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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: 19 h at pH 7 , 0.6 h at pH 4, 0.3 h at pH 9 and 20-25°C (QSAR)

Key value for chemical safety assessment

Additional information

A half-life value of approximately 19 h at 20-25°C and pH 7 was obtained using an accepted validated QSAR method (Peter Fisk Associates 2012a). The result is considered to be reliable and has been assigned as key study.

A QSAR that is currently being developed (Peter Fisk Associates 2012c) predicts half-lives at 20-25°C of 0.6 h at pH 4, 0.6 h at pH 5 and 0.3 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-3 and 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 3-(triethoxysilyl)propanethiol at pH 2 is therefore 0.006 hours (approximately 22 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) * e(0.08.(T-X))

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

Thus, for 3-(triethoxysilyl)propanethiol the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 7 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half-life is 0.0022 h (8 seconds).

The hydrolysis products are 3-(trihydroxysilyl)propanethiol and ethanol.

Hydrolysis of the read-across substance 3 -trimethoxysilylpropane-1 -thiol (CAS 4420 -74 -0)

Data for the substance3-trimethoxysilylpropane-1-thiol (CAS 4420-74-0)are read-across to the submission substance [3-(triethoxysilyl)propanethiol] for appropriate endpoints (see Section 1.4).The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For 3-trimethoxysilylpropane-1-thiol, hydrolysis half-lives at 25°C of 0.2 h at pH 4, 2.6 h at pH 7 and 0.1 h at pH 9 were determined using a validated QSAR estimation method.

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-life of 0.002 h (approximately 7 seconds) at pH 2 and 25°C, and 0.96 h at pH 7 and 37.5°C. 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 the substance at pH 2 and 37.5°C is approximately 5 seconds.