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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: 3.8 hours at pH 7 and 25°C

Key value for chemical safety assessment

Additional information

A reliable study according to OECD 111 is available for the substance

(3-chloropropyl)diethoxymethylsilane

. This substance has a half-life of 3.8 hours at pH 7 and 25°C. Additional information about this endpoint could be gathered through QSAR calculation.

A half-life of 9.5 hours for hydrolysis of (3-chloropropyl)diethoxymethylsilane was obtained using an appropriate calculation method. The result is considered to be reliable.

The initial hydrolysis products are (3-chloropropyl)methylsilanediol and ethanol. Silanols may undergo condensation reactions to form siloxane dimers, oligomers and polymers. The precise rate of this condensation and the species present are dependent on conditions.

 

A QSAR that is currently being developed (Peter Fisk Associates 2012) predicts half-lives at 20-25°C of 0.4 h at pH 4, 0.4 h at pH 5 and 0.2 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 uncatalysed 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+]=10-4mol dm-3and at pH2 [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

As a worst-case it can therefore be considered that the half-life for (3-chloropropyl)diethoxymethylsilane 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 (3-chloropropyl)diethoxymethylsilane the hydrolysis half-life 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 initial hydrolysis products are (3-chloropropyl)methylsilanediol and ethanol.