Trichloro(propyl)silane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

8.97 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

5 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.5 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:

0.23 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, propylsilanetriol, because the hydrolysis half-life of the registered substance is approximately 5 seconds at 20-25°C and pH 7. The substance will hydrolyse very rapidly in contact with water and atmospheric moisture to form propylsilanetriol 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 silanetriol are possible.

 

Propylsilanetriol is highly water soluble (1000000 mg/l at 20 °C) and has low log K_ow (-1.4 at 20 °C) and low vapour pressure (0.0046 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 K_ow of the resulting silanols and the chemical groups present in them. After hydrolysis has taken place the side chain of the registered and surrogate substances is a propyl chain with a methyl group at the end.

 

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) trichloro(propyl)silane (CAS 141-57-1), and the following substance used as surrogate for read-across (source substance), trimethoxy(propyl)silane (CAS 1067-25-0), are part of a class of chlorosilane and alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce the same Si hydrolysis product, propylsilanetriol, and another non-Si hydrolysis product.

 

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.

 

Read-across from trimethoxy(propyl)silane (CAS 1067-25-0) (source) to trichloro(propyl)silane (CAS 141-57-1) (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 trichloro(propyl)silane (CAS 141-57-1) (target) and the substance used as surrogate for read-across trimethoxy(propyl)silane (CAS 1067-25-0) (source) are part of a class of chlorosilane and alkoxysilane compounds which hydrolyse rapidly to produce propylsilanetriol and another non-Si hydrolysis product.

Both substances have a propyl side-chain attached to the silicon atom. In addition to the propyl side-chain, the target substance has three chloro groups bound to silicon. The source substance has three methoxy groups bound to silicon. The silanol hydrolysis product is propylsilanetriol for both target and source substances. The non-silanol hydrolysis products are hydrochloric acid for the target substance and methanol for the source substance.

The hydrolysis half-life of trichloro(propyl)silane (CAS 141-57-1) at pH 7 and 25°C is estimated to be approximately 5 seconds; the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-Cl bonds to give propylsilanetriol and hydrochloric acid.

The hydrolysis half-life of trimethoxy(propyl)silane (CAS 1067-25-0) is 2.6 hours at pH 7 and 25°C; the substance will therefore undergo rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-OCH3 bonds to give propylsilanetriol and methanol.

Both target and source substances share the same silicon containing hydrolysis product propylsilanetriol, which is the relevant silicon containing substance for environmental assessment.

 

The source substance trimethoxy(propyl)silane (CAS 1067-25-0) and the target substance trichloro(propyl)silane (CAS 141-57-1) are structurally similar substances. Both substances are transformed to the common compound propylsilanetriol and to the non-common compounds methanol and hydrochloric acid, respectively. The common compound propylsilanetriol is solely responsible for the effects to be predicted. The transformation of the parent substances is rapid and extensive and therefore, only negligible exposure to the parent substances occurs. Exposure to the non-common compounds methanol and hydrochloric acid does not influence the prediction of the property under consideration. The effects of the target substance trichloro(propyl)silane (CAS 141-57-1) are predicted to be equal to the effects of the source substance trimethoxy(propyl)silane (CAS 1067-25-0) for the property under consideration.

 

AE 1.2 Degradation of non-common compounds

The target substance trichloro(propyl)silane (CAS 141-57-1) and source substance trimethoxy(propyl)silane (CAS 1067-25-0) rapidly hydrolyse in water to produce propylsilanetriol 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 physicochemical 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 trichloro(propyl)silane (CAS 141-57-1) and the source substance trimethoxy(propyl)silane (CAS 1067-25-0) both rapidly hydrolyse on contact with water and moisture. Full and complete hydrolysis will have occurred prior to introduction of test organisms, during ecotoxicological tests with the source substance (test stock solution was prepared by stirring for 18 h, prior to addition of test organisms).

 

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 trimethoxy(propyl)silane (CAS 1067-25-0), the rapid hydrolysis (half-life 2.6 h at pH7 and 25°C) andthe media preparation (mixing of the test solution for a minimum of 18 hours prior to inclusion of test organisms) indicate that the organisms would have been exposed to the hydrolysis products propylsilanetriol and methanol.

Due to the rapid hydrolysis of the target registration substance trichloro(propyl)silane (CAS 141-57-1) (half-life 5 seconds at pH7 and 25°C), the Si hydrolysis product propylsilanetriol is the relevant silicon containing substance for environmental assessment. Therefore both source and target substances share identical substances upon which the chemical safety assessment is based.

 

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. The boundary composition of the target substance reports a purity of≥98 %. No impurities are described.

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 from trimethoxy(propyl)silane (CAS 1067-25-0):

Short-term fish: 96-hour LC₅₀ >746 mg/l, Danio rerio.

Short-term toxicity to invertebrates: 48-hour EC₅₀ >816 mg/l, Daphnia magna.

Algae: 72-hour ErC₅₀ >913 mg/l and NOEC of≥913 mg/l, Desmodesmus subspicatus.

 

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, trimethoxy(propyl)silane (CAS 1067-25-0), were selected because it rapidly 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 data available with a substance that hydrolyses to propylsilanetriol.

 

 

Read-across from trichloro(ethyl)silane (CAS 115-21-9) (source) to trichloro(propyl)silane (CAS 141-57-1) (target):

 

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. 

