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Hydrolysis

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Description of key information

Hydrolysis: Half-life < 1 min at 25°C and pH 4, 7 and 9 (analogue read-across)

Key value for chemical safety assessment

Additional information

No hydrolysis study is available for the submission substance. However, a reliable study according to OECD 111 is available for the related substance chlorotrimethylsilane. This substance is 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, as illustrated in table 4.1.2 below.

Table 4.1.2: Hydrolysis half-lives at pH 4, 7 and 9 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

Chlorotrimethylsilane

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.

 

Hydrolysis half-lives of 7 seconds at pH 4, 11 seconds at pH 7 and 8 seconds at pH 9 and 1.5°C were determined for chlorotrimethylsilane in accordance with OECD 111 (Dow Corning 2001).

In a preliminary study for another monochlorosilane, chlorotri(3-methyl-propyl)silane; the study was conducted at 50°C for 2.4 hours at pH 4, pH 7 and pH 9. The measured concentration of the test substance at t0 and 50°C, indicate a recovery that is less than the limit of determination of the analytical instrument used (GC). However, the authors (White and Mullee 2002) of the study used a limit value that is twice the baseline noise of the instrument to estimate the concentration of the test sample. This suggests that the test substance was rapidly hydrolysed in aqueous media. In addition, a degradant peak was observed in the chromatogram at approximately 3.6 minutes. The result from this study is used as a supporting data.

For other chlorosilane substances, quantitative half-life data at 1.5ºC are available; the measured half-lives at pH 4, 7 and 9 and 1.5ºC are all ≤17 seconds.

Given the very rapid hydrolysis rates in water 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 chloro(dimethyl)vinylsilane.

Reaction rate increases with temperature, and therefore hydrolysis will be faster at 25ºC and 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 that is quantitatively derived for chlorotrimethylsilane at 1.5ºC and pH 7 (11 seconds) the calculated hydrolysis half-life at 25ºC and pH 7 is approximately 2 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 submission substance at pH 7 and 25°C is approximately 5 seconds.

 

The calculated hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays) is approximately 5 seconds.

 

Using the half-life that is quantitatively derived for chlorotrimethylsilane at 1.5ºC and pH 4 (7 seconds) the calculated hydrolysis half-life at 25ºC and pH 4 is approximately 1 second. As a worst-case the half-life at pH 4 and 25°C is 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 uncatalyzed 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 -2 mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.

 

The half-life of a substance at pH 2 is calculated based on:

 

t1/2(pH 2) = t1/2(pH 4) / 100

 

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 approximately 5 seconds. At 37.5ºC and pH 5.5 (relevant for dermal exposure), the hydrolysis half -life will 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 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 1997), dichloro(dimethyl)silane was quantified in the exposure chamber using TCD detection and identification was confirmed using GC/MS. There was only about 15% of the nominal concentration of dichloro(dimethyl)silane left in the exposure chamber.  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 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 level, a stable test atmosphere of the substance could not be generated. Also, in dry air, 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 study 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 at 50% relative humidity would have more than 100 times the amount of water necessary for complete hydrolysis of chloro(dimethyl)vinylsilane:

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

Assuming a 50% humidity = 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 repeated exposure test

As chloro(dimethyl)vinylsilane has 1 Cl group this would be equivalent to 50 ppm

Molecular weight of chloro(dimethyl)vinylsilane = 120.65 g/mol

50 ppm chloro(dimethyl)vinylsilane is equivalent to 247 mg/m3 or 0.002 mmol/l

Therefore, it can be concluded that the registered substance will hydrolyse very rapidly under conditions relevant for environmental and human health risk assessment and no further testing is necessary. It is not possible or necessary to attempt a quantitative prediction of rate or half-life because the chemical safety assessment is not sensitive to this uncertainty within this range. Additional information is given in a supporting report (PFA 2013ab) attached in section 13 of the REACH technical dossier.

The hydrolysis products for the submission substance are dimethyl(vinyl)silanol and hydrochloric acid.

 Hydrolysis of the read-across substance Chlorotrimethylsilane (CAS 75-77-4)

Hydolysis data for the substance chlorotrimethylsilane (CAS 75-77-4) are read-across to the submission substance chloro(dimethyl)vinylsilane for appropriate endpoints (see Section 1.4 of the CSR).

Both substances are chlorosilanes belonging to the same analogue group and the hydrolysis rates of chlorotrimethylsilane are discussed in the hydrolysis half-lives of the submission substance above.

The hydrolysis products are trimethylsilanol and hydrochloric acid.

Hydrolysis of the read-across substance dichloro(methyl)(vinyl)silane (CAS 124-70-9)

Data for the substance dichloro(methyl)(vinyl)silane (CAS 124-70-9) are read-across to the submission substance chloro(dimethyl)vinylsilane for appropriate endpoints (see Section 1.4 of the CSR).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 dichloro(methyl)vinylsilane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 are read-across from other dichlorosilane analogous substances (see Table 4.1.2).

Using the half-life that is quantitatively derived for other dichlorosilane at 1.5ºC and pH 7 (10-17 seconds) the calculated hydrolysis half-life at 25ºC and pH 7 is approximately 2 -3 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 submission substance at pH 7 and 25°C is approximately 5 seconds.

Using the half-life that is quantitatively derived for other dichlorosilane at 1.5ºC and pH 4 (6 -10 seconds) the calculated hydrolysis half-life at 25ºC and pH 4 is approximately 1 -2 seconds. As a worst-case it can therefore be considered that the half-life of the submission substance at pH 4 and 25°C is approximately 5 seconds.

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 approximately 5 seconds. At 37.5ºC and pH 5.5 (relevant for dermal exposure), the hydrolysis half -life will 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 methylvinylsilanediol and hydrochloric acid.

Hydrolysis of the read-across substance trimethoxyvinylsilane (CAS No: 2768-02-7)

Data for the substance trimethoxyvinylsilane (CAS 2768-02-7) are read across to the submission substance, chloro(dimethyl)vinylsilane (CAS 2768-02-7) for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis products of the two substances are relevant for this read-across, as discussed in the appropriate Sections of the CSR for each endpoint (See CSR Section 1.4).

Trimethoxyvinylsilane has predicted half-lives at 25ºC of 0.1 h at pH 7, 0.04 h (2 minutes) at pH 4 and 0.004 h (14 seconds) at pH 9. The half-lives at different temperatures and pH values may be calculated as above. This gives half-lives of 2 minutes at pH 7 and 37.5ºC and 5 seconds at pH 2 and 37.5ºC.

The hydrolysis products are vinylsilanetriol and methanol.

Hydrolysis of the read-across substance trimethylsilanol (CAS No: 1066-40-6)

Data for the substance trimethylsilanol (CAS No: 1066-40-6) are read-across to the submission substance chloro(dimethyl)vinylsilane for appropriate endpoints (see Section 1.4 of the CSR). The submission substance undergoes very rapid hydrolysis in water to produce dimethyl(vinyl)silanol and hydrochloric acid.The silanol hydrolysis product of the submission substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

Trimethylsilanol does not undergo hydrolytic degradation.