Dichloromethyl(3,3,4,4,5,5,6,6,6-nonafluorohexyl)silane

Hydrolysis

Description of key information

Hydrolysis: Half-life approximately 5 seconds as a worst case at 25°C and pH 4, 7 and 9 (analogue read-across).

Additional information

No hydrolysis study is available for for [2-(perfluorobutyl)ethyl]dichloro(methyl)silane. However, reliable studies according to OECD 111 are available for other related dichlorosilane substances. These substances are fully hydrolysed within a few minutes at pH 4, 7 and 9 and 1.5°C. This read-across is made in the context of evidence from other available data for chlorosilane structural analogues.

[2-(Perfluorobutyl)ethyl]dichloro(methyl)silane contains the reactive group Si-Cl. Strong evidence based on read-across within the category of chlorosilanes is available to indicate that the Si-Cl bonds will rapidly hydrolyse to Si-OH with a half-life of ≤17 seconds at pH 4, 7 and 9 and 1.5°C (see below). The hydrolysis products are [2-(perfluorobutyl)ethyl](methyl)silanediol and hydrochloric acid.

For six substances, quantitative half-life data at 1.5ºC are available; a study with one further substance at 50ºC found no parent substance to be present at t₀, indicating extremely rapid hydrolysis. The measured

half-lives at pH 4, 7 and 9 and 1.5ºC are all ≤17s. Reliable hydrolysis studies according to OECD 111 are available for the related substances dichloro(dimethyl)silane, dichloromethyl(3,3,3-trifluoropropyl)silane and dichloro(diphenyl)silane. These substances are fully hydrolysed within less than a minute at pH 4, 7 and 9 and 1.5°C.

This read-across is made in the context of evidence from other available data for chlorosilane structural analogues, as shown in the following table.

Table: Hydrolysis data for chlorosilanes

CAS	Name	Result – half-life at pH 4 (seconds)	Result – half-life at pH 7 (seconds)	Result – half-life at pH 9 (seconds)	Temperature	Klimisch
75-77-4	Chloro(trimethyl)silane	7	11	8	1.5 ± 0.5˚C	2a
75-78-5	Dichloro(dimethyl)silane	10	17	7	1.5 ± 0.5˚C	2a
75-79-6	Trichloro(methyl)silane	7	9	6	1.5 ± 0.5˚C	2a
80-10-4	Dichloro(diphenyl)silane	6	10	8	1.5 ± 0.5˚C	2a
675-62-7	Dichloromethyl(3,3,3-trifluoropropyl)silane	8	12	9	1.5 ± 0.5˚C	2a
5578-42-7	Dichlorocyclohexylmethylsilane	<<27 min[1]	<<27 min[2]	<<27 min[3]	27°C	2a
4518-98-3	1,1,2,2-tetrachloro-1,2-dimethyldisilane	8	7	7	1.5 ± 0.5˚C	2a
13154-25-1	Chlorotri(3-methyl-propyl)silane	Not quantified[4]	Not quantified[5]	Not quantified[6]	50˚C	1a

[1]No parent substance was detected when the first measurement was taken.

[2]No parent substance was detected when the first measurement was taken.

[3]No parent substance was detected when the first measurement was taken.

[4]In this test the t₀analysis (50˚C) showed a recovery <LOD, suggestive of an extremely rapid reaction.

[5]In this test the t₀analysis (50˚C) showed a recovery <LOD, suggestive of an extremely rapid reaction.

[6]In this test the t₀analysis (50˚C) showed a recovery <LOD, suggestive of an extremely rapid reaction.

[1a] Study was conducted in accordance with relevant test method and in compliance with GLP

[2a] Study was conducted in accordance with relevant test method but not in compliance with GLP

 

Hydrolysis half-lives of 10 seconds at pH 4, 17 seconds at pH 7 and 7 seconds at pH 9 and 1.5°C were determined for dichloro(dimethyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Hydrolysis half-lives of 8 seconds at pH 4, 12 seconds at pH 7 and 9 seconds at pH 9 and 1.5°C were determined for dichloromethyl(3,3,3-trifluoropropyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Hydrolysis half-lives of 6 seconds at pH 4, 10 seconds at pH 7 and 8 seconds at pH 9 and 1.5°C were determined for dichloro(diphenyl)silane in accordance with OECD 111 (Dow Corning Corporation 2001).

