Diphenylsilanediol

Hydrolysis

Description of key information

Hydrolysis (diphenylsilanediol): not hydrolytically unstable 

Additional information

The substance is not susceptible to hydrolytic degradation. However, as discussed in Section 4.8 (water solubility), the substance may undergo condensation reaction to form siloxane dimers and linear and cyclic oligomers.

The hydrolysis half-lives of substances used as read-across in other areas/endpoints are discussed below:

Hydrolysis of the read-across substance trichloro(phenyl)silane (CAS No: 98-13-5)

Data for the substance trichloro(phenyl)silane (CAS No 98-13-5) are read-across to the submission substance diphenylsilanediol for appropriate endpoints. The structural similarity between the submission substance and the silanol hydrolysis product of the read-across substance is relevant to this read-across, as discussed in the appropriate sections for each endpoint.

 

For trichloro(phenyl)silane, hydrolysis half-lives are read-across from related substance, trichloro(methyl)silane. Hydrolysis half-lives of <1 minute at pH 4, pH 7 and pH 9 and 1.5°C were determined in accordance with OECD 111 (Dow Corning Corporation, 2001b).

In Scheiwe 2008, quantitative determination of half-lives was not performed. However, the spectra results indicate that the test substance, trichloro(phenyl)silane, hydrolyses rapidly; in the water solvent (i.e. deuterated water) there is no trichloro(phenyl)silane present at the time of measurement (approximately 20 minutes). Also, in acetone containing a trace of water, rapid formation of phenylsilanetriol was observed.

In addition, no loss of phenyl groups was observed due to polymer precipitation (condensation). At concentrations of up to 200 mg/kg of the silane, no meaningful formation of condensation products (oligomers and polymers) of the hydrolysis product, phenylsilanetriol was observed.

 

The measured hydrolysis half-lives at pH 4, pH 7 and pH 9 are all limit values. 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. 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_obsat pH 2 should be approximately 100 times greater than k_obsat pH 4.

In the absence of experimental evidence of differences across the pH range (due to very rapidly hydrolysis and limitations in the determination procedure), it is not necessary to adjust for this. The uncertainty does not impact the chemical safety assessment.

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

 

DT₅₀(XºC) = DT₅₀(T) x e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

 

Thus, for trichloro(phenyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood), at 37.5°C and pH 2 (relevant for conditions in the stomach following oral exposure) and at 37.5°C and pH 5.5 (relevant for dermal exposure); the hydrolysis half-life is estimated to be approximately 5 seconds.

 

The hydrolysis products are phenylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance dichloro(diphenyl)silane (CAS No: 80-10-4)

Data for the substance dichloro(diphenyl)silane (CAS No 80-10-4) are read-across to the submission substance diphenylsilanediol for appropriate endpoints. The silanol hydrolysis product of the read-across substance is relevant, as discussed in the appropriate sections for each endpoint.

 

For dichloro(diphenyl)silane, hydrolysis half-lives of 0.1 minute at pH 4, 0.167 minutes at pH 7 and 0.133 minutes at pH 9 and 1.5°C were determined in accordance with OECD 111 (Dow Corning Corporation, 2001b).

 

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. 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_obsat pH 2 should be approximately 100 times greater than k_obsat pH 4.

 

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

t_1/2(pH 2) = t_1/2(pH 4) / 100

 

The calculated half-life of dichloro(diphenyl)silane at pH 2 is therefore <1 second at 1.5°C. 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 for the substance at pH 2 and 1.5°C is approximately 5 seconds.

 

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

 

DT₅₀(XºC) = DT₅₀(T) x e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

 

Thus, for dichloro(diphenyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 5 seconds as a worst case.

 

The half-lives at pH 2 and 37.5°C (relevant for conditions in the stomach following oral exposure), it is not appropriate to apply any further correction for temperature to the limit value and the hydrolysis half-life is therefore approximately 5 seconds.

 

The half-lives at pH 4 and 37.5°C, the calculated half-life is 1 second, so worst case half-life of approximately 5 seconds is used. At pH 5.5 and 37.5°C (relevant for dermal exposure), the hydrolysis half-life will be between the half-lives at pH 4 and pH 7 and 37.5°C, and thus half-life of approximately 5 seconds is used as a worst case.

 

The hydrolysis products are diphenylsilanediol and hydrochloric acid.

Hydrolysis of the read-across substance dimethoxy(diphenyl)silane (CAS No: 6843-66-9)

Data for the substance dimethoxy(diphenyl)silane(CAS No. 6843-66-9) are read-across to the submission substance diphenylsilanediol for appropriate endpoints. The silanol hydrolysis product of the read-across substance is relevant, as discussed in the appropriate sections for each endpoint.

 

For dimethoxy(diphenyl)silane, hydrolysis half-lives at 20-25°C of 0.1 h at pH 4, 0.1 h at pH 5, 0.6 h at pH 7 and <5 seconds at pH 9 were determined using validated QSAR estimation method.

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. 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_obsat pH 2 should be approximately 100 times greater than k_obsat pH 4.

 

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

t_1/2(pH 2) = t_1/2(pH 4) / 100

 

The calculated half-life of dimethoxy(diphenyl)silane at pH 2 is therefore 0.001 hours (3.6 seconds). 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 for the substance at pH 2 and 37.5°C is approximately 5 seconds.

 

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

 

DT₅₀(XºC) = DT₅₀(T) x e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

 

Thus, for dimethoxy(diphenyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 0.22 hours.

The half-lives at pH 2 and 37.5°C (relevant for conditions in the stomach following oral exposure), it is not appropriate to apply any further correction for temperature to the limit value and the hydrolysis half-life is therefore approximately 5 seconds.

 

The half-lives at pH 4 and 37.5°C, the calculated half-life is 2.2 minutes. At pH 5.5 and 37.5°C (relevant for dermal exposure), the hydrolysis half-life will be between the half-lives at pH 4 and pH 7 and 37.5°C (i.e 0.04 - 0.22 hours).

The hydrolysis products are diphenylsilanediol and methanol.