Dichlorosilane

Hydrolysis

Description of key information

Hydrolysis: Half-life approximately 5 seconds as a worst case at 25°C and pH 4, 7 and 9 (analogue read-across). This half-life relates to degradation of the parent substance to give silanediol and HCl.  The Si-H bonds of silanediol are expected to react rapidly (<12 hours at 25°C and pH 7) to produce hydrogen and monosilicic acid. At concentrations above about 100-150 mg/l as SiO2, condensation products of silanediol and monosilicic acid can also form. In solution a mixture of the following species is expected to be present: silanediol, monosilicic acid and, at concentrations above about 100-150 mg/l as SiO2, their condensation products. The precise composition of this mixture will depend on concentration, temperature, other species present, and will change over time.

Additional information

The hydrolysis of dichlorosilane, and of the read-across substances used to fulfil other endpoint data requirements in this dossier, is discussed in detail below.

A standard hydrolysis study with dichlorosilane is considered technically unfeasible because the substance is a gas at normal temperature and pressure, and is expected to react violently with water.

The substance is expected to react rapidly and violently with water. The mechanism of the reaction is unknown; it may not be a true hydrolysis reaction. Potential reaction mechanisms are discussed in the document attached to Section 13 of IUCLID (PFA 2015ao). However, it is a chemical reaction with water and is referred to as hydrolysis in the remainder of this document

Dichlorosilane, H₂SiCl₂, contains two reactive groups: Si-Cl and Si-H.

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 silanediol and hydrogen chloride.

The Si-H bonds of silanediol are expected to react rapidly (<12 hours at 25°C and pH 7) to produce hydrogen and monosilicic acid. At concentrations above about 100-150 mg/l as SiO₂, condensation products of silanediol and monosilicic acid can also form. At concentrations >100-150 mg/l of SiO₂, monomeric monosilicic acid condenses into colloidal particles of polysilicic acid (silica sol) or a highly cross-linked network (silica gel). The condensation rate is dependent on temperature, concentration, and acidity/alkalinity (as in the pH) of the system. A dynamic equilibrium is established between monomeric monosilicic acid, oligomers and insoluble amorphous polysilicic acid. The composition of a solution is dependent upon conditions such as pH, temperature and the presence of ions.

Further discussion of reaction with water of substances in the structural class Silicon without carbon attached (“(Poly)silicic acid producers”), such as dichlorosilane can be found in the document attached to Section 13 of IUCLID (PFA 2015ao)

Si-Cl hydrolysis

No measured hydrolysis data are available for the registered substance. Therefore, data are read-across from other dichlorosilanes.

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 dichlorosilane.

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 likely that factors such as diffusion become rate-determining 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.

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.

k_obs= k₀+ k_H3O+[H3O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[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:

k_obs≈k_H3O+[H3O⁺]

At pH 4 [H₃O⁺] = 10^-4mol dm^-3and at pH 2 [H₃O⁺] = 10^-2mol dm^-3; therefore, k_obs at pH 2 should theoretically be approximately 100 times greater than k_obs 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.

Si-H hydrolysis

There is no measured value for the rate of reaction of the Si-H bond in dichlorosilane. However, studies have been carried out for silane and several siloxane compounds containing one or more Si-H bonds. The data are presented in the table below. The experiments investigated the rate of Si-H reaction by measuring the formation of gas (H₂) using headspace analysis. The half-lives obtained are in the range of hours to days.

The substances H4D4 and H5D5 are considered the most appropriate read-across for dichlorosilane as they produce the intermediate hydrolysis product methylsilanediol ((OH)₂Si(H)Me) rapidly (half-life for degradation of parent 2.2 - 4.2 minutes; half-life for intermediate hydrolysis steps (siloxane diol chain shortening reactions to produce methylsilanediol) 0.2 - 1h). Like the intermediate hydrolysis product of dichlorosilane hydrolysis (silanediol (HO)₂Si(H)₂), this is a silanediol with two -OH groups bound to silicon. The difference between the two intermediates is that methylsilanediol has one methyl group and one hydrogen bound to silicon, whereas silanediol has two hydrogens. Hydrolysis data are also available for the Si-H containing substance H-L3. H-L3 also produces the methylsilanediol intermediate but much more slowly (initial hydrolysis half-life 2.1 days).

For H4D4 and H5D5, 80% of the expected H₂formed afterapproximately 20 hours; assuming (pseudo) first order kinetics, this can be translated into a half-life of approximately 9 hours. Silanediol, (HO)₂Si(H)₂, might be expected to be more reactive than (HO)₂Si(H)R, where R is an organic group (for example methyl in this case) as the replacement of -H with -R makes the Si centre less electropositive and less susceptible to attack by nucleophiles, for example, OH^-. The presence of an R group attached to Si will slow these types of processes for steric reasons as well. The half-life for Si-H reactivity of dichlorosilane is therefore estimated as <12 hours (possibly much less than 12 hours) at pH 7 and 25°C.

