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Hydrolysis

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Description of key information

Hydrolysis half-life: <5 seconds at 25°C and pH 4, 7 and 9 (analogue read-across). This half-life relates to degradation of the parent substance to give 1,2-bis[dihydroxy(methyl)silyl]ethane and HCl.

Key value for chemical safety assessment

Additional information

No hydrolysis study is available for 1,2-bis[dichloro(methyl)silyl]ethane. 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.

1,2-bis[dichloro(methyl)silyl]ethane 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 1,2-bis[dihydroxy(methyl)silyl]ethane and hydrogen chloride.

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

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 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 t1/2. 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 1,2-bis[dichloro(methyl)silyl]ethane.

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 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 (Houghton 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 1,2-bis[dichloro(methyl)silyl]ethane:

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 repeated dose inhalation toxicity exposure test.

As 1,2-bis[dichloro(methyl)silyl]ethane has four Cl groups, 12.5 ppm of the substance would produce 50 ppm HCl, so 12.5 ppm of 1,2-bis[dichloro(methyl)silyl]ethane is the upper exposure limit for a repeated dose inhalation toxicity test.

 

Molecular weight of 1,2-bis[dichloro(methyl)silyl]ethane   = 256.11 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 12.5 ppm * 256.11 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 12.5 ppm 1,2-bis[dichloro(methyl)silyl]ethane                 ≡ 130.9 mg/m3 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.

 

Hydrolysis of the read-across substance dichloro(dimethyl)silane (CAS No. 75-78-5)

Data for the substance dichloro(dimethyl)silane (CAS No. 75-78-5) are read-across to the submission substance 1,2-bis[dichloro(methyl)silyl]ethane for appropriate endpoints (see Section 1.5 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 IUCLID for each endpoint.

For dichloro(dimethyl)silane, hydrolysis half-lives of approximately 0.2 minutes at pH 4, approximately 0.3 minutes at pH 7 and approximately 0.1 minutes at pH 9 and 1.5°C were obtained in accordance with a relevant test method (Dow Corning Corporation 2001). These half-lives relate to degradation of the parent substance to give dimethylsilanediol and hydrochloric acid.

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 products of hydrolysis are dimethylsilanediol and hydrochloric acid.

 

Hydrolysis of the read-across substance trichloro(methyl)silane (CAS No. 75-79-6)

Data for the substance trichloro(methyl)silane (CAS No. 75-79-6) are read-across to the submission substance 1,2-bis[dichloro(methyl)silyl]ethane for appropriate endpoints (see Section 1.5 of the CSR). The rate of hydrolysis of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

This substance is fully hydrolysed within a few minutes at pH 4, 7 and 9 and 1.5°C (SEHSC 2001). The hydrolysis products are methylsilanetriol and hydrochloric acid.

 

Hydrolysis of the read-across substance 1,2-bis(triethoxysilyl)ethane (CAS No. 16068-37-4)

Data for the substance 1,2-bis(triethoxysilyl)ethane (CAS No. 16068-37-4) are read-across to the submission substance 1,2-bis[dichloro(methyl)silyl]ethane for appropriate endpoints (see Section 1.5 of the CSR). The rate of hydrolysis of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

The hydrolysis half-lives of 1,2-bis(triethoxysilyl)ethane have been predicted using a validated QSAR estimation method to be 0.8 h at pH 4, 0.7 h at pH 5, 36 h at pH 7, and 0.5 h at pH 9 and 20-25°C (Peter Fisk Associates 2016).

The ultimate product of the hydrolysis reaction under dilute condition is 1,2-bis(trihydroxysilyl)ethane. The other hydrolysis product is ethanol.

Hydrolysis of read-across substance 1,6-bis(trimethoxysilyl)hexane (CAS No. 87135-01-1)

Data for the substance 1,6-bis(trimethoxysilyl)hexane (CAS No. 87135-01-1) are read-across to the submission substance 1,2-bis[dichloro(methyl)silyl]ethane for appropriate endpoints (see Section 1.5 of the CSR). The structural similarities of the parent and silanol hydrolysis products, and the rate of hydrolysis of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the IUCLID for each endpoint. The hydrolysis half-lives of the substance have been measured in accordance with OECD 111 Test Guideline and in compliance with GLP. The hydrolysis half-lives of the substance are: 0.08 h at pH 5 and 25°C, 5.2 h at pH 7 at 25.1°C and 0.15 h at pH 9 at 25°C (Dow Corning 2010). At 35°C, the half-lives are: 0.05 h at pH 5, 2.6 h at pH 7, and 0.06 h at pH 9. The results are considered to be reliable.

The products of hydrolysis are 1,6-bis(trihydroxysilyl)hexane and methanol.

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

Data for the substance dichloro(methyl)vinylsilane (CAS 124-70-9) are read-across to the submission substance for appropriate endpoints (see Section 1.5 of the CSR). The rapid hydrolysis 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 are read-across from other dichlorosilanes, as for the submission substance, and the conclusions regarding hydrolysis rates are the same. These half-lives relate to degradation of the parent substance to give (3- methylvinylsilanediol and HCl.

Hydrolysis of the read-across substance dichloro(3-chloropropyl)methylsilane(CAS 7787-93-1)

Data for the substance dichloro(3-chloropropyl)methylsilane(CAS 7787-93-1) are read-across to the submission substance for appropriate endpoints (see Section 1.5 of the CSR). The rapid hydrolysis of the two substances is relevant to this read-across, as discussed in the appropriate sections of the CSR for each endpoint.

 

For dichloro(3-chloropropyl)methylsilane, hydrolysis half-lives are read-across from other dichlorosilanes, as for the submission substance, and the conclusions regarding hydrolysis rates are the same. These half-lives relate to degradation of the parent substance to give (3-chloropropyl)methylsilanediol and HCl.

References

Holleman-Wiberg, (2001) Inorganic Chemistry,Academic Press, pp841 and 865-5.

Reconsile Consortium members, personal communication, 2013.

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.

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