Dichloromethyl(3,3,4,4,5,5,6,6,6-nonafluorohexyl)silane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of [2-(perfluorobutyl)ethyl]dichloro(methyl)silane (CAS 38436-16-7) is <1 minute at pH 7, 20-25⁰C based on read-across data. The substance will hydrolyse in water under dilute conditions to [2-(perfluorobutyl)ethyl]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”. TGD and ECHA guidance (EC 2003, ECHA 2016) 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.

 

READ-ACROSS JUSTIFICATION

Read-across is proposed to fulfil up to REACH Annex VII 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 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 silicon 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 4.8.  

The registration substance hydrolyses rapidly in water under dilute conditions and therefore the selection of surrogate substances is based on log K_ow of the resulting silanols and the chemical groups present in them. The surrogate substance, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7), hydrolyses to a structurally similar silanol to that of the registration substance, [2-(perfluorohexyl)ethyl]silanetriol. Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID dossier.

In the following paragraphs the read-across approach for [2-(perfluorobutyl)ethyl]dichloro(methyl)silane 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	38436-16-7	51851-37-7
Chemical Name	[2-(perfluorobutyl)ethyl]dichloro(methyl)silane	[2-(perfluorohexyl)ethyl]triethoxysilane
Si hydrolysis product	[2-(perfluorobutyl)ethyl]methylsilanediol	[2-(perfluorohexyl)ethyl]silanetriol
Molecular weight (parent) g/mol	361.11	510.37
Molecular weight (hydrolysis product) g/mol	324.22	426.21
log Kow (parent)	n/a	7.2 (QSAR prediction)
log Kow (silanol hydrolysis product)	3.3	2.7 (QSAR prediction)
Water sol (parent)	n/a	6.4E-06 mg/l (QSAR prediction)
Water sol (silanol hydrolysis product)	2.0 mg/l (QSAR prediction)	0.58 mg/l (QSAR prediction)
Vapour pressure (parent)	110 Pa (25°C) (QSAR prediction)	No data
Vapour pressure (hydrolysis product)	0.62 Pa (25°C) (QSAR prediction)	6.9E-03 Pa (25°C) (QSAR prediction)
Hydrolysis t1/2 at pH 7 and 25°C	5 seconds based on read-across data	12.5 h (20°C)
Hydrolysis t1/2 at pH 4 and 25°C	5 seconds based on read-across data	1.44 h (20°C)
Hydrolysis t1/2 at pH 9 and 25°C	5 seconds based on read-across data	5.18 h (20°C)
Short-term toxicity to fish (LC50)	no data	>3.1 mg/l (greater than the limit of solubility in water)
Short-term toxicity to aquatic invertebrates (EC50)	no data	>9 mg/l  (greater than the limit of solubility in water)
Algal inhibition (ErC50 and NOEC)	no data	EC50 >7.1 mg/l NOEC ≥7.1 mg/l  (greater than the limit of solubility in water)
Long-term toxicity to fish (NOEC)	no data	no data
Long-term toxicity to aquatic invertebrates (NOEC)	no data	no data
Long-term sediment toxicity (NOEC)	no data	no data
Short-term terrestrial toxicity (L(E)C50)	no data	no data
Long-term terrestrial toxicity (NOEC)	no data	no data

 

Read-across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorobutyl)ethyl]dichloro(methyl)silane:

The registration substance, [2-(perfluorobutyl)ethyl]dichloro(methyl)silane (CAS 38436-16-7), is a chlorosilane with a fluoroalkyl group attached to the silicon atom. In water under dilute conditions it hydrolyses rapidly (half-life approximately 5 seconds at 20-25°C and pH 7) to [2-(perfluorobutyl)ethyl]methylsilanediol and hydrochloric acid.

The surrogate substance, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7), is a trialkoxysilane with an alkyl and fluoro group attached to the silicon atom. In moist medium, [2-(perfluorohexyl)ethyl]triethoxysilane hydrolyses to [2-(perfluorohexyl)ethyl]silanetriol and ethanol (half-life of 12.5 hours at pH 7 and 20°C).