 Table Hydrolysis data for Si-H containing substances

CAS	Substance Name (acronym)	Parent substance	Initial hydrolysis product	Final hydrolysis product	Parent substance degradation t1/2at pH 7 and 22.5-25°C	Timescale of full siloxane hydrolysis	Timescale of H2evolution
2370-88-9	2,4,6,8-Tetramethylcyclotetrasiloxane (H4D4)	[OSi(Me)(H)]4	H[OSi(Me) (H)]4OH2	MeSi(OH)3	2.2 min	Estimated t1/2for siloxanediol chain shortening reactions 0.2 – 1 h	80% expected H2formed after approximately 20 h
			Followed by shorter oligomers then (HO)2Si(H)Me
6166-86-5	2,4,6,8,10-Pentamethylcyclopentasiloxane (H5D5)	[OSi(Me)(H)]5	H[OSi(Me) (H)]5OH	MeSi(OH)3	4.2 min	Estimated t1/2for siloxanediol chain shortening reactions 0.1 – 1 h	80% expected H2formed after approximately 20 h
			Followed by shorter oligomers then (HO)2Si(H)Me
3277-26-5	1,1,3,3-Tetramethyldisiloxane (H2L2)	Me3SiOSi(Me)(H)OH	Me2Si(H)OH	Me2Si(OH)2	11 min	n/a	Apparent half-life of 2.5 days for degradation of intermediate hydrolysis product observed
1873-88-7	1,1,1,3,5,5,5-Heptamethyltrisiloxane (HL3)	Me3SiOSi(Me)(H)OSiMe3	Me3SiOSi(Me)(H)OH and Me3Si(OH)3	MeSi(OH)3and Me3Si(OH)	2.1 d	Estimated t1/2for hydrolysis of Me3SiOSi(Me)(H)OH is 8 h.	8.9 days to 41% conversion to H2. Half-life of 17 days calculated but rate of H2formation did slow over time.
			Followed by (HO)2Si(H) Me
993-07-7	Trimethylsilane	Me3Si	n/a	Me3SiOH	4.2 d	n/a	t1/2= 4.2 d

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 octachlorotrisilane:

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 toxicityexposure test.

As dichlorosilane has two Cl groups, 25 ppm of the substance would produce 50 ppm HCl, so 6.25 ppm of octachlorotrisilane is the upper exposure limit for a repeated dose inhalation toxicity test.

 

Molecular weight of dichlorosilane   = 101.01 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, 25 ppm * 101.01 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, 25 ppm dichlorosilane                 ≡ 103 g/l (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 2015ao) attached in Section 13 of the REACH technical dossier.

 

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.

PFA, 2015ao, Peter Fisk Associates, The aquatic chemistry of inorganic silicic acid generators, PFA.404.001.001

Hydrolysis of the read-across substance trimethoxysilane (CAS Number: 2487-90-3)

Trimethoxysilane (CAS 2487-90-3) has hydrolysis half-lives at 2°C of 14 seconds at pH 4, 17 seconds at pH 7, and 14 seconds at pH 9 (measured).

The estimated hydrolysis half-lives at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays), at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), and at 37.5ºC and pH 5.5 (relevant for dermal exposure), are approximately 5 seconds.

These half-lives relate to degradation of the parent substance to give silanetriol and methanol.

The half-life for Si-H reactivity of silanetriol is expected to be <12 hours at 25°C and pH 7, to produce hydrogen and monosilicic acid.

At concentrations above about 100-150 mg/l as SiO₂, condensation products of silanetriol and monosilicic acid can also form.

Hydrolysis of the read-across substance trichlorosilane (CAS 10025 -78 -2)

Trichlorosilane hydrolysis half-lives are read-across from the chlorosilane analogue group and the conclusions regarding hydrolysis rates are the same as for the submission substance: Half-life approximately 5 seconds as a worst case at 25°C and pH 4, 7 and 9 (analogue read-across), and estimated half-lives at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays), at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), and at 37.5ºC and pH 5.5 (relevant for dermal exposure), of approximately 5 seconds.

These half-lives relate to degradation of the parent substance to give silanetriol and HCl.

The half-life for Si-H reactivity of silanetriol is expected be <12 hours at 25°C and pH 7 to produce hydrogen and monosilicic acid.

At concentrations above about 100-150 mg/l as SiO2, condensation products of silanetriol and monosilicic acid can also form.

Hydrolysis of the read-across substance tetraethyl orthosilicate (CAS 78 -10 -4)

Tetraethyl orthosilicate has hydrolysis half-lives at 25°C of 0.11 h at pH 4, 4.4 h at pH 7 and 0.22 h at pH 9 (measured).

The estimated half-life at

37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays) is 1.6 hours.

The estimated hydrolysis half-life at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure) and at pH 4 is approximately 5 seconds.

The half-life at 37.5ºC and pH 5.5 (relevant for dermal exposure), is estimated to be in between the half-lives at pH 4 and pH 7 at 37.5ºC.

The hydrolysis products are monosilicic acid and ethanol.

At concentrations above about 100-150 mg/l as SiO₂ condensation products of monosilicic acid can also form.

Hydrolysis of the read-across substance disodium metasilicate (CAS 6834 -92 -0) and synthetic amorphous silica (SAS)

Hydrolysis is not relevant for the read-across substances disodium metasilicate (CAS 6834-92-0) and synthetic amorphous silica (SAS). Disodium metasilicate and synthetic amorphous silica, once solubilised in water, will form an equilibrium between monosilicic acid and amorphous polysilicic acid.