Diacetoxydi-tert-butoxysilane

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

According to OECD Guideline 111. GLP study.

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Deviations:

yes

Remarks:

(test concentration 0.01 mol/L, see justification in section "Details on test conditions" )

Qualifier:

according to guideline

Guideline:

EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)

Deviations:

yes

Remarks:

(test concentration 0.01 mol/L, see justification in section "Details on test conditions" )

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Details on sampling:

MAIN TEST (NMR Spectrometry)
- Sampling intervals for the parent/transformation products: Measurements were performed 150 seconds after the initial contact of the test item with the aqueous phases. Number of scans were set to 16 (total determination time of approx. 180 seconds).
- Sampling method: Sampling was optimized for shortest sample preparation times to be able to start the measurements as quickly after the first contact of the test item with water as possible. In general first the test item was weighted exactly (to the nearest of 0.1 mg) into dry NMR tubes. Then the needed volumes of the previously prepared buffer solutions in D2O were measured in disposable 1 mL syringes and added in one portion into the NMR tubes. The tubes were then closed with a plastic cap and mixed thoroughly. Afterwards the tubes were placed into the spinner and inserted into the NRM spectrometer by automation. The average time from the initial contact of the test item with water until the beginning of the measurements was determined to be 150 seconds.
STEP WISE HYDROLYSIS (in acetone-d6)
A series of sample of the test item in acetone-d6 were prepared and increasing amounts of the 10 mg/g D2O stock solution in acetone-d6 were added. This addition of defined amounts of water led to further gradual hydrolysis resulting in a further decrease in the unhydrolysed acetoxy signal and a further increase in the signal of free acetic acid.

Buffers:

- pH: 1.2, 4.0, 7.0 and 9.0
- Composition of buffer:
pH 1.2: 19.06 mg of potassium chloride and 10.0 mg of 3-(trimethylsilyl)-propionic-2,2,3,3-d4 acid sodium salt were dissolved in 5 mL of deuterium oxide. The pH of the buffer was adjusted to 1.2 with deuterium chloride.
pH 4.0: 68.53 mg of potassium phosphate monobasic and 10.3 mg of 3-(trimethylsilyl)-propionic-2,2,3,3-d4 acid sodium salt were dissolved in 5 mL of deuterium oxide. The pH of the buffer was adjusted to 4.0 with deuterium chloride.
pH 7.0: 136.71 mg of potassium phosphate monobasic and 20.4 mg of 3-(trimethylsilyl)-propionic-2,2,3,3-d4 acid sodium salt were dissolved in 10 mL of deuterium oxide. The pH of the buffer was adjusted to 7.0 with deuterium chloride.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: NMR tubes and standard laboratory glassware for stock solution preparation.
- Sterilisation method: Due to the fast hydrolysis rates of the test item an biotic degradation does not have to be prevented by any special procedures. Anyhow only brand new NMR tubes were used and all other glassware and materials which were in contact with the buffering and measuring solutions were cleaned thoroughly in a laboratory cleaning machine at 80°C.
TEST MEDIUM
- Volume used/treatment: 1 mL.
- Preparation of test medium:
Main test (NMR spectrometry): Approximately 25 mg of test item was weighted (to the nearest of 0.1 mg) exactly into dry NMR tubes. Then the needed volumes of the previously prepared buffer solutions in D2O were measured to give concentrations of 0.1 mol/L in disposable 1 mL syringes.
Step wise hydrolysis (in acetone-d6): A series of sample weights containing 24.4 g to 25.5 g of the test item in acetone-d6 were prepared and increasing amounts of the 10 mg/g D2O stock solution in acetone-d6 were added. The amount of pure acetone-d6 for dissolving the test item was chosen individually for each sample to give a total volume of 1 mL (including the acetone-d6 from the D2O stock solutions). The added amounts of water were in the range of 20.2 mg to 895.5 mg of the D2O stock solution per NMR tube, corresponding to 0.226 mg to 10.012 mg of pure water per tube.
- Identity and concentration of co-solvent: Deuterium oxide and acetone-d6.
TEST CONCENTRATION (MAIN TEST): 0.1 mol/L.
According to the test guideline the maximum concentration of the test item should not exceed 0.01 mol/L (= 2.48 g/L for the test item). NMR spectroscopy is an analytical method that is optimized for the determination of chemical structures, not for trace analysis of substances. Therefore the sensitivity of NMR spectroscopy is much lower than the sensitivity of e. g. HPLC-UV methods. Good signal intensities can be reached with concentrations above 0.01 mol/L as the minimum concentrations. Due to the expected fast hydrolysis rates concentrations of the test item should be chosen high enough to be able to quantify at least remaining 10% of the initial concentration. Therefore to be able to reach an acceptable sensitivity of the NMR method the sample weights have to be increased compared to the values given in the OECD guideline 111. Approximately 25 mg of the test item have to be weighed into 1 mL of the NMR solvents to give concentrations of 0.1 mol/L which is 10 fold the concentration given in the guideline. A reduction of concentrations is technically not possible.