 

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, trichloro(propyl)silane (CAS 141-57-1) (target), and the substance used as surrogate for read-across, trichloro(ethyl)silane (CAS 115-21-9) (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; trichloro(propyl)silane (CAS 141-57-1) undergoes a very rapid hydrolysis reaction to produce propylsilanetriol and hydrochloric acid. Trichloro(ethyl)silane (CAS 115-21-9) reacts very rapidly in water to produce ethylsilanetriol 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 trichloro(propyl)silane (CAS 141-57-1) is 5 seconds at pH7 and 25°C, producing propylsilanetriol and hydrochloric acid; the substance will therefore undergo very rapid hydrolysis in contact with water to form propylsilanetriol and hydrochloric acid.

The hydrolysis half-life of the source substance trichloro(ethyl)silane (CAS 115-21-9) at pH 7 and 25 °C is <1 minute; the substance therefore reacts very rapidly in water to produce ethylsilanetriol 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, propylsilanetriol, 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, propylsilanetriol, and of the source substance, ethylsilanetriol. 

 

Ethylsilanetriol and propylsilanetriol share similar physicochemical properties: low log K_ow (-1.9 and -1.4, respectively), high water solubility (1.0E+06 mg/l limited to 1000 mg/l by condensation reactions, and miscible, respectively) and moderate molecular weight (MW 108.17 and 122.2 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, propylsilanetriol, and of the source substance, ethylsilanetriol.

 

Due to the rapid hydrolysis of the substance, during the long-term toxicity to invertebrates study conducted with the source substance trichloro(ethyl)silane (CAS 115-21-9), the organisms would have been exposed to the hydrolysis products ethylsilanetriol and hydrochloric acid.

 

The silanol hydrolysis products are structurally very similar, both with one silanetriol group (silicon atom attached to three silanol hydroxy groups). Propylsilanetriol has one propyl group, while ethylsilanetriol has one ethyl group. 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 K_owvalues.

 

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. 

 

AE A.3 Impact of impurities on the prediction

The source substance has a reported purity of 99.8%. There are no impurities described. The boundary composition of the target substance reports a purity of ≥98 %. No impurities are described.

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

Long-term toxicity to invertebrates data are read-across from trichloro(ethyl)silane (CAS 115-21-9) (data are presented here prior to molecular weight adjustment):

Long-term toxicity to invertebrates: 21-day NOEC ≥100 mg/l (highest concentration tested), Daphnia magna.

There are no other long-term toxicity to aquatic invertebrates data available with other structural analogues.

 

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(ethyl)silane (CAS 115-21-9), were selected because it hydrolyses to a structurally similar substance to the hydrolysis product of the target substance.

There are no other long-term toxicity to aquatic invertebrates data available with other structural analogues.  

 

 

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

CAS Number	141-57-1	1067-25-0	115-21-9
Chemical Name	Trichloro(propyl)silane	Trimethoxy(propyl)silane	Trichloro(ethyl)silane
Si hydrolysis product	propylsilanetriol	propylsilanetriol	ethylsilanetriol
Molecular weight (parent)	177.53	164.28	163.51
Molecular weight (hydrolysis product)	122.2	122.2	108.17
log Kow (parent)	n/a	1.7	n/a
log Kow (silanol hydrolysis product)	-1.4	-1.4	-1.9
Water sol (parent)	n/a	9200 mg/l	n/a
Water sol (silanol hydrolysis product))	Miscible	Miscible	1E+06 mg/L (but will be limited by condensation reactions)
Vapour pressure (parent)	1500 Pa	1900 Pa	4780 Pa at 20°C
Vapour pressure (hydrolysis product)	0.0046 Pa	0.0046 Pa	0.02 Pa at 25°C
Hydrolysis t1/2 at pH 7 and 25°C	< 1 min	2.6 h	<5 min
Hydrolysis t1/2 at pH 4 and 25°C	< 1 min	0.2 h	<1 min
Hydrolysis t1/2 at pH 9 and 25°C	< 1 min	0.1 h	<1 min
Short-term toxicity to fish (LC50)	No data	>745 mg/l	No data
Short-term toxicity to aquatic invertebrates (EC50)	No data	>816 mg/l	No data
Algal inhibition (ErC50 and NOEC)	No data	ErC50 >913 mg/l, NOEC ≥913 mg/l	No data
Long-term toxicity to fish (NOEC)	No data	No data	No data
Long-term toxicity to aquatic invertebrates (NOEC)	No data	No data	≥100 mg/l
Sediment toxicity (NOEC)	No data	No data	No data
Short-term terrestrial toxicity (L(E)C50)	No data	No data	No data
Long-term terrestrial toxicity (NOEC)	No data	No data	No data

 

 

References

OECD SIDS (2002) SIDS Initial Assessment Report for SIAM 15, Boston, USA, 22-25th October 2002, Hydrochloric acid, CAS 7647-01-0.

OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >746 mg/l in fish, >816 mg/l in invertebrates and >913 mg/l in algae based on read-across from a structural analogue with the same Si hydrolysis product. It has a reliable NOEC of ≥913 mg/l in algae and also a and reliable long-term NOEC of >100 mg/l in Daphnia, based on read-across from a structural analogue.

 

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

 

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.