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EC number: 208-765-4 | CAS number: 541-05-9
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Hydrolysis
Administrative data
Link to relevant study record(s)
Description of key information
Hydrolysis half-life (hexamethylcyclotrisiloxane): 23 min at pH 7, 2 min at pH 4, 0.4 min at pH 9 and 25°C
Key value for chemical safety assessment
Additional information
Hydrolysis half-lives and rate constants at pH 4, 7 and 9 and 10, 25 and 35°C were determined in a reliable study conducted according to OECD 111, and in compliance with GLP (Dow Corning Corporation, 2004). Hydrolysis half-lives of 2 minutes at pH 4 and 25°C, 23 minutes at pH 7 and 25°C, and 0.4 minutes at pH 9 and 25°C were determined.
Hexamethylcyclotrisiloxane (D3) is a cyclic siloxane with three silicon atoms connected by three oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. The stated half-lives are for removal of the registration substance due to hydrolysis. The products of this reaction are also unstable in water, and so further hydrolysis reactions will follow; the initial hydrolysis product is 1,1,3,3,5,5-hexamethyltrisiloxane-1,5-diol (CAS 3663-50 -1; L3-diol), with the final product being dimethylsilanediol (CAS 1066-42-8).
The supporting study (Spivack, 1994) determined that L3-diol hydrolyses to an equilibrium mixture in which dimethylsilanediol dominates. When dimethysilanediol, tetramethyldisiloxanediol (L2-diol) or L3-diol were dissolved in water at concentrations in the range of 1000 to 3000 ppm, a slow convergence over several days to an equilibrium position dominated by dimethylsilanediol was observed.
The following mechanism is suggested for the hydrolysis of D3:
|
k1 |
|
k2 |
|
k3 |
|
|
D3 |
↔ |
HOSi(Me2)(OSiMe2)2OH L3-diol |
↔ |
HOSi(Me2)(OSiMe2)OH L2-diol |
↔ |
HOSi(Me2)OH DMSD |
. |
Information on the hydrolysis rate of the intermediate hydrolysis products of D3 (L3-diol and L2-diol) may be read-across from studies with octamethylcyclotetrasiloxane (D4) and decamethyltetrasiloxane (L4).
Half-lives for the hydrolysis reaction intermediates of the cyclic siloxane octamethylcyclotetrasiloxane (D4; CAS 556-67-2) were measured using radio-HPLC (Dow Corning Corporation 2005). The following mechanism was proposed for hydrolysis:
|
k1and k2 |
|
k3 |
|
k4 |
|
k5 |
|
D4 |
↔ |
HO Si(Me2)(OSiMe2)3 OH L4 -diol |
→ |
HO Si(Me2)(OSiMe2)2 OH L3 -diol |
→ |
HOSi(Me2)(OSiMe2)OH L2 -diol |
→ |
HOSi(Me2)OH DMSD |
The first hydrolysis step is reversible; however the extent of reversibility under the dilute conditions of this study is minor. Non-linear regression analysis of the data was performed and the following rate constants at pH 7 and pH 9 for the individual reactions were estimated:
pH 7 k1=8E-03 h-1, k2= 1E-03 h-1, k3= 4E-03 h-1
pH 9 k1=0.9 h-1, k2= 0.1 h-1, k3= 0.4 h-1, k4= 0.3 h-1, k5= 0.4 h-1.
Using the rate constant k3 at pH 7, a half-life value of 173 h at pH 7 and 25°C is calculated for octamethyltetrasiloxanediol (L4-diol).
The half-life for the hydrolysis of L3-diol and L2-diol at pH 7 and 25°C may be calculated from the rate constants k4 and k5 at pH 9 using the following algorithm to convert the rate at pH 9 to a rate at pH 7 (see attached QMRF for further details):
Log(T1/2(pH9))= -1.63 + 0.84 * Log(T1/2(pH7))
Half-life values of 240 h and 170 h at pH 7 and 25°C are calculated for the hydrolysis of L3-diol and L2-diol respectively. (Using the same algorithm to convert the half-life derived from the rate constant k3 at pH 9 results in a half-life of 144 h for the hydrolysis of L4 -diol at pH 7, which is in reasonable agreement with the measured half-life of 173 h).
For the hydrolysis of decamethyltetrasiloxane (L4; CAS 141-62-8), the following mechanism was proposed (Dow Corning Corporation, 2009):
k1 |
k2 |
k3 |
|
|||
Me3Si(OSiMe2)2OSiMe3 L4 |
→ |
Me3Si(OSiMe2)2OH |
→ |
HOSi(Me2)(OSiMe2)OH L2 -diol |
→ |
HOSi(Me2)OH DMSD |
Non-linear regression analysis of the data was performed, and the following rate constants at pH 7 and 10°C, 25°C and 35°C for the individual reactions were estimated:
10°C k1= 1.8E-04 h-1, 25°C k1= 9.5E-04 h-1, 35°C k1= 3.2E-03 h-1
10°C k2= 2.3E-04 h-1, 25°C k2= 1.9E-03 h-1, 35°C k2= 5.2E-03 h-1
10°C k3= 5.3E-04 h-1, 25°C k3= 9.2E-03 h-1, 35°C k3= 27E-03 h-1
Using the rate constant k3 and 25°C, a hydrolysis half-life of 75 h at pH 7 and 25°C is calculated for tetramethyldisiloxanediol (L2-diol).
From the evidence collated above, using a precautionary basis the half-life for the hydrolysis of the L2-diol and L3-diol is approximately 200 h at pH 7 and 25°C. Using the algorithms in Peter Fisk Associates (2014) to convert the rate at pH 7 to a rate at pH 9 and pH 4, half-lives for hydrolysis of L2-diol and L3-diol are expected to be approximately 2 h at pH 4 and 25°C and approximately 2 h at pH 9 and 25°C.
Hydrolysis under physiologically relevant conditions
The hydrolysis half-life of D3 at 35ºC and pH 7 (relevant for lungs and blood) is approximately 8.5 minutes. At 35°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 4 (~45 seconds) and pH 7 (~8.5 minutes).
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 D3 at pH 2 and 35°C (relevant for conditions in the stomach following oral exposure) is therefore ≤5 seconds.
References:
Peter Fisk Associates (2014). Prediction of hydrolysis for Si-containing compounds at pH 4, 5 and 9. QSAR Model Reporting Format. Report no.: PFA.413.002.013. Owner company: Reconsile. Report date: 2014-04-15.
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