Isobutyldimethoxymethylsilane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of isobutyl(dimethoxy)methylsilane is predicted to be 1.7 hours at pH 7, 20-25°C. The registration substance will hydrolyse in contact with water and atmospheric moisture to isobutyl(methyl)silanediol and methanol. REACH guidance (ECHA 2012, 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 2012) 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, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

Read-across is used to fulfil 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 substances 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 and therefore the selection of surrogate substance is based on log K_ow of the resulting silanols and the chemical groups present in them. The registration substance and the substances used for read-across all contain alkyl groups attached to the silicon atom. Secondary and side chain features include a methyl group and a chloro- side chain feature.

Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID 5 dossier.

In the following paragraphs the read-across approach for isobutyl(dimethoxy)methylsilane 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: Physicochemical parameters and ecotoxicity data for the registered and surrogate substances

CAS Number	18293 -82 -8	18395-30-7	13501-76-3
Chemical Name	isobutyl(dimethoxy)methylsilane	isobutyl(trimethoxy)silane	3-chloropropyl(diethoxy)methylsilane
Si hydrolysis product	isobutyl(methyl)silanediol	(2-methylpropyl)silanetriol	(3-chloropropyl)(methyl)silanediol
Molecular weight (parent)	162.31	178.31	210.78
Molecular weight (hydrolysis product)	134.3	136.22	154.67
log Kow(parent)	3.4 at 20˚C (QSAR prediction)	2.1 at 20˚C (QSAR prediction)	4.2 at 20˚C (QSAR prediction)
log Kow(silanol hydrolysis product)	1.0 at 20˚C (QSAR prediction)	-1.0 at 20˚C (QSAR prediction)	0.8 at 20˚C (QSAR prediction)
Water sol (parent)	510 mg/l at 20-25°C (QSAR)	4900 mg/l at 20 -25˚C (QSAR prediction)	36 mg/l at 20 -25˚C (QSAR prediction)
Water sol (silanol hydrolysis product)	8.1E+04 mg/l at 20-25˚C (QSAR prediction) (limited by condensation reactions)	1.0E+06 mg/l at 20-25˚C (QSAR prediction) (limited by condensation reactions)	6000 mg/l at 20-25˚C (QSAR prediction) (limited by condensation reactions)
Vapour pressure (parent)	310 Pa at 25˚C (QSAR prediction)	190 Pa at 25˚C (QSAR prediction)	6.8 Pa at 25˚C (QSAR prediction)
Vapour pressure (hydrolysis product)	0.33 Pa at 25˚C (QSAR prediction)	0.0029 Pa at 25˚C (QSAR prediction)	0.021 Pa at 25˚C (QSAR prediction)
Hydrolysis t1/2 at pH 7 and 20-25°C	1.7 h (QSAR prediction)	4.1 h (QSAR prediction)	9.5 h (QSAR prediction)
Hydrolysis t1/2 at pH 4 and 20-25°C	0.2 h (QSAR prediction)	0.2 h (QSAR prediction)	0.4 h (QSAR prediction)
Hydrolysis t1/2 at pH 9 and 20-25°C	0.04 h (QSAR prediction)	0.1 h (QSAR prediction)	0.2 h (QSAR prediction)
Short-term toxicity to fish (LC50)	No data	>100 mg/l	574 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	No data	>864 mg/l	295 mg/l
Algal inhibition (ErC50and NOEC)	No data	ErC50 >1170 mg/l and NOEC 221 mg/l	ErC50 437 mg/l and NOEC 57 mg/l
Long-term toxicity to fish (NOEC)	No data	No data	No data
Long-term toxicity to aquatic invertebrates (NOEC)	No data	No data	No data

                                                                                                                                              

Read-across from isobutyl(trimethoxy)silane (CAS 18395-30-7) to isobutyl(dimethoxy)methylsilane (CAS 18293-82-8)

