Trimethoxy(2,4,4-trimethylpentyl)silane

Endpoint summary

Additional information

Note: Studies on the toxicity towards algae and aquatic microorganisms according to OECD guidelines 201 and 209, respectively, are currently ongoing for the analogue substance (i.e. trichloro(2,4,4 -trimethylpentyl)silane (CAS 18379-25-4)), which shares the same silanol hydrolysis product as the registered compound. This dossier will be updated with a read-across to the above mentioned analogue substance once the final study reports are available.

The registered substance trimethoxy(2,4,4-trimethylpentyl)silane (CAS 34396-03-7) is expected to hydrolyse rapidly in contact with water (DT50 = 5.7 h at pH 7 and 20 - 25 °C, QSAR) to (2,4,4-trimethylpentyl)silanetriol and methanol. REACH guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). TGD (EC, 2003) and REACH guidance R.16 (ECHA, 2016) also suggest that when the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity testing.

No acute and chronic aquatic toxicity data is available for the registered compound trimethoxy(2,4,4-trimethylpentyl)silane (CAS 34396-03-7) or its silanol hydrolysis product. Therefore, good quality data was read-across from the structurally analogue substances trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) (fish and daphnia data) and trichloro(phenyl)silane (CAS 98-13-5) (algae data). Both source substances hydrolyse rapidly under environmental conditions (DT50 < 1 min at 1.5°C and pH 7) to form a silanol hydrolysis product and hydrochloric acid as the second hydrolysis product instead of methanol (target). The source substance trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) shares the same silanol hydrolysis product as the target compound, (2,4,4-trimethylpentyl)silanetriol. While the source substance trichloro(phenyl)silane (CAS 98-13-5) forms a structurally similar silanol hydrolysis product, phenylsilanetriol, compared to the silanol hydrolysis product of the target compound. In fact, 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 Kow 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 Kow < 1.9 (PFA 2016). Additional information on CAS No. 18379-25-4 is given in a supporting read-across justification attached in IUCLID Section 13. The remaining source substances used in the current dossier (CAS No. 98-13-5 and CAS No. 142877-45-0) were not integrated into the analogue justification because studies are currently ongoing for the respective endpoints and the dossier will be updated once the final study reports are available.

 

Short-term toxicity data is available on fish and algae for the read-across source compounds trichloro(2,4,4-trimethylpentyl)silane and trichloro(phenyl)silane, respectively. Chronic aquatic data is available for algae and aquatic invertebrates for the read-across source compounds trichloro(phenyl)silane and trichloro(2,4,4-trimethylpentyl)silane, respectively. Due to the fast hydrolysis time (DT50 < 1 min at 1.5°C and pH 7) of the source compounds and the duration of the acute tests (72 to 96 hours), it is likely that the organisms were predominantly exposed to the hydrolysis products. The second hydrolysis product of both source substances hydrochloric acid is well characterised in the public domain literature and 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 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. Generally the changes in pH of the receiving water should stay within the natural range of the pH, and for this reason, adverse effects on the aquatic environment are not expected due to anthropogenic or naturally occurring hydrochloric acid (OECD SIDS, 2002). Thus, hydrochloric acid is not considered to contribute to the overall aquatic toxicity of the source substances. The second hydrolysis product formed by the target substance, methanol, is also well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies (OECD SIDS, 2004). Thus, methanol is not considered contributory to the overall aquatic toxicity of the target substance. In consequence, data for methanol and hydrochloric acid are not presented specifically for each environmental toxicity endpoint within this dossier.

  

All aquatic studies were performed according to accepted standard guidelines and GLP criteria. No acute effects were observed towards fish and algae up to the highest tested concentrations. Moderate chronic effects were observed in aquatic invertebrates with a 21-d NOEC of 32 mg/L.

 

Table: Overview on aquatic effect values used for the registration of trimethoxy(2,4,4-trimethylpentyl)silane.

	Parent	Silanol hydrolysis product	Source 1	Source 2	Non-silanol hydrolysis products
CAS	34396-03-7		18379-25-4	98-13-5	7647-01-0	67-56-1
Chemical Name	trimethoxy(2,4,4-trimethylpentyl)silane	(2,4,4-trimethylpentyl)silanetriol	trichloro(2,4,4-trimethylpentyl)silane	trichloro(phenyl)silane	Hydrochloric acid^a,*	Methanol^b
Acute fish (96-h LC50)	RA CAS 18379-25-4	RA CAS 18379-25-4	> 100 mg/L	n.a.	4.92 mg/L (pH 4.3)	> 15000 mg/L
Acute Invertebrates (48-h EC50)	n.a.	n.a.	n.a.	n.a.	0.492 mg/L (pH 5.3)	> 10000 mg/L
Algae (72-h EC50)	RA CAS 98-13-5	RA CAS 98-13-5	n.a.”	> 100 mg/L	0.492 mg/L (pH 5.3)	ca. 22000 mg/L
Algae (72-h NOErC)	RA CAS 98-13-5	RA CAS 98-13-5	n.a.”	≥100 mg/L	0.097 mg/L (pH 6)	No data
Chronic Invertebrates (21-d NOEC)	RA CAS 18379-25-4	RA CAS 18379-25-4	32 mg/L			No data

^a Source = OECD SIDS (2002)

^b Source = OECD SIDS (2004)

n.a. = not available

* = hydrolysis product formed by the source substances

" = study ongoing

 

No effects on activated sludge microorganisms were recorded up to a concentration of 1000 mg/L with the read-across substance (2,3-Dimethyl-2-butanyl)(trimethoxy)silane (CAS 142877-45-0). The 3-h EC50 and NOEC were found to be > 1000 mg/L and ≥1000 mg/L, respectively. On this basis, no toxic effects are anticipated by the target substance and the degradation process in sewage treatment plants is not considered to be inhibited.

 

References:

OECD, 2002. Hydrogen Chloride - SIDS Initial Assessment Report for SIAM 15, Boston, USA: UNEP Publications.

OECD SIDS, 2004. Methanol - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.