Ethane-1,2-diylbis[dichloromethylsilane]

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:

74 mg/L

Assessment factor:

10

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:

0.012 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 1,2-bis[dichloro(methyl)silyl]ethane (CAS 3353-69-3) is <1 minute at pH 7, 20-25⁰C based on read-across data. The substance will hydrolyse in water under dilute conditions to 1,2-bis[dihydroxy(methyl)silyl]ethane and hydrochloric acid. REACH guidance (ECHA 2016b, 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 substance itself”. TGD and ECHA guidance (EC 2003, ECHA 2016c, R.7b) 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 is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Section 4.8 of IUCLID, the silanol hydrolysis products may be susceptible to condensation reactions.

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex VIII requirements for the registration 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 is part of a class of compounds which act via a non-polar narcosis mechanism of toxicity. Substances in this group include alkoxy- and chloro- silanes having secondary features 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 the silanol hydrolysis products, their log Kow as well as the groups present on the side chains. The surrogate substance, 1,6-bis(trimethoxysilyl)hexane (CAS 87135-01-1), hydrolyses to a structurally similar silanol to that of the registration substance, 1,6-bis(trihydroxysilyl)hexane. The second surrogate substance, 4,4,7,7-tetraethoxy-3,8-dioxa-4,7-disiladecane (CAS 16068-37-3), also hydrolyses to a structurally similar silanol to that of the registration substance, 1,1,1,4,4,4-hexahydroxy-1,4-silabutane, but at a slower rate.

 

In the following paragraphs the read-across approach for 1,2-bis[dichloro(methyl)silyl]ethane is assessed for the surrogate substances taking into account structure, hydrolysis rate and physicochemical properties. Table 1 presents relevant physicochemical properties and the available ecotoxicological data.

 

Table 1: Summary of physicochemical and ecotoxicological properties of the registered and surrogate substances.

CAS Number	3353-69-3	87135-01-1	16068-38-3
Chemical Name	1,2-bis[dichloro(methyl)silyl]ethane	1,6-bis(trimethoxysilyl)hexane	4,4,7,7-tetraethoxy-3,8-dioxa-4,7-disiladecane
Si hydrolysis product	1,2-bis[dihydroxy(methyl)silyl]ethane	1,6-bis(trihydroxysilyl)hexane	1,1,1,4,4,4-hexahydroxy-1,4-silabutane
Non-Si hydrolysis product	Hydrogen chloride	methanol	ethanol
Molecular weight (parent)	256.11	326.54	354.59
Molecular weight (hydrolysis product)	182.32	242.38	186.27
log Kow (parent)	n/a	1.7	2.7 (QSAR prediction)
log Kow (silanol hydrolysis product)	-1.6	-3.5	-4 (QSAR prediction)
Water sol (parent)	n/a	n/a	20 mg/l (QSAR prediction)
Water sol (silanol hydrolysis product))	1 000 000 mg/l (limited by condensation reactions)	1 000 000 mg/l (limited by condensation reactions)	1 000 000 mg/l (QSAR prediction) (limited by condensation reactions)
Vapour pressure (parent)	11 Pa	0.081 Pa	0.3 Pa at 20°C
Vapour pressure (hydrolysis product)	2.4E-05 Pa	1.2E-09 Pa	4.4E-08 Pa at 25°C (QSAR prediction)
Hydrolysis t1/2 at pH 7 and 25°C	<1 min	5.2 h	36 h 20-25°C
Hydrolysis t1/2 at pH 4 and 25°C	<1 min	0.08 h	0.8 h 20-25°C
Hydrolysis t1/2 at pH 9 and 25°C	<1 min	0.15 h	0.5 h 20-25°C
Short-term toxicity to fish (LC50)	not available	>74 mg/l (highest concentration tested; nominal concentration, expressed in terms of the silanol hydrolysis product).	16 mg/l (exposure to parent substance) (undissolved test material present – reliability uncertain)
Short-term toxicity to aquatic invertebrates (EC50)	not available	>74 mg/l (highest concentration tested; nominal concentration, expressed in terms of the silanol hydrolysis product).	72.6 mg/l (exposure to parent substance)
Algal inhibition (ErC50 and NOEC)	not available	>74 mg/l and NOEC ≥74 mg/l (highest concentration tested; nominal concentration, expressed in terms of the silanol hydrolysis product).	ErC50 >100 mg/l and ErC10 92.9 mg/l (exposure to mixture of parent substance and HP)
Toxicity to microorganisms (EC50, EC10, NOEC)	not available	ASRI 3hr EC50 >1000 mg/l; NOEC ≥1000 mg/l. The EC50 is equivalent to >742 mg/l and the NOEC to ≥742 mg/l when expressed in terms of the silanol hydrolysis product.	16-h EC50 value of >8000 mg/l and NOEC of ≥8000 mg/l: cell multiplication of Pseudomonas putida.

