[3-(2,3-epoxypropoxy)propyl]dimethoxymethylsilane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65779-47-5) is 1.6 h at pH 7, 20-25⁰C. The substance will hydrolyse in contact with water and atmospheric moisture to [3-(2,3-epoxypropoxy)propyl]methylsilanediol (1 mole) and methanol (2 moles).

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 assessment is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Sections 4.8 and 5.1.2 of IUCLID, the silanol hydrolysis products may be susceptible to condensation reactions as well as subsequent hydrolysis reactions of the epoxide functional group.

There are no reliable measured data available with [3­(2,3­epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65779-47-5) therefore good quality data for the structurally-related substance, [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1), have been read across.

 

READ-ACROSS JUSTIFICATION

There are no available measured data with [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65779-47-5) for aquatic toxicity. Therefore, the Annex requirements are fulfilled with data from a structurally analogous substance. This document describes the analogue approach for fulfilling this endpoint by read-across from [3‑(2,3‑epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1), according to the Read-across Assessment Framework (RAAF)^[1].

Read-across is proposed in accordance with RAAF Scenario 2: “This scenario covers the analogue approach for which the read-across hypothesis is based on different compounds which have 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.”

The read-across justification is presented according to RAAF scenario 2 assessment elements (AE) as outlined in Table B1 of the RAAF1:

Table: RAAF scenario 2 assessment elements (AE) as given in Appendix B (Table B1) of the RAAF1

AE A.1	Characterisation of source substance
AE A.2	Link of structural similarity and differences with the proposed Prediction
AE A.3	Reliability and adequacy of the source study
AE 2.1	Compounds the test organism is exposed to
AE 2.2	Common underlying mechanism, qualitative aspects
AE 2.3	Common underlying mechanism, quantitative aspects
AE 2.4	Exposure to other compounds than to those linked to the prediction
AE 2.5	Occurrence of other effects than covered by the hypothesis and Justification
AE A.4	Bias that influences the prediction

 

 1.  AE A.1 Identity and characterisation of the source substance

The source substance [3‑(2,3‑epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1) is a diethoxysilane with methyl and 3-(2,3-epoxypropoxy)propyl side-chains. It has estimated hydrolysis half-lives at 20-25°C of 0.4 hours at pH 4, 11.7 hours pH 7 and 0.2 hours at pH 9. The products of hydrolysis are [3-(2,3-epoxypropoxy)propyl]methylsilanediol (1 mole) and ethanol (2 moles). The source substance has log K_ow of 2.7 at 20°C (estimated), predicted water solubility of 1200 and measured vapour pressure of 0.42 Pa at 25°C.

 

2.  AE A.2 Link of structural similarities and differences with the proposed prediction

The target substance, [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65779-47-5), is a dimethoxysilane with methyl and 3-(2,3-epoxypropoxy)propyl side-chains. It hydrolyses to [3-(2,3-epoxypropoxy)propyl]methylsilanediol (1 mole) and methanol (2 moles).

The source substance, [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane, (CAS 2897-60-1) is a diethoxysilane with methyl and 3-(2,3-epoxypropoxy)propyl side-chains. It hydrolyses to [3-(2,3-epoxypropoxy)propyl]methylsilanediol (1 mole) and ethanol (2 moles).

The target and source substances are both alkoxysilanes with a 3-(2,3-epoxypropoxy)propyl side-chain. They have the general structural formula shown in Figure 1 (attached).

For the target substance:       R1 is methyl,     R2 is methyl.

For the source substance:     R1 is methyl,     R2 is ethyl.

Therefore, all structural features of the target substance (3-(2,3-epoxypropoxy)propyl side-chain, dialkoxy(methyl)silane, methoxysilane) are present in the source substance.

The target substance and the source substance are both dialkoxysilanes with methyl and 3-(2,3-epoxypropoxy)propyl side-chains. The difference between these substances is that the two alkoxy groups are ethoxy for the source substance and methoxy for the target substance.

The target substance [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65779-47-5) and the source substance [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1), have similar moderate molecular weight (220 and 248, respectively) and low vapour pressure (3 Pa and 0.42 Pa at 25°C, respectively). They have low log K_ow (1.7 and 2.7, respectively) and moderate to high water solubility (1.2E+04 and 1200 mg/l, respectively. Both the substances hydrolyse rapidly, with half-lives of <12 h at pH 7 and 25°C (1.6 h and 11.7 h, respectively). The table below presents relevant physicochemical properties.

Table: Summary of physicochemical and ecotoxicological properties of the target and source substances.

CAS	65799-47-5	2897-60-1
Type	Dimethoxy	Diethoxy
Name	[3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane	[3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane
Hydrolysis products	[3-(2,3-epoxypropoxy)propyl]methylsilanediol (1 mole) and methanol (2 moles)	[3-(2,3-epoxypropoxy)propyl]methylsilanediol (1 mole) and ethanol (2 moles)
MW	220.34	248.4
WS / mg/l	1.2E+04	1200
Log Kow	1.7	2.7
Hydrolysis half-life at pH 7 and 25°C	1.6 h	11.7 h
Hydrolysis half-life at pH 7 and 37.5°C	0.6 h	4.4 h
Hydrolysis half-life at pH 2 and 37.5°C	5 s	5 s
VP at 25°C / Pa	3.0	0.42 (measured)
HP MW	192.29	192.29
HP WS / mg/l	1E+06 (limited to around 1000 mg/l by condensation reactions)	1E+06 (limited to around 1000 mg/l by condensation reactions)
HP Log Kow	-0.7	-0.7
HP VP at 25°C / Pa	2.8E-04	2.8E-04

 

3.  AE A.3 Reliability and adequacy of the source study

The short-term toxicity to invertebrates study was conducted according to the OECD Guideline 202 (Daphnia sp. Acute Immobilisation Test), in compliance with GLP (Sewell, 2004). Daphnia magna were exposed to nine test substance concentrations of 1.0, 1.8, 3.2, 5.6, 10, 18, 32, 56 and 100 mg/l, for a duration of 48 hours. The study reported a 48-hour EC₅₀ nominal value of 20 mg/l, for the effects of the test substance on mobility of the freshwater invertebrate Daphnia magna.

