Reaction mass of trimethyl(octadecyloxy)silane and octadecanol

Endpoint summary

Description of key information

In the key acute oral toxicity study, conducted according to an appropriate OECD test guideline and in compliance with GLP, the reported LD₅₀ value for the related substance, dimethylbis(octadecyloxy)silane, was greater than 2000 mg/kg bw OECD Guideline 401, GLP compliant (RTC, 1996).

In the key acute dermal toxicity study, conducted according to an appropriate OECD test guideline and in compliance with GLP, the reported LD₅₀ value for trimethyl(octadecyloxy)silane was greater than 2000 mg/kg bw (WIL Research, 2016).

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

LD50

Value:

2 000 mg/kg bw

Acute toxicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

no study available

Acute toxicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

LD50

Value:

2 000 mg/kg bw

Additional information

In the key acute oral toxicity study, which was conducted according to OECD 401, compliant with GLP, the LD₅₀ for the structural analogous substance dimethylbis(octadecyloxy)silane was >2000 mg/kg bw (RTC, 1996).

Following oral gavage administration of 2000 mg/kg bw test substance to 5 male and 5 female rats, no clinical signs of toxicity, effects on body weight or abnormal macroscopic findings were observed. The substance tested was of a lower purity than the analogous substance; this is not considered to affect the outcome of the study.

In addition, the registered substance, Reaction mass of octadecane-1-ol and trimethyl(octadecyloxy)silane, was administered daily by oral gavage to male and female Wistar Han rats at dose levels of 100, 300 and 1000 mg/kg for up to 54 days, followed by a 14-day treatment-free recovery period (Charles River, 2017). No mortality was observed at any dose level, which supports the use of the read-across acute oral study to conclude that the registration substance is not acutely harmful by the oral route. The study was conducted according to an appropriate OECD test guideline and in compliance with GLP.

In the key acute dermal toxicity study, conducted according to an appropriate OECD test guideline and in compliance with GLP, the reported LD50value for trimethyl(octadecyloxy)silane was greater than 2000 mg/kg bw (WIL Research, 2016).

Following a 24 -hour occluded dermal application of undiluted test item onto the dorsal skin of 5 male and 5 female rats, flat or hunched posture, ptosis, chromodacryorrhoea and/or lethargy were noted in all the animals on day 1. Additionally, chromodacryorrhoea was noted in two animals until Day 4 or Day 9. Scales were seen on the left flank in one animal on Day 5. The changes in body weight gain were within the range expected in rats used in this type of study. No macroscopic abnormalities were noted at post-mortem observations.

According to ECHA's disseminated dossier, octadecan-1-ol has been tested for acute dermal toxicity in a reliable study, conducted according to appropriate OECD test guideline and in compliance with GLP. No deaths were reported in test animals with intact skin. The reported LD50 value was greater than 8000 mg/kg bw.

 

Read-across justification

There are no available acute oral toxicity data forreaction mass of trimethyl(octadecyloxy)silane and octadecan-1-olfor acute oral toxicity. Therefore, read-across data are used to fulfil Annex requirements. This document describes the analogue approach for fulfilling this endpoint by read-across from the source substance, dimethylbis(octadecyloxy)silane (CAS 29043-70-7),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 (Table 5.6.4) according to RAAF scenario 2 assessment elements (AE) as outlined in Table B1 of the RAAF1:

Table 1: 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

 

AE A.1 Identity and characterisation of the source substance

The source substance,dimethylbis(octadecyloxy)silane (CAS 29043-70-7), has a single silicon atom connected to two octadecyloxy and two methyl groups. The substance has hydrolysis half-life values of 0.5 h at pH 4, 15 h at pH 7 and 0.2 h at pH 9 and 20-25°C. These values were obtained using an accepted calculation method. The hydrolysis products are dimethylsilanediol (1 mole) and octadecanol (2 moles). The substance has vapour pressor of 4E-04 Pa at 25°C, partition coefficient of 9 at 20°C (QSAR) and water solubility of 2.5E-14 mg/L at 20°C (QSAR).