 

 

Read-across from 1,6-bis(trimethoxysilyl)hexane to 1,2-bis[dichloro(methyl)silyl]ethane

The registration substance, 1,2-bis[dichloro(methyl)silyl]ethane, is a chlorosilane. In water under dilute conditions, it undergoes very rapid hydrolysis with a half-life of <1 minute at 25˚C and pH 7, to form hydrochloric acid and 1,2-bis[dihydroxy(methyl)silyl]ethane.

The surrogate substance, 1,6-bis(trimethoxysilyl)hexane (CAS 87135-01-1) is an alkoxysilane that hydrolyses rapidly (t_1/2 of 5.2 h at 25°C and pH 7, half-life considerably shorter at higher pH) to produce 1,6-bis(trihydroxysilyl)hexane and methanol.

The silanol hydrolysis product of the registration substance, 1,2-bis[dihydroxy(methyl)silyl]ethane, has structural similarity to the silanol hydrolysis product of the surrogate substance, 1,6-bis(trihydroxysilyl)hexane, in that both have two silicon atoms, each with at least two OH groups, connected by an alkyl bridge. The registration substance also has Si-Me groups present. The two silanols share some similar physicochemical properties: both have log K_ow <0 (-1.6 and -3.5, respectively; note that in this range of log K_ow the absolute difference between the two values is not expected to be significant) and have very high water solubility (1E+06 mg/L (QSAR prediction), with solubility only limited by possible condensation reactions).

Due to the very rapid hydrolysis rate of 1,2-bis[dichloro(methyl)silyl]ethane, the chemical safety assessment is based on its silanol hydrolysis product. The hydrolysis rate of the surrogate substance, 1,6-bis(trimethoxysilyl)hexane, is slower than that of the registration substance. However, during the tests carried out with the surrogate substance the observations in the study are attributed to the hydrolysis products in the test system, because under the pH conditions of the test medium, the parent substance would only have been present at the beginning of the exposure period. See the endpoint summaries for further discussion of each endpoint and study for further information.  

Because the registration substance has such a rapid hydrolysis half-life, organisms are unlikely to be exposed to the parent substance. It is therefore considered reasonable to expect that no effects observed in the studies conducted with 1,6-bis(trimethoxysilyl)hexane, in which the test organisms were predominantly exposed to 1,6-bis(trihydroxysilyl)hexane, suggests that no effects at the equivalent concentration would be expected if 1,2-bis[dihydroxy(methyl)silyl]ethane itself were to be tested.

The effects of hydrochloric acid are limited to effects in an unbuffered media and are assessed below. The non-silicon hydrolysis products, hydrogen chloride and methanol respectively, do not cause effects in aquatic organisms at relevant concentrations and under appropriately buffered conditions as discussed below.

The substance is used to read-across to short-term toxicity to fish, invertebrates and algae endpoints, and toxicity to soil microorganisms. E(L)C₅₀ values of >74 mg/L (fish, invertebrates and algae) and >742 mg/L (soil microorganisms) (all values in terms of the equivalent concentration of

1,6-bis(trihydroxysilyl)hexane in the test system) have been determined.

Consequently, it is considered appropriate to use these studies as read-across data in the registration data set of 1,2-bis[dichloro(methyl)silyl]ethane.

