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Hydrolysis half-life: ca.0.3 h at 20-25°C and pH 7 (QSAR)

Key value for chemical safety assessment

Additional information

A half-life value of approximately 0.3 h at 20-25°C and pH 7 was obtained using an accepted validated QSAR method (Peter Fisk Associates 2012a). A QSAR that was developed by Peter Fisk Associates (2012a,b) predicts half-lives at 20-25 °C of 0.1 h at pH 4 and 0 h at pH 9. Since the hydrolysis reaction could be acid or base catalysed, the rate of reaction is expected to be slowest at pH 7 and increases with an increase or decrease of the pH.

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.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[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. This is supported by studies for various organosilicon compounds, in which calculation of kH3O+ and kOH- from the experimental results at pH 4 and 9, respectively, resulted in reasonable estimates of the half-life at pH 7 (Peter Fisk Associates, 2012b).

Therefore, at low pH:

kobs≈kH3O+[H3O+]

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

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

t1/2(pH 2) = t1/2(pH 4) / 100

The calculated half-life of dimethoxymethylsilane 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. As a worst-case it can therefore be considered that the half-life for dimethoxymethylsilane 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) x e(0.08.(T-X))

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

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

The hydrolysis products are methylsilanediol then methylsilanetriol and methanol.