Since the hydrolysis was so rapid relative to the timescale of the analytical measurement, there was insufficient data to determine rate constants for the hydrolysis reactions of these chlorosilanes using regression modelling. However, the data was adequate for estimating the upper limit of t_1/2. Half-lives were estimated as 0.1t, where t=time for complete hydrolysis.

Measured hydrolysis half-lives of <<27 mins at pH 4, pH 7 and pH 9 and 27°C were determined for dichlorocyclohexylmethylsilane in a study conducted according to generally acceptable scientific principles (Haas 2012). Only a preliminary study was carried out and a more precise knowledge of the half-life is needed for use in the chemical safety assessment. Therefore, this data is not used as part of the weight-of-evidence.

Given the very rapid hydrolysis rates in water ( ≤17 seconds at 1.5°C and pH 4, 7 and 9) observed for all tested dichlorosilanes, and the lack of significant variation in the half-lives for the different substances, it is considered appropriate to read-across this result to [2-(perfluorobutyl)ethyl]dichloro(methyl)silane.

 

Reaction rate increases with temperature, and therefore hydrolysis will be faster at 25ºCand at physiologically-relevant temperatures. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT₅₀(XºC) = DT₅₀(T°C) *e^(0.08.(T-X))

 

Where T = temperature for which data are available and X = target temperature.

 

Using the longest half-life measured for the dichlorosilanes at 1.5ºC and pH 7 (17 seconds) the estimated hydrolysis half-life at 25ºC and pH 7 is 2.6 seconds. However, it is not appropriate or necessary to attempt

to predict accurately when the half-life is less than 5-10 seconds. As a worst-case it can therefore be considered that the half-life of the substance at pH 7 and 25°C is approximately 5 seconds

 

The estimated hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays) is 1 second, so worst case approximately 5 seconds.

 

Using the longest half-life measured for the dichlorosilanes at 1.5ºC and pH 4 (10 seconds), the estimated hydrolysis half-life at 37.5ºC and pH 4 is 0.6 second, so worst case approximately 5 seconds.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at pH 7 and increase as the pH is raised or lowered (this is consistent with the data presented above).

The half-life at pH 4 and 37.5°C is estimated to be around 5 s, therefore, the half-life at pH 2 and 37.5°C (relevant for oral exposure) is also estimated as 5 s as a worst case. At 37.5ºC and pH 5.5 (relevant for

dermal exposure), the hydrolysis half -life is estimated to be in between the half-lives at pH 4 and pH 7 at 37.5ºC, and thus also approximately 5 seconds as a worst case.

 

Hydrolysis in air

 

The above hydrolysis studies were carried out with the substance dissolved in water.

 

Consideration of the rates of reaction with moisture in air is relevant for inhalation exposure assessment. Experience in handling and use, as well as the extremely rapid rates observed in the available water-based studies, would suggest that rates of reaction in moist air will also be rapid. If any unreacted chlorosilane were to reach the airways, it would rapidly hydrolyse in this very moist environment.

 

A simulated nose-only exposure study (Dow Corning Corporation 2013) has been conducted to determine hydrolytic stability of dichloro(dimethyl)silane under conditions typical of nose-only vapour inhalation exposures. The vapour generation was on 1 day for 3 hrs 14 minutes; concentrations of parent material were measured at 30 minute intervals using gas chromatography (GC). The nominal concentration was 50 ppm. The mean temperature was 21.6°C and the relative humidity (RH) was 57%. 24% parent concentration remaining in the test atmosphere relative to nominal concentration was measured by GC. This indicates 76% hydrolysis of the parent substance had taken place by the time the test atmosphere reached the GC. It was concluded that at least 20-29% of the parent test article would be present in the breathing zone relative to the nominal concentration under typical conditions used for nose-only inhalation exposure of rats. It is therefore possible to expose rats in a nose-only study to parent chlorosilane, because the transit time from the substance reservoir to the nose is very rapid (<1 second), however, this is not considered to be representative of human exposure conditions.

 

The authors of this summary have used the information from this study to estimate a half-life for dichloro(dimethyl)silane in air of approximately 7 seconds (95% confidence limit = 3 -11 seconds), which is comparable to the half-life in water.