Isobutyl(dimethoxy)methylsilane (CAS 18293-82-8) and isobutyl(trimethoxy)silane (CAS 18395-30-7) are both predicted to hydrolyse rapidly in contact with water (1.7 h at pH 7, 20-25°C, and 4.1 h at pH 7, 20-25°C, respectively). The organosilicon hydrolysis products are isobutyl(methyl)silanediol and (2-methylpropyl)silanetriol respectively. The non-silicon hydrolysis product of both the registration and surrogate substance is methanol. Isobutyl(methyl)silanediol and (2-methylpropyl)silanetriol are structural analogues, both containing a 2-methylpropyl group. The additional methyl group in isobutyl(methyl)silanediol means only that it has a slightly higher log K_ow than the analogous silanetriol but both values are still low (see below). They share similar physicochemical properties: high water solubility, low log K_ow (1.0 and -1.0 respectively), low vapour pressure (0.33 Pa and 0.029 Pa respectively) and similar molecular weight (134.3 and 136.22 respectively), and they are both within the low functionality analogue group (PFA 2016y).

Isobutyl(trimethoxy)silane is used to read-across to short-term toxicity to invertebrates and algae endpoints. E(L)C₅₀ values of >864 mg/l and >1170 mg/l, respectively, have been determined. During these aquatic ecotoxicity studies the test organisms would have been predominantly exposed to (2-methylpropyl)silanetriol, and read-across to provide information on isobutyl(methyl)silanediol is considered to be valid for these endpoints.

The aquatic toxicity data for the surrogate substance indicates that it is not acutely toxic to aquatic organisms. 

 

Read-across from 3-chloropropyl(diethoxy)methylsilane (CAS 13501-76-3) to isobutyl(dimethoxy)methylsilane (CAS 18293-82-8)

Isobutyl(dimethoxy)methylsilane (CAS 18293-82-8) and 3-chloropropyl(diethoxy)methylsilane (CAS 13501-76-3) are both predicted to hydrolyse rapidly in contact with water (1.7 h at pH 7, 20-25°C, and 9.5 h at pH 7, 20-25°C respectively). The organosilicon hydrolysis products are isobutyl(methyl)silanediol and (3-chloropropyl)(methyl)silanediol respectively. The non-silicon hydrolysis products are methanol and ethanol, respectively. Isobutyl(methyl)silanediol and (3-chloropropyl)(methyl)silanediol are structural analogues, both containing an alkyl group, which in the case of (3-chloropropyl)(methyl)silanediol is chlorinated. They share similar physicochemical properties: high water solubility, low log K_ow (1.0 and 0.8 respectively), low VP (0.33 and 0.021 respectively) and similar MW (134.3 and 154.67 respectively). During these aquatic ecotoxicity studies the test organisms would have been predominantly exposed to (3-chloropropyl)(methyl)silanediol, and read-across to provide information on isobutyl(methyl)silanediol is considered to be valid for these endpoints.

3-Chloropropyl(diethoxy)methylsilane (CAS 13501-76-3) is used as an additional read-across to isobutyl(trimethoxy)silane (CAS 18395-30-7) because it also shares a structurally analogous hydrolysis product, with a closer log K_ow, to that of the registration substance, and is also within the low functionality analogue group (PFA 2016y).

3-Chloropropyl(diethoxy)methylsilane is used to read-across to short-term toxicity to invertebrates and algae endpoints. E(L)C₅₀ values of 295 and 437 mg/l, respectively, have been determined.

The aquatic toxicity data for the surrogate substance indicates that it is not acutely toxic to aquatic organisms. 

 

Considerations on the non-silanol hydrolysis products:

Methanol and ethanol are well characterised in the public domain literature and are not hazardous to aquatic organisms at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for these substances are in excess of 1000 mg/l (OECD2004a - SIDS for methanol, CAS 67-56-1, OECD 2004b - SIDS for ethanol, CAS 64-17-5).Therefore, at the loading rates experienced in these tests it is unlikely that the presence of either would significantly affect the results of the tests.

Reference:

ECHA 2012: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1, October 2012 (Section R.16.5.4.1. Hydrolysis)

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >864 mg/l and 295 mg/l in invertebrates and >1170 mg/l and 437 in algae based on read-across from structural analogues of the hydrolysis product. It has reliable NOEC values of 221 mg/l and 57 mg/l in algae.

The registration substance hydrolyses rapidly in water and its silanol hydrolysis product has a low log K_ow.

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.