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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis: half-life approximately. 1 h at 25°C and pH7, ≤15 minutes at pH 5 and <1 h at pH 9 (analogue read-across, supported by prediction). 

Key value for chemical safety assessment

Additional information

No hydrolysis study is available for the submission substance. However, a reliable study according to EPA guideline N 161-1 (Hydrolysis) is available for the related substance3-chloropropyltrimethoxysilane (CAS 2530-87-2). A measured half-life value of 53 minutes (0.88 h) at pH 7, 14.6 minutes (0.24 h) at pH 5, and 22.4 minutes (0.37 h) at pH 9 and 25°C was determined for the substance in accordance with a relevant test method. The result is considered to be reliable and has been assigned as key study.

3-chloropropyltrimethoxysilane is a structural analogue of (3-chloropropyl)dimethoxymethylsilane in which a Si-OCH3group is present rather than the Si-CH3group. On the basis of the proposed mechanisms of the hydrolysis reaction, the dialkoxysilane would be expected to hydrolyse more rapidly than the trialkoxysilane at acid pH and more slowly at alkaline pH (Fisk, 2009). At pH 7, both acid- and alkali-catalysed hydrolysis may occur and it is difficult to predict a precise value of the half-life for (3-chloropropyl)dimethoxymethylsilane based on the data for 3-chloropropyltrimethoxysilane. However, based on consideration of the available data for the category of alkoxysilanes (Fisk, 2009), half-lives at pH 7 for the two substances are expected to be of the same order of magnitude. Therefore, the half-life of the substance (3-chloropropyl)dimethoxymethylsilane is estimated to be approximately 1 hour. The half-life at pH 4 is estimate to be ≤15 minutes and the half-life at pH 9 is estimated to be between 20 minutes and <1 hour.

A half-life value of approximately 1.3 h at 20-25°C and pH 7 was obtained using an accepted validated QSAR method (Peter Fisk Associates 2012a).

A QSAR that is currently being developed (Peter Fisk Associates 2012c) predicts half-lives at 20-25°C of 0.1 h at pH 4, 0.2 h at pH 5 and 0.03 h at pH 9 for the submission substance.

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.

 

Therefore, at low pH:

kobs≈kH3O+[H3O+]  

At pH 4 [H3O+]=10-4mol dm-3and at pH 2 [H3O+]=10-2mol 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-chloropropyl)dimethoxymethylsilane at pH 2 is therefore 0.001 hours (3.6 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 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 (3-chloropropyl)dimethoxymethylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 0.37 hours (1330 seconds). 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 products of hydrolysis are (3-chloropropyl)methylsilanediol and methanol.

Hydrolysis of the read-across substance dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1)

Data for the substance dichloro(3-chloropropyl)methylsilane(CAS 7787-93-1) are read-across to the submission substance (3-chloropropyl)dimethoxymethylsilane 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 dichloro(3-chloropropyl)methylsilane, hydrolysis half-lives at 25°C of ≤15 minutes at pH 5, 1 h at pH 7 and <1 h at pH 9 were determined using a relevant method.

Hydrolysis of the read-across substance (3-chloropropyl)diethoxymethylsilane (CAS 13501-76-3)

Data for the substance(3-chloropropyl)diethoxymethylsilane(CAS 13501-76-3) are read-across to the submission substance (3-chloropropyl)dimethoxymethylsilane 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-chloropropyl)diethoxymethylsilane, hydrolysis half-lives at 25°C of 0.4 h at pH 4, 9.5 h at pH 7 and 0.2 h at pH 9 were determined using a validated QSAR 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 0.004 h (14 seconds) at pH 2 and 25°C, and 3.5 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.