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Additional information

The hydrolysis half-life of 3-[dimethoxy(methyl)silyl]propanethiol is 1.4 h at pH 7, 0.1 h at pH 4, 0.03 h at pH 9 and 20-25°C (QSAR).The registered substance will hydrolyse in contact with water and atmospheric moisture to 3-(dihydroxymethylsilyl)propanethiol and methanol. 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, 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 4.8, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to fulfil REACH Annex VII 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, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour.

The registered substance and the substance used as surrogate for read-across are part of a class of alkoxysilanes (and their silanol hydrolysis products) that have a thiol functional group present on an alkyl silicon side chain. The group of organosilicon substances in this group contain a terminal thiol (SH) group present on a silicon side chain. The silanol hydrolysis products may be susceptible to condensation reactions as discussed in Section 4.8 (water solubility). The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substance is based on the log Kow of the resulting silanols and the other chemical groups that are present in them. The alkyl side-chain features of the registered substance and read-across substances are propanethiol groups, the registration substance additionally includes a methyl group in its structure.

Additional information is given in a supporting report (PFA 2013w) attached in Section 13.

In the following paragraphs the read-across approach is assessed for the surrogate substance taking into account structure, hydrolysis rate and physicochemical properties.

Read-across from 3-trimethoxysilylpropane-1-thiol (CAS 4420-74-0) to 3-[dimethoxy(methyl)silyl]propanethiol (CAS 31001-77-1)

3-[Dimethoxy(methyl)silyl]propanethiol (CAS 31001-77-1) and 3-trimethoxysilylpropane-1-thiol (CAS 4420-74-0) both hydrolyse rapidly in contact with water (1.4 h at pH 7 and 2.6 h at pH 7 respectively and 20-25°C). The organosilicon hydrolysis products are 3-(dihydroxymethylsilyl)propanethiol and 3-(trihydroxysilyl)propanethiol respectively. 3-(Dihydroxymethylsilyl)propanethiol and 3-(trihydroxysilyl)propanethiol are structural analogues, both containing a propanethiol group. The additional methyl group in 3-(dihydroxymethylsilyl)propanethiol means only that it has a slightly higher log Kow than the analogous silanetriol but both values are still very low (see below). They also share similar physicochemical properties: high water solubility, low log Kow (0.5 and -1.4 respectively) and low molecular weight (152.28 g/mol and 154.26 g/mol respectively). The aquatic toxicity data for the surrogate substance indicates that it is acutely toxic to invertebrates but not to algae. The thiol group is a recognised structural alert for potential toxicity to aquatic invertebrates (e.g. ECHA Guidance Part R.10 citing von der Ohe et al., 2005) and the results seen are fully consistent with this.

Reference:

Von der Ohe PC, Kühne R, Ebert R-U, Altenburger R, Liess M, Schüürmann G. 2005. Structural alerts – a new classification model to discriminate excess toxicity from narcotic effect levels of organic compounds in the acute Daphnid assay. Chem. Res. Toxicol. 18: 536-555.

Considerations on the non-silanol hydrolysis products:

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 for this substance is in excess of 1000 mg/l (OECD 2004 - SIDS for methanol, CAS 67-56-1). Therefore, at the loading rates experienced in these tests it is unlikely that the presence of methanol would significantly affect the results of the tests.

Table: Physicochemical parameters and ecotoxicity data for the registered and surrogate substances

CAS Number

31001-77-1

4420-74-0

Chemical Name

3-[dimethoxy(methyl)silyl]propanethiol

3-trimethoxysilylpropane-1-thiol

Si hydrolysis product

3-(dihydroxymethylsilyl)propanethiol

3-(trihydroxysilyl)propanethiol

Molecular weight (parent) (g/mol)

180.34

196.34

Molecular weight (hydrolysis product) 9g/mol)

152.28

154.26

log Kow(parent)

2.9

1.7

log Kow (silanol hydrolysis product)

0.5

-1.4

Water sol (parent)

290 mg/l

2.8E+03 mg/l

Water sol (silanol hydrolysis product))

4.7E+04 mg/l

1E+06 mg/l

Vapour pressure (parent)

23 Pa

8 Pa

Vapour pressure (hydrolysis product)

0.0092Pa

1.1E-04 Pa

Hydrolysis t1/2at pH 7 and 25°C

1.4 h

2.6 h

Hydrolysis t1/2at pH 4 and 25°C

0.1 h

0.2 h

Hydrolysis t1/2at pH 9 and 25°C

0.03 h

0.1 h

Short-term toxicity to fish (LC50)

not available

439 mg/l

Short-term toxicity to aquatic invertebrates (EC50)

not available

6.7 mg/l

Algal inhibition (ErC50and NOEC)

not available

ErC50: 931 mg/l; ErC10: 92 mg/l; NOEC: 40 mg/l

Long-term toxicity to fish (NOEC)

not available

not available

Long-term toxicity to aquatic invertebrates (NOEC)

not available

not available

Long-term sediment toxicity (NOEC)

not available

not available

Short-term terrestrial toxicity (L(E)C50)

not available

>1000 mg/kg dwt

Long-term terrestrial toxicity (NOEC)

not available

not available

References:

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

ECHA (2016) Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Estimation, Version 3.0, February 2016

Conclusion on classification

The substance has reliable short-term E(L)C50 values of 6.7 mg/l in invertebrates and 931 mg/l in algae based on read-across from a structural analogue. It has a reliable ErC10 value of 92 mg/l in algae. The EC50 in invertebrates, and the ErC50 and ErC10 in algae are equivalent to 5.3 mg/l, 731 mg/l and 72 mg/l respectively when expressed in terms of 3-(trihydroxysilyl)propanethiol).

The substance hydrolyses rapidly in water and is not readily biodegradable. The effects seen are attributed to the hydrolysis product, which is not rapidly degradable.

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Category 2