane is negligible, therefore the substance is not considered further. 

 

AE A.3 Impact of impurities on the prediction

The source substance has a reported analytical purity of 99.6%. There are no impurities described in the studies that are read across. The Substance Identification Profile reports a purity of ≥95%, with chlorosilane impurities. For confidential reasons the impurities are not further described, however, none of the impurities impact the classification and labelling of the substance. 

The Substance Identification Profile for the target substance reports a purity of ≥98%, with chlorosilane impurities. None of the impurities impact the classification and labelling of the substance. 

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 data are read-across from trichloro(phenyl)silane (CAS 98-13-5) (data are presented here prior to molecular weight adjustment):

Short-term toxicity to fish: 96 h LC50 >100 mg/l, Oncorhynchus mykiss.

Short-term toxicity to invertebrates: 48 h EC50 >100 mg/l, Daphnia magna.

Toxicity to aquatic algae: 72 h EC50 >100 mg/l and NOEC100 mg/l, Pseudokirchneriella subcapitata

These short-term toxicity to fish, invertebrates and algae experimental results are consistent with the expected low toxicity of the silanol hydrolysis product, phenylsilanetriol, predicted using the ECOSAR toolbox for neutral organics, which gives the following short-term values:

Short-term toxicity to fish 96-hour LC50 7.18E+3 mg/l

Short-term toxicity to aquatic invertebrates 48-hour EC50 3.41E+3 mg/l

Toxicity to aquatic algae 96-hour EC50 1.21E+3 mg/l.

 

AE A.5 Reliability and adequacy of the source data

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

 

AE A.6 Bias that influences the prediction

Data with the source substance, trichloro(phenyl)silane (CAS 98-13-5), were selected because it hydrolyses to a structurally similar substance to the hydrolysis product of the target substance, including the phenyl side-chain.

Short-term toxicity to aquatic organisms data available with other structural analogues have also been included in the chemical safety assessment and are discussed above. 

 


Table 7.1 Summary of the physicochemical and ecotoxicological properties of the registered and surrogate substance.

CAS Number

149-74-6

7787-93-1

13501-76-3

98-13-5

Chemical Name

Dichloro(methyl)(phenyl)silane

Dichloro(3-chloropropyl)methylsilane

3-chloropropyl(diethoxy)methylsilane

Trichloro(phenyl)silane

Si hydrolysis product

Methylphenylsilanediol

(3-chloropropyl)(methyl)silanediol

(3-chloropropyl)(methyl)silanediol

Phenylsilanetriol

Molecular weight (parent)

191.13

191.56

210.78

211.6

Molecular weight (hydrolysis product)

154.24

154.67

154.67

156.2

log Kow (parent)

n/a

n/a

4.2

n/a

log Kow (silanol hydrolysis product)

0.8

0.8

0.8

-0.02

Water sol (parent)

n/a

n/a

36 mg/l

n/a

Water sol (silanol hydrolysis product))

34 000 mg/l

60 000 mg/l

60 000 mg/l

1 000 000 mg/l

Vapour pressure (parent)

47 Pa

83 Pa

6.8 Pa

44.3 Pa

Vapour pressure (hydrolysis product)

3.63E-03 Pa

0.018 Pa

0.018 Pa

44.3 Pa

Hydrolysis t1/2 at pH 7 and 25°C

<1 min

< 1 min

9.5 h

<1 min

Hydrolysis t1/2 at pH 4 and 25°C

< 1 min

< 1 min

0.4 h

<1 min

Hydrolysis t1/2 at pH 9 and 25°C

< 1 min

< 1 min

0.2 h

<1 min

Short-term toxicity to fish (LC50)

n/a

>107 mg/l

574 mg/l

>100 mg/l

Short-term toxicity to aquatic invertebrates (EC50)

>100 mg/l

no reliable data

295 mg/l

>100 mg/l

Algal inhibition (ErC50 and NOEC)

n/a

no reliable data

ErC50: 437 mg/l; EC10 137 mg/l; NOEC: 57 mg/l

>100 mg/l

Long-term toxicity to fish (NOEC)

n/a

n/a

n/a

n/a

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

n/a

n/a

n/a

Long-term sediment toxicity (NOEC)

n/a

n/a

n/a

n/a

Short-term terrestrial toxicity (L/EC50)

n/a

n/a

n/a

n/a

Long-term terrestrial toxicity (NOEC)

n/a

n/a

n/a

n/a

Conclusion on classification

The substance has a reliable short-term EC50 value of >100 mg/l in invertebrates based on data for the registration substance, and E(L)C50 values of >107 mg/l, >100 mg/l and 574 mg/l in fish, and >100 mg/l and 437 mg/l in algae, as well an algal NOEC of ≥100 mg/l and ErC10 of 137 mg/l, based on read-across from relevant analogues.

 

The substance hydrolyses (very) rapidly in water and is not readily biodegradable but the silanol hydrolysis product has low log Kow.

 

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.