## Registration Dossier

Environmental fate & pathways

## Hydrolysis

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### Description of key information

Hydrolysis half life: 2.6 hours at 20-25°C

### Key value for chemical safety assessment

Half-life for hydrolysis:
2.6 h
at the temperature of:
20 °C

A hydrolysis half-life of 2.6 hours was obtained for the substance using an appropriate calculation method. The result is considered to be reliable.

A QSAR that is currently being developed (Peter Fisk Associates, 2012) predicts half-lives at 20-25°C of 0.2 h at pH 4, 0.3 h at pH 5 and 0.1 h at pH 9.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.

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.

Therefore, at low pH:

kobs≈kH3O+[H3O+]

At pH 4 [H3O+]=10-4 mol dm-3and at pH 2 [H3O+]=10-2 mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at 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 trimethoxy(propyl)silane at pH 2 is therefore 0.002 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 trimethoxy(propyl)silane 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.

For trimethoxy(propyl)silane the hydrolysis half-life 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 initial hydrolysis products are propylsilanetriol and methanol.