Read-across from 4,4,7,7-tetraethoxy-3,8-dioxa-4,7-disiladecane (CAS 16068-37-3) to 1,2-bis[dichloro(methyl)silyl]ethane

The registration substance, 1,2-bis[dichloro(methyl)silyl]ethane, undergoes very rapid hydrolysis on contact with water (half-life of <1 minute at 25˚C and pH 7) to form hydrochloric acid and 1,2-bis[dihydroxy(methyl)silyl]ethane.

The surrogate substance, 4,4,7,7-tetraethoxy-3,8-dioxa-4,7-disiladecane (CAS 16068-37-3) is an alkoxysilane that has a moderate hydrolysis rate (t_1/2of 36 h at 25°C and pH 7, half-life considerably shorter at higher pH) and produces 1,1,1,4,4,4-hexahydroxy-1,4-silabutane and ethanol.

The silanol hydrolysis product of the registration substance, 1,2-bis[dihydroxy(methyl)silyl]ethane, has structural similarity to the silanol hydrolysis product of the surrogate substance, 1,1,1,4,4,4-hexahydroxy-1,4-silabutane, in that both have two silicon atoms, each with at least two OH groups, connected by an alkyl bridge. The two silanols share some similar physicochemical properties: both have log K_ow <0 (-1.6 and -4.0, respectively; note that in this range of log K_ow the absolute difference between the two values is not expected to be significant) and have very high water solubility (1E+06 mg/L (QSAR prediction), with solubility only limited by possible condensation reactions).

Due to the very rapid hydrolysis rate of  the registration substance, the chemical safety assessment is based on its silanol hydrolysis product. The hydrolysis rate of the surrogate substance, 4,4,7,7-tetraethoxy-3,8-dioxa-4,7-disiladecane, is slower than that of the registration substance and therefore consideration of the parent material would be relevant in that case. Test data are available with the surrogate substance for short-term toxicity to fish, invertebrates and algae. However, the test organisms were predominantly exposed to the parent substance during the fish and invertebrate tests. The algae test involved significantly longer preparation time, and a static exposure regime over 72 hours, and so the algae were exposed to a mixture of parent substance and hydrolysis product. Therefore, only the algae test has been read across from 4,4,7,7-tetraethoxy-3,8-dioxa-4,7-disiladecane.

 

The non-silicon hydrolysis products, hydrogen chloride and ethanol respectively, do not cause effects in aquatic organisms at relevant concentrations and under appropriately buffered conditions as discussed below.

The substance is used to read-across to toxicity to the toxicity to algae endpoint. An EC₅₀ value of >100 mg/L and EC₁₀ of 92.9 mg/L have been determined.

 

Considerations on the non-silanol hydrolysis products:

Methanol and ethanol

Methanol and ethanol 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 methanol and ethanol are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1 and OECD (2004b): SIDS for Ethanol, CAS 64-17-5).

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 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.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because:

• effects arising from exposure to HCl are related to changes in pH and not true chemical toxicity
• the silicon-containing hydrolysis products are not toxic to aquatic organisms at the highest concentration tested (for a structural analogue) in short-term studies.

Reference:

ECHA 2016b: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Assessment. Version: 3.0, February 2016 (Section A.16-3.2.2 part (A): Hydrolysis)

ECHA 2016c: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.7b: Endpoint specific guidance. Version: 3.0, February 2016 (Section R.7.9.5.2, subsection on Assessment of the potential persistence of metabolites)

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

Conclusion on classification

The substance hydrolyses very rapidly in water to form 1,2-bis[dihydroxy(methyl)silyl]ethane and hydrogen chloride. The environmental classification is based on consideration of the toxicity of the hydrolysis product 1,2-bis[dihydroxy(methyl)silyl]ethane.

 

1,2-bis[dihydroxy(methyl)silyl]ethane has reliable short-term E(L)C50 values of >74 mg/l in fish, >74 mg/l in invertebrates and >74 mg/l in algae based on read-across from a structural analogue (no effects observed at the highest concentration tested, in fish, invertebrates or algae) and is not readily biodegradable.

 

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.