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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis: 0.04 h at pH 4, 0.1 h at pH 7 and 0.004 h at pH 9 and 20-25°C (QSAR)

Key value for chemical safety assessment

Additional information

Hydrolysis half-lives at 20 -25°C of 0.04 h at pH 4, 0.1 h at pH 7 and 0.004 h at pH 9 were determined for the substance using a validated QSAR estimation method. Also, in a preliminary study conducted in accordance with OECD 111, hydrolysis half-lives of <10 minutes at pH 4, <2.4 h at pH 7 and <10 minutes at pH 9 and 50°C were determined for the substance. Both results were considered to be reliable and used as weight of evidence. A half-life of 0.24 h at pH 7 and 23°C, from a study of non-assignable reliability adds further weight of evidence.

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-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 the substance at pH 2 is therefore less than 2 seconds. However, it is not appropriate or necessary to attempt to predict accurately 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) * e(0.08.(T-X))

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

Thus, for trimethoxy(vinyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 2 minutes. 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 hydrolysis products in this case are vinylsilanetriol and methanol.

The hydrolysis data for substances used in this dossier for read-across purposes for other endpoints are now discussed.

Hydrolysis of the read-across substance trichloro(vinyl)silane (CAS Number: 75-94-5)

Data for the substance trichloro(vinyl)silane (CAS No. 75-94-5) are read-across to the submission substance trimethoxy(vinyl)silane for appropriate endpoints (see Section 1.5 of the CSR). The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trichloro(vinyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were read-across from trichloro(methyl)silane in accordance with OECD Test Guideline 111 (Dow Corning Corporation 2001).

The hydrolysis products are vinylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance diethoxy(dimethyl)silane (CAS Number: 78-62-6)

Data for the substance diethoxy(dimethyl)silane (CAS No. 78-62-6) are read-across to the submission substance trimethoxy(vinyl)silane for appropriate endpoints (see Section 1.5 of the CSR). The rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For diethoxy(dimethyl)silane, hydrolysis half-lives at 20-25°C of 0.3 hours at pH 4, 5.5 hours at pH 7 and 0.3 hours at pH 9 were determined using a validated QSAR estimation method.

The hydrolysis products are dimethylsilanediol and ethanol.