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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: <0.083 h at pH 4 and <0.1 h at pH 9 and 25°C. At pH 7, half-lives are 4.5 h at 20°C, 3.1 h a 25°C, 1 h at 40°C and 0.51 h at 50°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:
3.1 h
at the temperature of:
25 °C

Additional information

Hydrolysis half-lives at 25°C of <0.083 h (<5 minutes) at pH 4 and <0.1 h at pH 9 and 25°C. At pH 7, half-lives of 4.5 h at 20°C, 3.1 h a 25°C, 1 h at 40°C and 0.51 h at 50°Cwere measured for 3-chloropropyltrimethoxysilane in accordance with OECD 111 test method and in compliance with GLP. At pH 4 and pH 9, the hydrolysis half-lives were estimated at 25°C because the test substance was hydrolytically unstable. The result is considered to be reliable and selected as key study. In a non-standard study (Gorman 1995), the disappearance of the methoxy groups was monitored by the use of H-NMR; hydrolysis half-lives of 0.88 h at pH 7 and 0.37 h at pH 9 and 25°C was determined for 3-chloropropyltrimethoxysilane. In the supporting study, 3-chloropropyltrimethoxysilane was an impurity in another substance and the study was designed primarily to determine the half-life of the major constituent.

The key study was selected as:

- The study most recently conducted (Schaap 2014)

- The study with the highest reliability

- The study conducted according to a relevant test guideline (OECD 111)

The available data are in agreement that the substance undergoes rapid hydrolytic degradation.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around 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.

 

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 the substance at pH 2 and 25°C is therefore approximately 3 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 room temperature 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) * e(0.08.(T-X))

 

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

 

Thus, for 3-chloropropyltrimethoxysilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 1.1 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. At 37.5ºC and pH 5.5 (relevant for dermal exposure), the hydrolysis half -life will be in between the half-lives at pH 4 and pH 7 at 37.5ºC.

 

The hydrolysis products in this case are 3-chloropropylsilanetriol.and methanol.