The silanol hydrolysis product of the registration substance, [2-(perfluorobutyl)ethyl]methylsilanediol, has structural similarity to the silanol hydrolysis product of the surrogate substance, [2-(perfluorohexyl)ethyl]silanetriol. They share some similar physicochemical properties: both have log K_ow <4 (3.3 and 2.7, respectively) and have low water solubility (2 mg/l and 0.58 mg/l (QSAR prediction)), respectively). They both have a chemical structure that contains a highly fluorinated alkyl chain; fluorocarbons have very low solubility in water, relative to hydrocarbons of equivalent chain length, (e.g. Horvath 1982), so it would be expected that these substances would be of low solubility. The effective limits of solubility of both the parent substances and monomeric silanol hydrolysis products are very low. Condensation phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The rates of hydrolysis for the registration substance relate to dilute conditions. These factors are important in any consideration of aqueous behaviour and properties.

Due to the rapid hydrolysis rate of [2-(perfluorobutyl)ethyl]dichloro(methyl)silane the chemical safety assessment is based on its silanol hydrolysis product. The hydrolysis rate of the surrogate substance, [2-(perfluorohexyl)ethyl]triethoxysilane, is slower than that of the registration substance. However, during the tests carried out with the surrogate substance it is likely that the test organisms were exposed to a mixture of the test substance and the hydrolysis products at the beginning of the test, and after this it is likely to be predominantly the hydrolysis products. See section 6.1 Aquatic toxicity endpoint summary for further discussion.  

Because the registration substance has such a rapid hydrolysis half-life, organisms are unlikely to be exposed to the parent substance. In the tests with the surrogate substance, organisms are likely to have been exposed to a mixture of parent substance and hydrolysis products at the beginning of the tests. The parent substance is assumed to be more toxic than the hydrolysis products in terms of intrinsic narcosis effects, if expressed. It is therefore considered conservative to read across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorobutyl)ethyl]dichloro(methyl)silane but read across is justified for the following reasons:

the registration substance is extremely difficult to test. It has very low water solubility and the presence of fluorocarbon chains indicates the potential for some surface activity. The amount of silanol in test solution depends on the amount of dissolved parent substance and condensation phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The short-term toxicity values from the studies of the surrogate substance show measured EC₅₀ values in excess of the solubility of the substance; however, best use has been made of the existing studies for [2-(perfluorohexyl)ethyl]triethoxysilane, although it is not clear how much parent or silanol hydrolysis product is in solution. No effects were seen in any of the tests. New studies are not recommended due to difficulties in testing the substance and avoiding physical effects associated with condensation/precipitation of insoluble oligomers, and surface activity related effects. Therefore read-across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorobutyl)ethyl]dichloro(methyl)silane is considered to be valid.

[2-(perfluorohexyl)ethyl]triethoxysilane is used to read-across to short-term toxicity to invertebrates and algae endpoints. E(L)C₅₀ values of >9 mg/l and >7.1 mg/l, respectively, have been determined.

Consideration of the non-silanol hydrolysis products:

Ethanol

Ethanol 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 for the substances are in excess of 1000 mg/l (OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, 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;

•            silanetriol and monosilicic acid have predicted first dissociation constants around 10 and so do not significantly affect the pH of an aqueous solution;

 

References:

EC 2003: European Union Technical Guidance Document on Risk Assessment for New and Existing Substances, Part II, European Chemicals Bureau, 2003.

ECHA 2016: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 3.0. February 2016.

Horvath, A.L. (1982) Halogenated Hydrocarbons. Solubility-Miscibility with Water, Marcel Dekker, New York, 1982.

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

OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, Ethanol, CAS 64-17-5.

PFA, 2016y, Peter Fisk Associates, Analogue Report - Ecotoxicity of substances with ‘low-functionality’ side chains attached to silicon, PFA. 404.003.004.R1

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >9 mg/l (greater than limit of solubility) in invertebrates and >7.1 mg/l (greater than limit of solubility) in algae based on read-across data from the structural analogue [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7). It has a reliable NOEC value of >=7.1 mg/l (greater than limit of solubility) in algae. The available aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.

The registration substance hydrolyses rapidly to [2-(perfluorobutyl)ethyl]methylsilanediol, which is not readily biodegradable, based on read-across data, and has low water solubility, which could mean it is likely to persist in the environment. However the log K_ow is <4 and so the 'safety net' criteria for classification does not apply.

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