Registration Dossier

Environmental fate & pathways

Hydrolysis

Currently viewing:

Administrative data

Link to relevant study record(s)

Reference
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
See attached QMRFs/QPRFs
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details).

The model for hydrolysis at pH 7 has been developed for, and applies specifically to di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and apply specifically to organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
Key result
pH:
4
DT50:
0.2 h
Remarks on result:
other: 20-25°C
Key result
pH:
5
DT50:
0.3 h
Remarks on result:
other: 20-25°C
Key result
pH:
7
DT50:
4 h
Remarks on result:
other: 20-25°C
Key result
pH:
9
DT50:
0.1 h
Remarks on result:
other: 20-25°C
Conclusions:
Hydrolysis half-life values at 20-25°C of 0.2 h at pH 4, 4 h at pH 7 and 0.1 h at pH 9 were obtained using an accepted calculation method. The result is considered to be reliable.

Description of key information

Hydrolysis: half-life at 20-25°C approximately 4 h at pH 7, 0.2 h at pH 4 and 0.1 h at pH 9 (QSAR)

Key value for chemical safety assessment

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

Additional information

Hydrolysis half-lives of 0.2 h at pH 4, 0.3 h at pH 5, 4 h at pH 7 and 0.1 h at pH 9 and 20-25°C were obtained using an accepted validated QSAR method. The results are considered to be reliable and has been assigned as key study.

 

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 pH 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 trimethoxy(octyl)silane at pH 2 is therefore 0.002 hours (0.12 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 for trimethoxy(octyl)silane 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 trimethoxy(octyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 1.5 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.

 

The hydrolysis products for the registered substance are octylsilanetriol and methanol.

 

The hydrolysis half-lives of the read-across substances used in other areas are discussed below.

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 trimethoxy(octyl)silane for the following endpoints; short-term toxicity to fish and long-term toxicity to aquatic invertebrates. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

 

For trichloro(2,4,4-trimethylpentyl)silane, hydrolysis half-lives have been read-across from an analogue trichlorosilane, trichloro(methyl)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 and hydrochloric acid.

 

Hydrolysis of the read-across substance trichloro(phenyl)silane (CAS 98-13-5)

Data for the substance trichloro(phenyl)silane (CAS 98-13-5) are read-across to the submission substance trimethoxy(octyl)silane for the following endpoint; toxicity to aquatic algae. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate section for the endpoint.

 

For trichloro(phenyl)silane, hydrolysis half-lives have been read-across from an analogue trichlorosilane, trichloro(methyl)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 phenylsilanetriol and hydrochloric acid.

 

Hydrolysis of the read-across substance triethoxy(octyl)silane (CAS 2943-75-1)

Data for the substance triethoxy(octyl)silane (CAS 2943-75-1) are read-across to the submission substance trimethoxy(octyl)silane for the following endpoints; biodegradation in water; short-term toxicity to fish; short-term toxicity to aquatic invertebrates; toxicity to algae; toxicity to microorganisms; repeated dose toxicity oral; mammalian cytogenicity; mammalian mutagenicity; toxicity to reproduction and developmental toxicity . The structural similarity, hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

 

For triethoxy(octyl)silane, 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, 0.0026 h (9 seconds) at pH 2 and 37.5°C and 11 h at pH 7 and 37.5°C.

 

The hydrolysis products are octylsilanetriol and ethanol.

 

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

Data for the substance trimethoxy(2,4,4-trimethylpentyl)silane (CAS 34396-03-7) are read-across to the submission substance trimethoxy(octyl)silane for the following endpoint; repeated dose toxicity inhalation. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate section for the 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 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 0.003 h (11 seconds) at pH 2 and 25°C and 2.1 h at pH 7 and 37.5°C. 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 for the substance at pH 2 and 37.5°C is approximately 5 seconds.

 

The hydrolysis products are (2,4,4-trimethylpentyl)silanetriol and methanol.

 

Hydrolysis of the read-across substance triethoxy(2,4,4-trimethylpentyl)silane (CAS 35435-21-3)

Data for the substance triethoxy(2,4,4 -trimethylpentyl)silane (CAS 35435-21-3) are read-across to the submission substance trimethoxy(octyl)silane for the following endpoints; skin sensitisation; repeated dose toxicity oral and developmental toxicity. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

 

For triethoxy(2,4,4-trimethylpentyl)silane, hydrolysis half-lives at 20-25°C of 0.9 h at pH 4, 42.5 h at pH 7 and 0.5 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 0.009 h (32 seconds) at pH 2 and 25°C, 0.003 h (11 seconds) at pH 2 and 37.5°C and 16 h at pH 7 and 37.5°C.

 

The hydrolysis products are (2,4,4-trimethylpentyl)silanetriol and ethanol.