 

In a study of the acute toxicity to rats via the inhalation route (Dow Corning Corporation 1997), dichloro(dimethyl)silane was quantified in the exposure chamber using Thermal Conductivity Detection and identification was confirmed using GC/MS. The relative humidity (RH) in the exposure chamber was 30 -35%. The mean measured concentrations in the exposure chambers during exposure (1 hour) was only about 15% of the nominal concentration of dichloro(dimethyl)silane.  The test atmosphere contained an amount of chloride consistent with the nominal concentration of test article as determined via electrochemical detection.  Thus, the majority of parent had hydrolysed in the test atmosphere at only 30-35% relative humidity.

Similarly, in a study to assess stability of dichloro(dimethyl)silane vapour in air using gas-sampling FTIR (Dow Corning 2009), dichloro(dimethyl)silane was observed to be extremely unstable in high relative humidity atmospheres. At 75% relative humidity (RH)level, a stable test atmosphere of the substance could not be generated. In dry air (<5% RH), the substance had achieved 28% loss after 1 hour and 71% loss after 3.2 hours.

 

The significant extent of chlorosilane hydrolysis demonstrated in the studies with dichloro(dimethyl)silane is in good agreement with the theoretical capacity for hydrolysis in air under conditions typical of a rat repeated exposure test.

 

Theoretically, air at 20°C and 50% relative humidity would have more than 100 times the amount of water necessary for complete hydrolysis of [2-(perfluorobutyl)ethyl]dichloro(methyl)silane:

 

Water content of air at 20°C and 100% relative humidity = 17.3 g/m³ 

Assuming a 50% humidity, the water content would be 8.65g water/m³= 8.65 mg water/l

Molecular weight of water = 18 g/mole; So 8.65 mg water/l = 0.48 mmol water/l

50 ppm HCl is the estimated upper exposure limit based on HCl corrosivity for a repeateddose inhalation toxicityexposure test.

As [2-(perfluorobutyl)ethyl]dichloro(methyl)silanehas two Cl groups, 25 ppm of the substance would produce 50 ppm HCl, so 6.25 ppm of [2-(perfluorobutyl)ethyl]dichloro(methyl)silane is the upper exposure limit for a repeated dose inhalation toxicity test.

 

Molecular weight of [2-(perfluorobutyl)ethyl]dichloro(methyl)silane   = 361.11 g/mol;

1 mole of an ideal gas under relevant conditions (standard pressure and temperature 25°C) has a volume of 24.45 l. So 25 ppm * 361.11 g/mol gives mass of substance per mole of air, then dividing by 24.45 gives the mass of substance per volume of air.

So 25 ppm [2-(perfluorobutyl)ethyl]dichloro(methyl)silane                 ≡ 369 g/l (or 0.001 mmol/l).

 

Therefore, it can be concluded that the registered substance is expected to hydrolyse very rapidly under conditions relevant for environmental and human health risk assessment. Additional information is given in

a supporting report (PFA 2013ab) attached in section 13 of the REACH technical dossier.

 

References

Dow Corning Corporation (1997). An acute whole body inhalation toxicity study of dimethyldichlorosilane in Fischer 344 rats. Testing laboratory: Dow Corning Corporation, MI 48686-0994. Report no.: Internal Report No. 1997-I0005-43537. Report date: 1997-12-22.

The hydrolysis half-lives of substances used for read-across in other areas are stated below:

Hydrolysis of the read-across substance [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No: 51851-37-7)

Data for the substance [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No 51851-37-7) are read-across to the submission substance [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No 85857-16-5) for appropriate endpoints. The hydrolysis half-life and the silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections for each endpoint.

 

For [2-(perfluorohexyl)ethyl]triethoxysilane, hydrolysis half-lives of 1.44 hours at 20°C, 0.9 hours at 30°C, 0.474 hours at 40°C and pH 4; 12.5 hours at 20°C, 5.25 hours at 30°C, 2.74 hours at 40°C and pH 7 and 5.18 hours at 20°C, 2.61 hours at 30°C, 1.38 hours at 40°C and pH 9 were determined in accordance with OECD 111 Test Guideline and in compliance with GLP (Lange 2016).

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the submission substance above. This gives a half-life of 0.0144 hours (approximately 52 seconds) at pH 2 and 20°C and 3.1 hours at pH 7 and 37.5°C. At pH 7 and 40°C, measured half-life is 2.7 hours. At pH 2 and 37.5°C, the half-life is 0.0036 hours (approximately 13 seconds).

 

The hydrolysis products are [2-(perfluorohexyl)ethyl]silanetriol and ethanol.