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Hydrolysis

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Description of key information

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

Key value for chemical safety assessment

Additional information

No measured hydrolysis data are available for [2-(perfluorohexyl)ethyl]trichlorosilane. However, a reliable study according to OECD 111 is available for the related substance, trichloro(methyl)silane (CAS No. 75 -79 -6). This substance is fully hydrolysed within a few minutes at pH 4, 7 and 9 and 1.5°C.

For six substances, quantitative half-life data at 1.5ºC are available; two studies at 25°C and 27°C gave limit half-lives, one 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 ≤17 s.

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 No

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

Chlorotrimethylsilane

7

11

8

1.5 ± 0.5˚C

2

75-78-5

Dichloro(dimethyl) silane

10

17

7

1.5 ± 0.5˚C

2

75-79-6

Trichloro(methyl) silane

7

9

6

1.5 ± 0.5˚C

2

80-10-4

Dichloro(diphenyl) silane

6

10

8

1.5 ± 0.5˚C

2

675-62-7

Dichloromethyl(3,3,3-trifluoropropyl) silane8

12

9

 

1.5 ± 0.5˚C

2

5578-42-7

Dichlorocyclohexylmethylsilane

<<27 min[1]

<<27 min[2]

<<27 min[3]

27°C

2

18379-25-4

Trichloro(2,4,4-trimethylpentyl)silane

<<2 min[7]

<<2 min[7]

<<2 min[7]

27°C

2

4518-98-3

1,1,2,2-tetrachloro-1,2-dimethyldisilane

8

7

7

1.5 ± 0.5˚C

2

13154-25-1

Chlorotri(3-methyl-propyl) silane

Not quantified[4]

Not quantified[5]

Not quantified[6]

50˚C

1

[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.

[7] determination of hydrolysis kinetics was not possible, very fast hydrolysis and precipitation of solids were observed.

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

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 t1/2. Half-lives were estimated as 0.1t, where t=time for complete hydrolysis.

Measured hydrolysis half-lives of <<2 mins at pH 4, pH 7 and pH 9 and 25°C were determined for trichloro(2,4,4 -trimethylpentyl)silane in a study conducted according to generally acceptable scientific principles (Altmann 2015). 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 chlorosilanes, 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-(perfluorohexyl)ethyl]trichlorosilane.

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 half-life measured for the trihlorosilane analogue at 1.5ºC and pH 7 (9 seconds) the estimated hydrolysis half-life at 25ºC and pH 7 is 1.4 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 as a worst case can be considered to be approximately 5 seconds.

Using the half-life measured for the trichlorosilane analogue at 1.5ºC and pH 4 (7 seconds), the estimated hydrolysis half-life at 37.5ºC and pH 4 is 0.4 second, so worst case approximately 5 seconds.

The hydrolysis reaction may be acid or base catalysed, and the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalysed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

kobs≈kH3O+[H3O+]

At pH 4 [H3O+] = 10 -4 mol dm-3 and at pH 2 [H3O+] = 10-2mol dm-3; therefore, kobs at pH 2 should theoretically be approximately 100 times greater than kobs at pH 4. However, at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), it is not appropriate to apply any further correction for pH to the limit value at 37.5ºC and pH 4 and the hydrolysis half -life is therefore estimated to be approximately 5 seconds. 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.

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

 

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 Corporation 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-(perfluorohexyl)ethyl]trichlorosilane:

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/mol; 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 repeated dose inhalation toxicity exposure test.

As [2-(perfluorohexyl)ethyl]trichlorosilane has three Cl groups, this would be equivalent to 17 ppm.

Molecular weight of [2-(perfluorohexyl)ethyl]trichlorosilane = 481.54 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 17 ppm * 481.54 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 17 ppm [2-(perfluorohexyl)ethyl]trichlorosilane ≡ 335 g/l (or 0.0007 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.

The hydrolysis half-life of substances used for read-across in other areas are discussed 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]trichlorosilane (CAS No 78560 -45 -9) 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.

Hydrolysis of the read-across substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane  (CAS No: 73609-36-6)

Data for the substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane (CAS No 73609-36-6) are read-across to the submission substance [2-(perfluorohexyl)ethyl]trichlorosilane (CAS No 78560 -45 -9) 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 of the CSR for each endpoint.

 

For [2-(perfluorohexyl)ethyl]dichloro(methyl)silane, hydrolysis half-lives are read-across from analogous dichlorosilane substances. 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).

 

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.

 

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-(perfluorohexyl)ethyl]dichloro(methyl)silane.

 

Since rate of hydrolysis is faster at increased temperature, at ambient conditions (20 - 25°C), relevant to the environment, the hydrolysis half-lives are expected to be faster. Therefore, half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C, as a worst case considered to be approximately 5 seconds.

 

The hydrolysis products are [2-(perfluorohexyl)ethyl]methylsilanediol and hydrochloric acid.

Hydrolysis of the read-across substance [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No: 85857 -16 -5)

Data for the substance [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No 85857 -16 -5) are read-across to the submission substance [2-(perfluorohexyl)ethyl]trichlorosilane (CAS No 78560 -45 -9) 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]trimethoxysilane, hydrolysis half-lives of 0.4 h at pH 4, 0.4 h at pH 5, 8.8 h at pH 7 and 0.1 h at pH 9 at 20 -25°C were predicted for the substance.

 

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 0.004 hours (approximately 14 seconds) at pH 2 and 20 -25°C and 3.2 hours at pH 7 and 37.5°C. At pH 2 and 37.5°C, the half-life is approximately 4 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 2 and 37.5°C is approximately 5 seconds.

 

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

 

 

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.