Isobutyldimethoxymethylsilane

Hydrolysis

Description of key information

Hydrolysis half-life: 0.2 h at pH 4, 0.2 h at pH 5, 1.7 h at pH 7, 0.04 h at pH 9 and 20-25°C

Additional information

Hydrolysis half-lives at 20 - 25°C of 0.2 h at pH 4, 0.2 h at pH 5, 1.7 h at pH 7 and 0.04 h at pH 9 were determined for the substance using a validated QSAR estimation method. The result is considered to be reliable.

As the hydrolysis reaction may be acid or base-catalysed, 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 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 the substance at pH 2 is therefore less than 2 seconds at 25°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 of the substance at pH 2 and room temperature 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°C) * e^(0.08.(T-X))

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

Thus, for isobutyl(dimethoxy)methylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 0.6 hours. 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 temperature to the limit value and the hydrolysis half -life is therefore approximately 5 seconds.

The hydrolysis products in this case are isobutyl(methyl)silanediol and methanol.

Hydrolysis of the read-across substance isobutyl(trimethoxy)silane (CAS 18395-30-7)

Data for the substance isobutyl(trimethoxy)silane (CAS 18395-30-7) are read-across to the registration substance isobutyl(dimethoxy)methylsilane for appropriate endpoints (see the read-across justification report attached in Section 13). The silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate endpoint summaries.

Isobutyl(trimethoxy)silane hydrolyses rapidly, with calculated hydrolysis half-lives of 0.2 hours at pH 4, 4.1 hours at pH 7 and 0.1 hours at pH 9 and 25°C. At 37.5°C, the calculated half-lives are approximately 1.5 hours at pH 7, 5 seconds at pH 2 and 0.07 hours at pH 4.The products of hydrolysis are (2-methylpropyl)silanetriol and methanol.

Hydrolysis of the read-across substance (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3)

Data for the substance (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3) are read-across to the registration substance isobutyl(dimethoxy)methylsilane for appropriate endpoints (see the read-across justification report attached in Section 13). The silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate endpoint summaries.

(3 -Chloropropyl)diethoxymethylsilane hydrolyses rapidly, with calculated hydrolysis half-lives of 0.4 hours at pH 4, 9.5 hours at pH 7 and 0.2 hours at pH 9 and 25°C. At 37.5°C, the calculated half-lives are approximately 3.5 hours at pH 7, 5 seconds at pH 2 and 0.1 hours at pH 4. The products of hydrolysis are (3-chloropropyl)(methyl)silanediol and ethanol.