Dimethoxymethylvinylsilane

Hydrolysis

Description of key information

Hydrolysis half-lives: <<2 min at pH 4, 18.3 min at pH 7 and 0.7 min at pH 9 and 25°C (EC Method C.7)

Key value for chemical safety assessment

Half-life for hydrolysis:

18.3 min

at the temperature of:

25 °C

Additional information

Two reliable hydrolysis rates are available for the substance. Hydrolysis half-lives at 20-25°C of 0.1 h at pH 4, 0.1 h at pH 5, 0.3 h at pH 7 and 0.01 h at pH 9 were determined for the substance using validated QSAR estimation method. Similarly, hydrolysis half-lives of <<2 minutes at pH 4, 18.3 minutes at pH 7 and 0.7 minutes at pH 9 and 25°C were determined in a non-guideline study (screening method based on ¹H-NMR) conducted in accordance to a generally accepted scientific principle and in compliance with a known quality system. The available data are considered to be reliable and used as weight of evidence.

As the hydrolysis reaction may be acid or base-catalysed, the rate of reaction is expected to be slowest at around pH 7 and increases 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 the substance at pH 2 and 25°C is 0.001 h (approximately 4 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 2 and 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°C) * e^(0.08.(T-X))

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

Thus, for dimethoxymethylvinylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 0.1 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 methylvinylsilanediol and methanol.