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Environmental fate & pathways

Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Remarks:
The study was conducted according to an appropriate OECD test guideline but it was not conducted under GLP.
Qualifier:
according to
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
GLP compliance:
no
Analytical monitoring:
yes
Buffers:
o Buffer solutions were not sterilized.
o Co-solvent: <1% acetonitrile
o Buffer volume for hydrolysis: 50 mL

Target pH Buffer System Measured pH (before addition of test material
4.0 Acetic acid/sodium hydroxide 4.00
7.0 Sodium dihydrogen phosphate/sodium hydroxide 7.01
9.0 Boric acid/sodium hydroxide 8.99

Details on test conditions:
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 Co-solvent: <1% acetonitrile
Number of replicates:
One at pH 4, 7, and 9
Statistical methods:
• 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).
Transformation products:
yes
No.:
#1
No.:
#2
Key result
pH:
4
Temp.:
1.5 °C
DT50:
< 1 min
Key result
pH:
7
Temp.:
1.5 °C
DT50:
< 1 min
Key result
pH:
9
Temp.:
1.5 °C
DT50:
< 1 min
Details on results:
Nominal: 1x10-3 M (149 mg/L) Methyltrichlorosilane

Measured value (the value with units preferably mg/L):
3.0x10-3 M (106 mg/L) Chloride ion at pH 4
3.2x10-3 M (113 mg/L) Chloride ion at pH 7
2.7x10-3 M (96 mg/L) Chloride ion at pH 9

Half-life (t(1/2) in seconds at a specific pH (pH 4, 7, 9, or other) at 1.5±0.5 °C:
pH 4.0: 7 seconds
pH 7.0: 9 seconds
pH 9.0: 6 seconds

Remarks Field for Results
• Values of upper limit on t1/2 (shown above) 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 the rate constants (k1, k2,
and k3) for the hydrolysis reactions by regression modelling.
• Rate constants and half-lives could not be determined quantitatively, although the data is certainly
adequate for estimating the upper limit of t1/2.
• The chloride ion concentration measured was stoichiometrically equivalent to the methyltrichlorosilane
concentration added to each buffer. This confirmed the quantitative completion of hydrolysis.
• 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), b) the inherent limitation caused
by measuring co-product concentration, and c) the relationship between k1, k2, and k3 is not known.
• Breakdown products from hydrolysis: Hydrogen chloride and silanol. 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 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.2
Conclusions:
The test material was found to have a half-life of <1 min at pH 4, 7 and 9 and 1.5°C in a reliable study not conducted under GLP.

Description of key information

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

Key value for chemical safety assessment

Additional information

No measured hydrolysis data are available for trichloro(propyl)silane. However, a reliable study according to OECD 111 is available for the related substance, trichloro(methyl)silane (CAS No. 75-79-6). Trichloro(methyl)silane is fully hydrolysed within a few minutes at pH 4, pH 7 and pH 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, pH 7 and pH 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 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 <<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 trichloro(propyl)silane.

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 measured for the trichlorosilane 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+] = 10-2 mol 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 propylsilanetriol 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 trichloro(propyl)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/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 trichloro(propyl)silane has three Cl groups, this would be equivalent to 17 ppm.

Molecular weight of trichloro(propyl)silane = 177.53 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 * 177.53 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 trichloro(propyl)silane ≡ 123 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 trichloro(methyl) silane (CAS 75-79-6)

Data for the substance trichloro(methyl)silane (CAS 75-79-6) are read-across to the submission substance trichloro(propyl)silane for appropriate endpoints. 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.

The hydrolysis half-lives of trichloro(methyl)silane is discussed under the hydrolysis half-lives of the submission substance.

The hydrolysis products are methylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance trichloro(ethyl)silane (CAS 115-21-9)

Data for the substance trichloro(ethyl)silane (CAS 115-21-9) are read-across to the submission substance trichloro(propyl)silane for appropriate endpoints. The silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections of the CSR for each endpoint.

For trichloro(ethyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were read-across from the study on the analogue substance, trichloro(methyl)silane (Dow Corning Corporation 2001).

The hydrolysis products are ethylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance trimethoxy(propyl) silane (CAS 1067-25-0)

Data for the substance trimethoxy(propyl)silane (CAS 1067-25-0) are read-across to the submission substance trichloro(propyl)silane for appropriate endpoints (see Section 1.5 of the CSR). The hydrolysis half-lives and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trimethoxy(propyl)silane, hydrolysis half-lives at 20-25°C of 0.2 h at pH 4, 0.3 h at pH 5, 2.6 h at pH 7 and 0.1 h at pH 9 were determined using a validated QSAR estimation method.

The hydrolysis products are propylsilanetriol and methanol.

Hydrolysis of the read-across substance trimethox(methyl)silane (CAS 1185-55-3)

Data for the substance trimethoxy(methyl)silane (CAS 1185-55-3) are read-across to the submission substance trichloro(propyl)lsilane for appropriate endpoints. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trimethoxy(methyl)silane, hydrolysis half-lives of 0 - <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 and 25°C were obtained in accordance with OECD 111 test method.

The hydrolysis products are methylsilanetriol and methanol.

Hydrolysis of the read-across substance dichloro(dimethyl)silane (CAS 75-78-5)

Data for the substance dichloro(dimethyl)silane (CAS 75-78-5) are read-across to the submission substance trichloro(propyl)silane for appropriate endpoints . The silanol hydrolysis product of the two substances are 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 at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were read-across from the studies on the analogue dichlorosilane substances.

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 registration substance above. This gives a half-lives of approximately 5 seconds.

The hydrolysis products are dimethylsilanediol and hydrochloric acid.

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.