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Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2001-03-26 to 2001-10-26
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Remarks:
The study was conducted according to the appropriate OECD test guideline, but was not in compliance with GLP.
Qualifier:
according to guideline
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
GLP compliance:
no
Analytical monitoring:
yes
Buffers:
Buffer volume for hydrolysis: 50 mL
Buffers:
Target pH Buffer System Measured pH (before addition of test material)
4.0 Acetic acid/sodium hydroxide 4.00
7.0 Sodium dihydrogen 7.01
phosphate/sodium hydroxide
9.0 Boric acid/sodium hydroxide 8.99
Estimation method (if used):
Data treatment: For given solution conditions, the hydrolysis of parent was followed to completion as indicated by a stable chloride ion concentration measurement (2% change between concentration readings). The elapsed time between the addition of the test material to the aqueous buffer solution and the observation of a stable chloride ion concentration was used to estimate an
upper limit on t1/2 (seconds) assuming that 10 half-lives represents exhaustive hydrolysis (99.9%
complete).
Details on test conditions:
o Test substance stock solutions in acetonitrile were prepared inside a nitrogen gas purged glove bag.
o Plastic, instead of glass containers were used since it is known that the SiOH layer on glass will react with SiCl compounds.
o Temperature: 1.5 ± 0.5 °C
o Vessels: Low-density polyethylene bottles of 90-mL capacity with screw caps. Vessels were not sterilized.
o Buffer solutions were not sterilized.
o Co-solvent: <1% acetonitrile
Duration:
2.5 min
pH:
7
Duration:
3.8 min
pH:
9
Duration:
2 min
pH:
4
Number of replicates:
o Replicates: One at pH 4, 7, and 9
Transformation products:
yes
No.:
#1
No.:
#2
Details on hydrolysis and appearance of transformation product(s):
Breakdown products from hydrolysis: Hydrogen chloride and silanol (trimethylsilanol). For given solution conditions, the degradation product hydrogen chloride was observed to be stable during data collection. Consequently, HCl was considered stable. The stability of the silanol was not measured, however silanols will undergo condensation reactions to form siloxanes. The stabilities of silanols lie in the order R3SiOH > R2Si(OH)2 > RSi(OH)3, with the bulkier R groups lending more stability to the SiOH function (Smith, A. L., The Analytical Chemistry of Silicones 1991, 112, 12.)
pH:
4
Temp.:
1.5 °C
DT50:
ca. 0.3 min
Remarks on result:
other: This value represents an estimate of the upper limit for the hydrolysis half-life.
pH:
7
Temp.:
1.5 °C
DT50:
ca. 0.3 min
Remarks on result:
other: This value represents an estimate of the upper limit for the hydrolysis half-life.
pH:
9
Temp.:
1.5 °C
DT50:
ca. 0.3 min
Remarks on result:
other: This value represents an estimate of the upper limit for the hydrolysis half-life.
Other kinetic parameters:
A rate constant and half-life could not be determined quantitatively, although the data are certainly adequate for estimating the upper limit of t1/2.

First order or pseudo-first order behaviour could not be confirmed because: a) sparse nature of the data during the critical portion of the process (20-70% hydrolyzed), and b) the inherent limitation caused by measuring co-product concentration.

Details on results:
Values of upper limit on t1/2 (shown below) refer to disappearance of test material, i.e. complete hydrolysis, from measurement of chloride ion concentration formed.

Since the hydrolysis is so rapid, there is insufficient data to determine a rate constant (k) for the hydrolysis reaction by regression modelling.

The chloride ion concentration measured was stoichiometrically equivalent to the trimethylchlorosilane concentration added to each buffer. This confirmed the quantitative completion of hydrolysis.


Nominal

9.7x10-4 M (106 mg/L) Trimethylchlorosilane

9.2x10-4 M (33 mg/L) Chloride ion at pH 4

1.0x10-3 M (35 mg/L) Chloride ion at pH 7

8.7x10-4 M (31 mg/L) Chloride ion at pH 9

Half-life t1/2 in seconds at 1.5 +/- 0.5°C at different pH

pH 4.0: 7 seconds

pH 7.0: 11 seconds

pH 9.0: 8 seconds

Breakdown products

Yes

Conclusions:
The test material was found to have a half-life of <0.3 min at 1.5°C and pH 4, 7 and 9 in a reliable study conducted according to an appropriate test protocol, but was not in compliance with GLP.

Description of key information

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

Key value for chemical safety assessment

Half-life for hydrolysis:
1 min
at the temperature of:
25 °C

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 (CAS No. 75-77-4). This substance is fully hydrolysed within a few minutes at pH 4, pH 7 and pH 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 the table below.

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

8

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 was observed.

For six substances, quantitative half-life data at 1.5ºC are available; one study at 27°C gave a limit half-life, 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.

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 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 0.1 t, where t=time for complete hydrolysis.

In a preliminary study for another monochlorosilane, chlorotri(isobutyl)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 supporting data.

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 tert-butyl(chloro)dimethylsilane.

 

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.

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.

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.

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 tert-butyl(chloro)dimethylsilane:

 

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 tert-butyl(chloro)dimethylsilane has 1 Cl group this would be equivalent to 50 ppm

Molecular weight of tert-butyl(chloro)dimethylsilane = 150.73 g/mol

50 ppm tert-butyl(chloro)dimethylsilane is equivalent to 308.24 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.

 

The hydrolysis products for the registration substance are tert-butyl(dimethyl)silanol (CAS No. 18173-64-3) and hydrochloric acid.

Hydrolysis of the read-across substance butyl(chloro)dimethylsilane CAS No. 1000-50-6

Data for the substance, butyl(chloro)dimethylsilane (CAS No. 1000-50-6) are read-across to the submission substance tert-butyl(chloro)dimethylsilane for appropriate endpoints. The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate Sections for each endpoint.

The hydrolysis half-lives of butyl(chloro)dimethylsilane are read-across from chlorotrimethylsilane (CAS No. 75-77-4) as discussed for the submission substance above.

The hydrolysis products in this case are butyl(dimethyl)silanol and hydrochloric acid.

Hydrolysis of the read-across substance chlorotri(isobutyl)silane CAS No. 13154-25-1

Data for the substance, chlorotri(isobutyl)silane (CAS No. 13154-25-1) are read-across to the submission substance tert-butyl(chloro)dimethylsilane for appropriate endpoints. The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate Sections for each endpoint.

The hydrolysis of chlorotri(isobutyl)silane has been determined in a preliminary study; 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 hydrolysis products in this case are tri(isobutyl)silanol and hydrochloric acid.