 

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

The target and source substance have similar physico-chemical properties as well as hydrolysis rates (Table 2). The target substance Reaction mass of octadecan-1-ol and trimethyl(octadecyloxy)silane is composed of 35-48% stearyl alcohol (1-octadecanol) and 50-65% trimethyl(octadecyloxy)silane. Trimethyl(octadecyloxy)silane has a single silicon atom connected to one octadecyloxy group and three methyl groups, while the source substance dimethylbis(octadecyloxy)silane (CAS 29043-70-7)hastwo octadecyloxy groups and two methyl groups bonded to a silicon atom. The hydrolysis half-lives for both substances have been predicted using a validated QSAR estimation method. Hydrolysis half-lives are estimated to be 15 hours at pH 7 and 0.5 h at pH 4 and 20-25°C. As the hydrolysis reaction may be acid or base-catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. Therefore, the hydrolysis rate is expected to be < 0.5 hours at pH 1.5 - 3.5 (pH of stomach).  The very high log Kow and very low water solubility of the substance may limit the rate of hydrolysisin vivo.

The source substance hydrolyses to form dimethylsilanediol (1 mole) and octadecanol (2 moles) whereas the target substance forms trimethylsilanol (1 mole) and octadecanol (1 mole). The second constituent of the target substance is 1-octadecanol, which is the also the common hydrolysis product of the source and target substances. Dimethylsilanediol and trimethylsilanol are structurally similar. They are both small molecule alkylsilanols. Dimethylsilanol has two methyl groups and two hydroxy groups bound to a silicon atom; trimethylsilanol has three methyl groups and one hydroxy group bound to a silicon atom.

Table 2:Physico-chemical properties

Property	Target substance	Source substance
Substance name	Reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol	Dimethylbis(octadecyloxy)silane
CAS number	None105	29043-70-7
Hydrolysis half-life	< 0.5 hours at pH 1.5 – 3.5	< 0.5 hours at pH 1.5 – 3.5
Silanol hydrolysis product	trimethylsilanol	dimethylsilanediol
Non-Si hydrolysis product	1-octadecanol	1-octadecanol
LogKow value (parent)	9 at 20°C (QSAR)	9 at 20°C (QSAR)
LogKow value (Si-product)	1.19 at 25^oC	-0.4 at 20^oC
Vapour pressure (parent)	2.7E-03 Pa at 25°C (QSAR)	4E-04 Pa at 25°C
Vapour pressure (Si-product)	1900 Pa at 25^oC	7.0 Pa at 25^oC
Water solubility (parent)	2.2E-06 mg/L at 20°C (QSAR)	2.5E-14 mg/L at 20°C (QSAR)
Water solubility (Si-product)	995 mg/l at 20^oC	1E+06 mg/l at 20^oC

 

AE A.3 Reliability and adequacy of the source study

The key acute oral toxicity study for the source substance was conducted according to OECD 401 and in compliance with GLP. The reported LD₅₀ value for dimethylbis(octadecyloxy)silane was >2000 mg/kg bw (RTC, 1996). Following oral gavage administration of 2000 mg/kg bw test substance to 5 male and 5 female rats, no clinical signs of toxicity, effects on body weight or abnormal macroscopic findings were observed. The substance tested was of a lower purity than the analogous substance; this is not considered to affect the outcome of the study.

AE A.4 Bias that influences the prediction

Data on dimethylbis(octadecyloxy)silane (CAS 29043-70-7) are read-across to the registered substance Reaction mass of octadecan-1-ol and trimethyl(octadecyloxy)silane. The source substance and the target substance are structurally similar and have similar physico-chemical properties. They also share one common hydrolysis product. The non-common hydrolysis products, trimethylsilanol and dimethylsilanediol, are not acutely harmful via the oral route based on measured data from guideline studies. Therefore, their toxicological properties are considered to be similar, with identical acute toxic effects. No other data for relevant substances are available.

