Potassium methylsilanetriolate

Hydrolysis

Description of key information

Potassium methylsilanetriolate is not susceptible to hydrolytic degradation.

Additional information

The substance as registered is an aqueous solution and is stable in this solution (at pH >12). Potassium methylsilanetriolate dissociates completely into potassium and methylsilanetriolate ions in aqueous solution. The substance as sold is only stable at high pH (pH >12). When the pH is reduced, the concentration of the non-ionised form (methylsilanetriol) increases. The first pKa of methylsilanetriol is approximately 10. At pH 9.0 and below, the substance is no longer in the ionised form and methylsilanetriol predominates. Methylsilanetriol undergoes reversible condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers. The overall rate and extent of condensation is dependent on the amount of substance present (the nominal loading), temperature, pH of the system and what else is present in the solution.

Potassium methylsilanetriolate and methylsilanetriol contain no chemical groups that are susceptible to hydrolytic degradation.

Hydrolysis of the read-across substance trimethoxymethylsilane (CAS 1185-55-3)

Data for the substance trimethoxy(methyl)silane(CAS 1185-55-3) are read-across to the submission substance potassium methylsilanetriolate for appropriate endpoints (see discussion of read-across in the data matrix attached to Section 13 of the IUCLID). The hydrolysis half-life and the silanol hydrolysis product of the read-across substance is relevant to this read-across, as discussed in the appropriate Endpoint Summaries of IUCLID.

For trimethoxymethylsilane, hydrolysis half-lives at 25°C of <0.0033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2004).

For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed 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 pH2 [H₃O⁺]=10^-2mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at 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 trimethoxymethylsilane at pH 2 is therefore 0.000033 hours (0.12 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 for trimethoxymethylsilane at pH 2 and 20-25°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) * e^(0.08.(T-X))

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

Thus, for trimethoxymethylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 0.8 hours. At 37.5ºC and pH2 (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 hydrolysis products are methylsilanetriol and methanol.

Hydrolysis of the read-across substance phenysilanetriol (CAS 3047-74-3)

Phenylsilanetriol is an organosilicon compound with no hydrolysable groups.

 

Hydrolysis of the read-across substance trimethylsilanol (CAS 1066-40-6)

Trimethylsilanol is an organosilicon compound with no hydrolysable groups.

Hydrolysis of the read-across substance triethoxy(methyl)silane (CAS 2031-67-6)

A hydrolysis half-life of approximately 5.5 hours at pH 7 and 20-25°C was obtained for the substance using an appropriate calculation method.

Hydrolysis half-lives of approximately 0.3 h at pH 4 and pH 5 and 0.1 h at pH 9 and 20-25°C were obtained for the substance using an appropriate calculation 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 uncatalyzed 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^-4 mol dm^-3 and at pH 2 [H₃O⁺] = 10^-2 mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at 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 triethoxy(methyl)silane at pH 2 is therefore 5 seconds. However, it is likely that factors such as diffusion become rate-determining when the half-life is less than 5-10 seconds.

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions and the half-lives at 37.5ºC (relevant for in vivo studies) are expected to be faster than those at 20-25°C. 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 triethoxy(methyl)silanethe hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 2 hours. At 37.5ºC and pH 2 (relevant for the conditions in the stomach following oral exposure) is 5 seconds.

The initial hydrolysis products are methylsilanetriol and ethanol.

 

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 potassium methylsilanetriolate for appropriate endpoints (see discussion of read-across in the data matrix attached to Section 13). The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate endpoint summaries.

For trichloro(ethyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were obtained based on read-across for the analoque trichloro(methyl)silane (CAS No. 79-75-6) in accordance with OECD 111 test method (Dow Corning Corporation, 2001).

The hydrolysis products are ethylsilanetriol and hydrochloric acid.