The toxicity to aquatic algae and cyanobacteria study was conducted according to the OECD Guideline 201 (Alga, Growth Inhibition Test), in compliance with GLP (Mead and McKenzie, 2004). Desmodesmus subspicatus were exposed to seven test substance concentrations of 0.32, 1.0, 3.2, 10, 32, 100 and 320 mg/l, for a duration of 72 hours. The study reported a 72-hour ErC₅₀ nominal value of >32 mg/l and a NOEC value of 3.2 mg/l, for the effects of the test substance on growth rate of the freshwater green algae Desmodesmus subspicatus.

 

4.  AE A.4 Bias that influences the prediction

Data on the source substance [3‑(2,3‑epoxypropoxy)propyl]diethoxy(methyl)silane were read-across to the registered (target) substance [3‑(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane. The source substance and the target substance have similar chemical structure and physico-chemical properties. Both substances hydrolyse at comparable rates and produce the same silicon-containing hydrolysis product, [3-(2,3-epoxypropoxy)propyl]methylsilandiol and a different non-silanol hydrolysis product, ethanol and methanol. Structural features of the target substance (3-(2,3-epoxypropoxy)propyl side-chain, dialkoxy(methyl)silane, methoxysilane) are present in the source substance. Both substances have the same structural alerts. Therefore, their ecotoxicological properties are expected to be similar, with similar acute toxicity. The source substance is the closest structural analogue to the target substance.

 

5.  AE A.2.1 Compounds the test organism is exposed to

The target substance, [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65779-47-5) and the source substance [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1) both hydrolyse rapidly in contact with water (1.6 h and 11.7 h at pH 7, 25°C, respectively). They share the same organosilicon hydrolysis product, [3-(2,3-epoxypropoxy)propyl]methylsilanediol, with the non-silicon hydrolysis products being methanol and ethanol, respectively.

During the tests with the source substance, [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1), it is likely that the test organisms were exposed to a mixture of the hydrolysis products of the substance and the parent substance. The invertebrate study report concludes that the substance had a hydrolysis half-life of approximately 3 hours under the test conditions, attributed by the reviewer to the elevated pH conditions of the test media (pH 8.0 - 8.1). The stability analyses conducted in the algal test showed the test material to be unstable in culture medium over the 72-hour test period and that the half-life of the test material was determined to be approximately 2 hours. It is reasonable to conclude, for the reasons discussed above, that the test organisms were predominantly exposed to the hydrolysis products of the test substance

 

6.  AE A.2.2 and A.2.3 Common underlying mechanism, qualitative and quantitative aspects

No toxicity data are available for the target substance [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65799-47-5), therefore data are read-across from the structurally analogous substance [3‑(2,3‑epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1). The substances hydrolyse at comparable rates to produce 1 mole of the silanol hydrolysis product, [3-(2,3-epoxypropoxy)propyl]methylsilanediol, and 2 moles of the non-silanol hydrolysis product. The non-silanol hydrolysis products are methanol and ethanol, respectively, which are of known toxicity and not expected to contribute to the effects observed for this endpoint. Moreover, the target and source substances have similar predicted physico-chemical properties, and share the same silanol hydrolysis product. Thus, both substances are expected to have similar ecotoxicity profiles.

 

7.  AE 2.4 Exposure to other compounds than to those linked to the prediction

The registration (target) substance, [3-(2,3-epoxypropoxy)propyl]dimethoxy(methyl)silane (CAS 65799-47-5) has a purity of 98%.

The test substance in the studies with the source substance [3‑(2,3‑epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1), has a purity of 97.8%.

Neither the target substance nor the source substances have impurities of ecotoxicological concern.

 

8.  AE 2.5 Occurrence of Other Effects than Covered by the Hypothesis and Justification

Not relevant.

 

^[1]European Chemicals Agency (ECHA) (2015) Read-across Assessment Framework. Appendix B, Scenario 2.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of 20 mg/l in invertebrates and >32 mg/l in algae (15.5 mg/l and >25 mg/l respectively based on the hydrolysis product [3-(2,3-epoxypropoxy)propyl]methylsilanediol). It has a reliable NOEC value of 3.2 mg/l, or 2.5 mg/l as [3-(2,3-epoxypropoxy)propyl]methylsilanediol, in algae.

The substance hydrolyses rapidly in water and is not readily biodegradable.

The hydrolysis product [3-(2,3-epoxypropoxy)propyl]methylsilanediol is susceptible to further hydrolysis reactions and the ultimate hydrolysis product 3-{3-[dihydroxy(methyl)silyl]propoxy}propane-1,2-diol is considered unlikely to exhibit significant ecotoxic effects based on QSAR estimated E(L)C₅₀s >>100 mg/l. The rate of reaction under environmental conditions is uncertain. [3-(2,3-Epoxypropoxy)propyl]methylsilanediol may meet the criteria of a ‘rapidly degradable’ substance in the context of classification and labelling for the environment; however, in the absence of definitive evidence, a precautionary approach has been applied.

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

Acute toxicity: Not classified.

Chronic toxicity: Category Chronic 3.