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Environmental fate & pathways

Hydrolysis

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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
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)
GLP compliance:
no
Transformation products:
yes
No.:
#1
No.:
#2
Key result
pH:
7
DT50:
5.5 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
pH:
4
DT50:
0.3 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
pH:
5
DT50:
0.3 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
pH:
9
DT50:
0.1 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
Conclusions:
A hydrolysis half life of approximately 5.5 h at 20-25°C and pH 7 was obtained using an accepted calculation method. The result is considered to be reliable.

Description of key information

Diethoxy(dimethyl)silane CAS 78-62-6: hydrolysis half-life = 5.5 h at 20-25 °C and pH7 (QSAR)

Key value for chemical safety assessment

Half-life for hydrolysis:
5.5 h

Additional information

Once released into the water phase, hydrolysis contributes significantly to the degradation of this substance. A QSAR that was developed (Peter Fisk Associates 2012) predicts half-lives at 20-25 °C of 0.3 h at pH 4, 0.3 h at pH 5, 5.5 h at pH 7 and 0.1 h at pH 9. 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 extreme pH values 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+]=10-4mol dm-3and at pH2 [H3O+]=10-2mol dm-3; therefore, kobsat pH 2 should be approximately 100 times greater than kobsat pH 4.

The half-life of a substance at pH 2 is calculated based on:

t1/2(pH 2) = t1/2(pH 4) / 100

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 diethoxy(dimethyl)silane CAS 78-62-6, the hydrolysis half-life at 37.5°C and pH 2 (relevant for oral exposure) is < 5 s.

The initial hydrolysis products are and dimethylsilanediol and ethanol. It can be concluded that the submission substance will hydrolyse rapidly under environmentally relevant conditions.

Reference:

Mabey W. & Mill T. (1978). Critical review of hydrolysis of organic compounds in water under environmental conditions. J Phys Chem Ref Data. 7(2): 383-415.