Registration Dossier

Environmental fate & pathways

Hydrolysis

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

Hydrolysis: half-life 0.9 h at pH 4, 43 h at pH 7 and 0.5 h at pH 9 and 20-25°C (QSAR)

Key value for chemical safety assessment

Additional information

There are no reliable measured hydrolysis data for the submission substance. The hydrolysis half-life of the substance has been predicted using a validated QSAR estimation method.

Hydrolysis half-lives of 0.9 h at pH 4, 43 h at pH 7 and 0.5 h at pH 9 and 20-25°C were predicted for the substance using a validated QSAR estimation method.

Hydrolysis reactions of alkoxysilanes can be catalysed by both acid and base. The rate of hydrolysis is slowest close to pH 7 and increases 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 triethoxy(2,4,4-trimethylpentyl)silane at pH 2 and 20 - 25°C is therefore 0.009 hours (approximately 32 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 triethoxy(2,4,4-trimethylpentyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 16 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half-life is 0.003 h (approximately 11 seconds). At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life will be between half-lives at pH 4 and pH 7 (0.2 - 16 hours).

The hydrolysis products for the submission substance are (2,4,4-trimethylpentyl)silanetriol (1 moles)and ethanol (3 moles).

A supporting study for the read-across substance methyltrimethoxysilane is presented. In this study, the hydrolysis of methyltrimethoxysilane was investigated under conditions designed to mimic the rat stomach after dosing the substance in corn oil. The half-life for disappearance of methyltrimethoxysilane applied in corn oil to gastric simulation buffer was 33 mins at pH 3 and 37°C and appears to be determined by phase transfer. The data suggest that, in the investigated system, hydrolysis occurs rapidly once methyltrimethoxysilane comes into contact with the aqueous layer and the rate determining step is the transfer of the methyltrimethoxysilane from the corn oil to the water. Combined recoveries of methyltrimethoxysilane and methanol (in mole equivalents of methyltrimethoxysilane; 3 moles methanol to 1 mole methyltrimethoxysilane assumed) were 87.5 to 104% and methanol content increased proportionally to the decrease in methyltrimethoxysilane. The study was conducted according to an appropriate test protocol and is considered reliable.

Methyltrimethoxysilane an dtriethoxy(2,4,4-trimethylpentyl)silaneare both trialkoxysilanes with an alkyl side-chain; the differences between the two substances are that the alkyl chain is C1 for methyltrimethoxysilane and C8 for triethoxy(2,4,4-trimethylpentyl)silaneand that the co-product of hydrolysis is methanol for methyltrimethoxysilane and ethanol for triethoxy(2,4,4-trimethylpentyl)silane. Therefore, it is not intended to read-across the half-life for methyltrimethoxysilane to triethoxy(2,4,4-trimethylpentyl)silane but to indicate the type of behaviour that might be observed fortriethoxy(2,4,4-trimethylpentyl)silane. Thus, in animal studies using corn oil as vehicle, triethoxy(2,4,4-trimethylpentyl)silane could potentially hydrolyse more slowly than its predicted half-life at pH 2 and 37°C would suggest. This effect might be more pronounced for substances with lower water solubility and higher log KOWthan methyltrimethoxysilane.  It is uncertain to what extent the in vitro test results are relevant for conditions present in the rat stomach (for example composition of the applied gastric simulant was highly simplified, type/speed of mixing is expected to strongly influence phase partitioning).

The hydrolysis of substances that are used for read-across in other endpoints is discussed below.

Hydrolysis of the read-across substance trimethoxy(2,4,4-trimethylpentyl)silane (CAS Number 34396-03-7)

Data for the substance trimethoxy(2,4,4-trimethylpentyl)silane (CAS No: 34396-03-7) are read-across to the submission substance, triethoxy(2,4,4-trimethylpentyl)silane for appropriate endpoints (see Section 1.4 of the CSR). 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 trimethoxy(2,4,4-trimethylpentyl)silane, hydrolysis half-lives at 20 - 25°C of 0.3 h at pH 4, 5.7 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 pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives half-life of 0.003 h (approximately 11 seconds) at pH 2 and 25°C. at pH 2 and 37.5°C, the half-life (relevant for conditions in the stomach), the half-life is 0.001 h (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 for the substance at pH 2 and 37.5°C is approximately 5 seconds.

 

At pH 7 and 37.5°C (relevant for lungs and blood), the half-life is approximately 2 hours.

 

The products of hydrolysis are (2,4,4-trimethylpentyl)silanetriol (1 mole) and methanol (3 moles).

 

Hydrolysis of the read-across substance trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4)

Data for the substance trichloro(2,4,4-trimethylpentyl)silane(CAS 18379-25-4) are read-across to the submission substance triethoxy(2,4,4-trimethylpentyl)silane 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 trichloro(2,4,4-trimethylpentyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2001).

 

The hydrolysis products are (2,4,4-trimethylpentyl)silanetriol (1 mole) and hydrochloric acid (3 moles).

 

Hydrolysis of the read-across substance triethoxyoctylsilane (CAS 2943-75-1)

Data for the substance triethoxyoctylsilane(CAS 2943-75-1) are read-across to the submission substance triethoxy(2,4,4-trimethylpentyl)silane for appropriate endpoints (see Section 1.4).The hydrolysis half-life and 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 triethoxyoctylsilane, hydrolysis half-lives at 20-25°C of 0.7 h at pH 4, 30 h at pH 7 and 0.4 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 0.007 h (25 seconds) at pH 2 and 25°C and 11 h at pH 7 and 37.5°C. At pH 2 and 37.5°C, the half-life is 0.0026 h (9 seconds).

 

The hydrolysis products are octylsilanetriol (1 mole) and ethanol (3 moles).

Hydrolysis of the read-across substance trimethoxyoctylsilane (CAS 3069 -40 -7)

Data for the substance trimethoxyoctylsilane (CAS 3069 -40 -7) are read-across to the submission substance triethoxy(2,4,4-trimethylpentyl)silane for appropriate endpoints (see CSR Section 1.4). The hydrolysis half-life and 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 trimethoxyoctylsilane, hydrolysis half-lives at 20-25°C of 0.2 h at pH 4, 4 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 (7 seconds) at pH 2 and 25°C and 1.5 h at pH 7 and 37.5°C. At pH 2 and 37.5°C, the half-life is 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 for the substance at pH 2 and 37.5°C is approximately 5 seconds.

 

The hydrolysis products are octylsilanetriol (1 mole) and methanol (3 moles).