Trimethoxy(2,4,4-trimethylpentyl)silane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.64 mg/L

Assessment factor:

50

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

6.4 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.064 mg/L

Assessment factor:

500

Extrapolation method:

assessment factor

PNEC marine water (intermittent releases):

6.4 mg/L

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

4.3 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.43 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.48 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Conclusion on classification

Classification related key information

The registered compound trimethoxy(2,4,4-trimethylpentyl)silane (CAS No. 34396-03-7) is expected to hydrolyse rapidly in contact with water under environmentally relevant conditions (half-life approximately 5.7 hours at 20-25°C and pH 7). Therefore, the environmental hazard and risk assessment is based on the silanol hydrolysis product, i.e. (2,4,4-trimethylpentyl)silanetriol in accordance to REACH guidance R.16 (ECHA, 2016). The other, alcohol hydrolysis product, methanol, is not expected to affect the environmental hazard profile of the parent substance (OECD SIDS, 2004) and thus can be disregarded in the environmental hazard assessment.

 

Because no ecotoxicity data is available on the registered compound or its silanol hydrolysis product, good quality data was read-across from the structurally analogue substances trichloro(2,4,4-trimethylpentyl)silane (CAS No. 18379-25-4) and trichloro(phenyl)silane (CAS No. 98-13-5). 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, which is formed by the target substance. The source substance trichloro(2,4,4-trimethylpentyl)silane (CAS No. 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 No. 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 the source substance trichloro(2,4,4-trimethylpentyl)silane (CAS No. 18379-25-4) is given in a supporting read-across justification attached in IUCLID Section 13.

Due to the rapid hydrolysis of the source compounds and the duration of the aquatic toxicity tests (72 hours to 21 days), 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 (caused by an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterised 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). 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 read-across source substances.

 

 

Abiotic degradation - Hydrolysis

DT50 = 5.7 h at pH 7 and 20 – 25 °C (QSAR)

 

Biodegradation in water

Not biodegradable (13% in 28 days, OECD 310, RA CAS No. 35435-21-3)

 

Bioaccumulation

Trimethoxy(2,4,4-trimethylpentyl)silane (parent): Log Kow = 4.0 (QSAR)

(2,4,4-Trimethylpentyl)silanetriol (hydrolysis product): Log Kow = 0.9 (QSAR)

Methanol (hydrolysis product): Log Kow = - 0.82 to -0.64 (OECD SIDS, 2004)

 

Aquatic acute toxicity:

Fish

LC50 (96 h) > 100 mg/L (nominal, OECD 203; RA CAS No. 18379-25-4)

 

Aquatic invertebrates

Not available

 

Algae

ErC50 (72 h) > 100 mg/L (nominal, OECD 201; RA CAS No. 98-13-5)

  

Chronic aquatic toxicity

Algae

NOErC (72 h)≥100 mg/L (nominal, OECD 201; RA CAS No. 98-13-5)

 

Aquatic invertebrates

NOEC (21-d) = 32 mg/L (mean initial measured, OECD 211; RA CAS No. 18379-25-4)

 

Classification according to CLP

Trimethoxy(2,4,4-trimethylpentyl)silane (CAS No. 34396-03-7) is expected to hydrolyse rapidly in water (DT50 = 5.7 h at pH 7 and 20 - 25 °C), to the hydrolysis products (2,4,4-trimethylpentyl)silanetriol and methanol. The silanol hydrolysis product is not considered to be readily biodegradable, while the second hydrolysis product methanol is readily biodegradable (OECD SIDS, 2004). Acute aquatic toxicity data are available for fish and algae based on read-across to structurally analogue substances trichloro(2,4,4-trimethylpentyl)silane (CAS No. 18379-25-4) and trichloro(phenyl)silane (CAS No. 98-13-5). No acute aquatic effects were recorded at the highest concentrations tested (100 mg/L). Chronic data is available for daphnia and algae based on read-across to structurally analogue substances trichloro(2,4,4-trimethylpentyl)silane (CAS No. 18379-25-4) and trichloro(phenyl)silane (CAS No. 98-13-5). While no chronic effects were observed on algae, a moderate chronic toxicity was observed in daphnia magna with a 21-day NOEC of 32 mg/L. On this basis, trimethoxy(2,4,4-trimethylpentyl)silane and its silanol hydrolysis product is not classified as hazardous to the environment according to the consolidated version of Regulation (EC) No 1272/2008 (CLP) and all further amendments (ATPs).

 

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.