Trimethoxy(2,4,4-trimethylpentyl)silane

Endpoint summary

Additional information

The hydrolysis half-life oftrimethoxy(2,4,4-trimethylpentyl)silaneis approximately 5.7 h at 20-25⁰C and pH 7; the substance will therefore undergo rapid hydrolysis in contact with water to form(2,4,4-trimethylpentyl)silanetrioland methanol.

REACH guidance (ECHA 2016c, 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 guidance, (Eurpoean Commission, 2003) and R.16 (2016c) also suggests that when the hydrolysis half-life is less than 102 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 1, condensation reactions of the silanols are possible.

The silanol hydrolysis product, (2,4,4-trimethylpentyl)silanetriol, is a weak, nonionic surface active substance (see Section 1 for further details). The mechanism of toxicity of nonionic surfactants is considered to be the same as that of non-surfactant unreactive organic substances, i.e. non polar narcosis. In addition, due to the silanol hydrolysis product only being a weak surfactant, micelles are thought to be formed only at high concentrations, which are not environmentally realistic. The synergistic effects of micelles and other toxic compounds in the environment are therefore not considered further.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfill up to REACH Annex IX 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 since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

In moist medium, trimethoxy(2,4,4-trimethylpentyl)silane hydrolyses very rapidly (half-life 5.7 h at 20-25°C and pH 7) to (2,4,4-trimethylpentyl)silanetriol. (2,4,4-trimethylpentyl)silanetriol is a silanetriol with a log K_ow of 0.9. The non-silanol hydrolysis product methanol, is not expected to contribute to any adverse effects at the relevant dose levels. This is discussed further below.

In the following paragraphs the read-across approach for trimethoxy(2,4,4-trimethylpentyl)silane is assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

·        Read-across from trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) totrimethoxy(2,4,4-trimethylpentyl)silane (CAS 34396-03-7):

Trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) is a trialkoxysilane with an isooctyl side chain that hydrolyses rapidly (<1 min at 25°C and pH 4, 7 and 9) to form (2,4,4-trimethylpentyl)silanetriol and hydrochloric acid. The effects of hydrochloric acid are related to effects in an unbuffered media and are assessed below. Both, the registered substance and the read across substance, share the same hydrolysis product, (2,4,4-trimethylpentyl)silanetriol. Short- and long-term data available with trichloro(2,4,4-trimethylpentyl)silane indicate that this substance is of low toxicity to aquatic organisms.

·        Read-across from trichloro(phenyl)silane (CAS 98-13-5) to trimethoxy(2,4,4-trimethylpentyl)silane (CAS 34396 -03 -7):

Trichloro(phenyl)silane (CAS 98-13-5) is a trichlorosilane with an aryl side chain that hydrolyses very rapidly (<1 min at 25°C and pH 4, 7 and 9) to produce phenylsilanetriol and hydrochloric acid. The effects of hydrochloric acid are related to effects in an unbuffered media and are assessed below. Phenylsilanetriol and (2,4,4-trimethylpentyl)silanetriol are considered part of the same analogue group as they are both trisilanols and have similar physical properties with low log K_ow values of -0.02 and 0.9 respectively. Even though the side chain differs, there is evidence that the phenyl group does not contribute to the toxicity and that no effects are seen with silanols with a log K_ow< 1.9 (PFA 2012, PFA, 2013y).

Short-term toxicity data for algae with trichloro(phenyl)silane indicate that no effects are recorded up to 100 mg/l.

Considerations on the non-silanol hydrolysis products:

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 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).

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.

Table 7.1.1 Key physicochemical parameters and ectoxicological data for the registered and surrogate substances.

CAS Number	CAS 034396-03 -7	CAS 18379-25-4	98-13-5
Chemical Name	trimethoxy(2,4,4-trimethylpentyl)silane	Trichloro(2,4,4-trimethylpentyl)silane	Trichloro(phenyl)silane
Si hydrolysis product	(2,4,4-trimethylpentyl)silanetriol	(2,4,4-trimethylpentyl)silanetriol	Phenylsilanetriol
Molecular weight (parent)	234.4	247.7	211.6
Molecular weight (silanol hydrolysis product)	n/a	n/a	156.2
log Kow(parent)	4 (QSAR)	4 (QSAR)	n/a
log Kow (silanol hydrolysis product)	0.89 at 20°C	0.89 at 20°C	-0.02
Water sol (parent)	n/a	n/a	n/a
Water sol (silanol hydrolysis product))	2.4E+05 mg/l (QSAR)	2.4E+05 mg/l (QSAR)	1.0E+06 mg/l
Vapour pressure (parent)	8.6 Pa at 25°C (QSAR)	45 Pa at 20°C	44.3 Pa
Vapour pressure (hydrolysis product)	4.9 Pa at 25°C (QSAR)	4.9 Pa at 25°C (QSAR)	5.3E-05 Pa
Hydrolysis t1/2 at pH 7 and 25°C	5 h at 20-25°C	< 1 min at 1.5°C	<1 min
Hydrolysis t1/2 at pH 4 and 25°C	0.3 h at 20-25°C	< 1 min at 1.5°C	<1 min
Hydrolysis t1/2 at pH 9 and 25°C	0.1 h at 20-25°C	< 1 min at 1.5°C	<1 min
Short-term toxicity to fish (LC50)	n/a	> 100 mg/l	>100 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	n/a	>100 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	n/a	EC50 (72 h): > 100 mg/l NOEC (72 h): >= 100 mg/l
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	NOEC (21 d): 32 mg/l	n/a