Dichloro(3-chloropropyl)methylsilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.43 mg/L

Assessment factor:

500

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

4.3 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.043 mg/L

Assessment factor:

5 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

4 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

2.7 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.27 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:

0.44 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The substance hydrolyses very rapidly (see section 4 for further details) in contact with water to give (3-chloropropyl)methylsilanediol 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 Technical Guidance R.16 (2016) 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, condensation reactions of the silanediols are possible.

READ-ACROSS JUSTIFICATION

In order to reduce animal 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 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 silanol hydrolysis products may be susceptible to condensation reactions, see Section 1.3. 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. Both the registered substance and the surrogate substance are chloroalkyls with the same side chains and having fast hydrolysis rates.

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

In the following paragraphs the suitability of the read-across based on hydrolysis rates and structure of the surrogate and parent substances are presented below.

• Read-across from (3-chloropropyl)dimethoxymethylsilane to dichloro(3-chloropropyl)methylsilane

Dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) is a dichlorosilane with chloropropyl and alkyl (methyl) side chains. The substance hydrolyses very rapidly in water (<1 min at pH 4, 7 and 9 and 25°C, category read-across) to form chloropropyl(methyl)silanediol and hydrogen chloride. The effects of hydrogen chloride are restricted to pH effects in an unbuffered media and are given further consideration below. (3-Chloropropyl)dimethoxymethylsilane (CAS 18171-19-2) is a dialkoxysilane with chloropropyl and alkyl (methyl) side chains. (3-Chloropropyl)dimethoxymethylsilane has a half-life of approximately 1 hour at 25°C and pH 7, and will also produce chloropropyl(methyl)silanediol, and methanol. Both the source and target substances will hydrolyse to chloropropyl(methyl)silanediol under the conditions relevant to the studies, therefore read-across is considered to be directly relevant.

Short-term toxicity data available with the surrogate substance indicates that it is of low short-term toxicity to aquatic organisms.

Data have been read-across to short-term toxicity to aquatic invertebrates.

 

• Read-across from (3-chloropropyl)diethoxymethylsilane to dichloro(3-chloropropyl)methylsilane

Dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) is a dichlorosilane that hydrolyses very rapidly in water (<1 min at pH 4, 7 and 9 and 25°C, category read-across, (see above)). (3-Chloropropyl)diethoxymethylsilane (CAS 13501-76-3) is a dialkoxysilane with alkyl (methyl) and chloropropyl side chains. The hydrolysis half-life of (3-chloropropyl)diethoxymethylsilane is 9.5 h at pH 7 and 25°C. Under the conditions relevant to the studies, both substances will hydrolyse to chloropropyl(methyl)silanediol and form hydrogen chloride and ethanol respectively. Consideration of the non-silanol hydrolysis products is given below. Therefore, read-across from (3-chloropropyl)diethoxymethylsilane to dichloro(3-chloropropyl)methylsilane is considered to be directly relevant.

Short-term toxicity data available with the surrogate substance indicates that it is of low short-term toxicity to aquatic organisms.

Data have been read-across to short-term toxicity to fish, aquatic invertebrates and algae.

 

• Considerations on the non-silanol hydrolysis products:

The registered substance will produce hydrochloric acid upon hydrolysis. Effects on aquatic organisms arising from exposure to HCl 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. 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 No. 7647-01-0, hydrochloric acid). 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.

Methanol and ethanol, produced upon hydrolysis of the surrogate substances, are well characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for these substances are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1, OECD 2004b - SIDS for ethanol, CAS 64-17-5).

 

Table 7.1 Summary of the physico-chemical and ecotoxicological properties of the registered and surrogate substance.

CAS Number	7787-93-1	13501-76-3	18171-19-2
Chemical Name	Dichloro(3-chloropropyl)methylsilane	(3-chloropropyl)diethoxymethylsilane	(3-chloropropyl)dimethoxymethylsilane
Si hydrolysis product	3-chloropropyl)methylsilanediol	3-chloropropyl)methylsilanediol	3-chloropropyl)methylsilanediol
Molecular weight (parent)	191.56	210.78	182.72
Molecular weight (hydrolysis product)	154.67	154.67	154.67
log Kow (parent)	n/a	4.2	3.2
log Kow (silanol hydrolysis product)	0.8	0.8	0.8
Water sol (parent)	n/a	36 mg/l	370 mg/l
Water sol (silanol hydrolysis product))	60 000 mg/l	60 000 mg/l	60 000 mg/l
Vapour pressure (parent)	83 Pa	6.8 Pa	52 Pa
Vapour pressure (hydrolysis product)	0.021 Pa	0.021 Pa	0.021 Pa
Hydrolysis t1/2 at pH 7 and 25°C	< 1 min	9.5 h	1 hour
Hydrolysis t1/2 at pH 4 and 25°C	< 1 min	0.4 h	15 min at pH 5
Hydrolysis t1/2 at pH 9 and 25°C	< 1 min	0.2 h	22 min
Short-term toxicity to fish (LC50)	>107 mg/l	574 mg/l	n/a
Short-term toxicity to aquatic invertebrates (EC50)	>100 mg/l	295 mg/l	>100 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	ErC50: 437 mg/l; NOEC: 57 mg/l	n/a
Long-term toxicity to fish (NOEC)	n/a	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a	n/a
Long-term sediment toxicity (NOEC)	n/a	n/a	n/a
Short-term terrestrial toxicity (L/EC50)	n/a	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a	n/a

Conclusion on classification

Dichloro(3-chloropropyl)methylsilane hydrolyses very rapidly and the silanol hydrolysis product is not readily biodegradable.

Dichloro(3-chloropropyl)methylsilane is not classified in the EU according to the Regulation (EC) No 1272/2008 because data for the substance and appropriate read-across data indicate that the silanol hydrolysis product is not toxic up to a concentration of 100 mg/l, i.e. lowest E(L)C50 is >100 mg/l. Additionally, the silanol hydrolysis product has log K_ow <3.