AE A.2.1 Compounds the test organism is exposed to

The source substance as well as the target substance have 15-hour half-lives at pH 7 and 0.5 h at pH 4.As the hydrolysis reaction may be acid or base-catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. Therefore, the hydrolysis rates are expected to be < 0.5 hours at pH 1.5 - 3.5 (pH of stomach). The very high log Kow and very low water solubility of the substance may limit the rate of hydrolysisin vivo. It is considered that following oral ingestion the test organism may be exposed to both the parent substances and the hydrolysis products dimethylsilanediol and octadecanol or trimethylsilanol and octadecanol. All the hydrolysis products are found to be not acutely toxic in reliable studies.

The acute oral toxicity of dimethylsilanediol was evaluated in a single-dose study in rats. Initially, the test article was administered once orally via gavage at the limit test level of 2000 mg/kg to a single fasted female albino rat. No mortality was observed and 4 additional animals were dosed at 2000 mg/kg. Mortality, clinical observations and body weight changes were evaluated over a 14-day observation period. All treated animals were subjected to a gross necropsy. One animal was found dead on study day 2. Clinical findings limited to this animal included impaired equilibrium, partial closure of the right and left eyes, hypoactivity, decreased respiration, soft feces, pale body and extremities and body cool to touch. Clinical findings for all animals included wet yellow material on the urogenital, anogential areas and/or ventral trunk. There were no remarkable body weight changes. There were no remarkable gross necropsy findings. Based on the results of this study, the estimated LD50 of dimethylsilanediol is greater than 2000 mg/kg when administered once orally via gavage to fasted female albino rats.

In an acute oral toxicity study conducted using a protocol comparable to the now deleted OECD 401, but not to GLP (Bayer, 1985), the LD₅₀for trimethylsilanol was 3.5 ml/kg bw (2835 mg/kg bw based on a density of 0.81 g/cm3) in rats (0, 0, 10, 40, 70 and 100% mortality at 0.5, 1.0, 2.0, 3.1, 5.0, 6.3 ml/kg bw, respectively). There was a general deterioration in the animals' health, anaesthesia, lateral positioning and ruffled fur. All surviving animals appeared symptom-free after ten days

The acute oral toxicity of octadecanol was evaluated in an acute oral toxicity study, conducted according to the appropriate OECD test guideline, and in compliance with GLP. Single dose of 2000 mg/kg of test material dissolved in arachis oil was administered to 5 male and 5 female rats. Body weight changes and clinical signs of toxicity were noted regularly during the 14 -day study period. Necropsy was performed at the end of the study. There were no deaths during the 14 -day study period. No signs of systemic toxicity were observed. The expected gain in body weight was observed in all animals. No macroscopic abnormalities were observed at necropsy. The estimated LD₅₀value of octadecanol is greater than 2000 mg/kg bw.

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

No acute toxicity data are available for the target substance reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol, therefore data are read-across from the structurally analogous substance dimethylbis(octadecyloxy)silane (CAS 29043-70-7). Both substances share a common hydrolysis product, and the non-common hydrolysis products are not acutely toxic. Moreover, the parent substances have similar physicochemical properties. Thus, both substances are expected to have similar toxicity profiles.

No deaths were observed in an acute oral toxicity study for the structural analogous substance dimethylbis(octadecyloxy)silane (CAS 29043-70-7). The reported LD₅₀value was >2000 mg/kg bw (RTC, 1996).

In addition, the registered substance, reaction mass of trimethyl(octadecyloxy)silane and octadecan-1-ol, was administered daily by oral gavage to male and female Wistar Han rats at dose levels of 100, 300 and 1000 mg/kg for up to 54 days, followed by a 14-day treatment-free recovery period (Charles River, 2017). No mortality was observed at any dose level, which supports the findings of the acute oral study. The study was conducted according to an appropriate OECD test guideline and in compliance with GLP.

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

Neither the target substance, Reaction mass of octadecan-1-ol and trimethyl(octadecyloxy)silane, nor the source substance, dimethylbis(octadecyloxy)silane (CAS 29043-70-7), have impurities of toxicological concern.

AE2.5 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.

Justification for classification or non-classification

Based on the available data for trimethyl(octadecyloxy)silane, no classification is required for acute toxicity according to Regulation (EC) No 1272/2008.