Duration:

3 min

Initial conc. measured:

0.1 mol/L

Number of replicates:

1 replicate.

Positive controls:

no

Negative controls:

no

Preliminary study:

The preliminary test was not performed due to the known instability of the test item.

Transformation products:

yes

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

Step wise hydrolysis of the test item in acetone-d6:
The mechanism of hydrolysis was analyzed by visualizing the signal intensities of the unhydrolysed acetoxy signal and the signal of free acetic acid versus the molar amount of additional water. The areas of these two signals correlated in a reciprocal way. The reduction of unhydrolysed test item caused an increase in free acetic acid concentration. By this experiment it was proved, that the test item reacts to form free acetic acid as soon as it gets in contact with water. It could also be seen that the increase in free acetic acid concentration was not linear compared to the added amount of water. What could be observed at low amounts of water added was an increase in the free acetic acid concentration that even was higher than the molar amount of water added. Hence one molecule of water seems to be able to hydrolyse more than one acetoxy group of the test item. This observation is only explainable by arguing with the formation of dimers or even oligomers of the hydrolysis products (silanols). A hydrolysed free silanol (R3Si-OH) contains an available free alcohol functionality which itself can lead to hydrolysis of another unhydrolysed acetoxy group. With increasing amounts of hydrolysed acetoxy groups the probability of further intermolecular hydrolysis reactions decreases. This experiment proved the stated mechanism of hydrolysis – with free acetic acid as the main hydrolysis product – and it also showed the presence as well as the fast kinetics of polycondensation reactions of the forming silanols.

Key result

pH:

1.2

Temp.:

22 °C

DT50:

< 0.625 min

Key result

pH:

4

Temp.:

22 °C

DT50:

< 0.625 min

Key result

pH:

7

Temp.:

22 °C

DT50:

< 0.625 min

Key result

pH:

9

Temp.:

22 °C

DT50:

< 0.625 min

Other kinetic parameters:

Due to the extremely fast hydrolysis reaction of propyltriacetoxysilane, a detailed determination of rate constants and half life times is technically not possible.

Details on results:

MAIN TEST (NMR spetrometry):
TEST CONDITIONS
- pH: 1.2, 4, 7, 9
- Temperature: 22 ± 2 ºC

MAJOR TRANSFORMATION PRODUCTS
Independent from the pH of the buffering solutions the samples showed clear spectra with 100% of free acetic acid (integrals of 8.9679 to 9.0092) and none of the unhydrolysed test item remaining. In analogy to the tests in acetone-d6 (see section "details on hydrolysis and appearance of transformation products"), the unhydrolysed test item must have given a signal with a different chemical shift than the signal of free acetic acid. Due to the fast hydrolysis this signal could never be observed. Therefore hydrolysis was finished – independent from pH – at least 150 seconds after the initial contact of the test item with water. Assuming that a minimum of four half life cycles were completed during the time from the initial contact of the test item with water until the beginning of the measurement, the maximum half life time of the test item would be 150 sec / 4 = 37.5 sec.

PATHWAYS OF HYDROLYSIS
- Description of pathways: The step wise hydrolysis of the test item in acetone-d6 proved the stated mechanism of hydrolysis – with free acetic acid as the main hydrolysis product – and it also showed the presence as well as the fast kinetics of polycondensation reactions of the forming silanols.
- Figures of chemical structures attached: No

Main test (NMR spectrometry):

The final results of the determination of hydrolysis rates are given in the following table:

 

pH value	Estimated half life (mean 22°C)
1.2	< 37.5 seconds
4.0	< 37.5 seconds
7.0	< 37.5 seconds
9.0	< 37.5 seconds

Step wise hydrolysis of the test item in acetone-d6:

Signal areas of the test item and acetic acid in the step wise hydrolysis:

Peak Area Test Item	Peak Area Free Acetic acid	Mol% of water
6.5391	1.8949	0
5.6321	2.4678	2.86
5.1779	2.8543	5.9
4.5423	3.7352	9.32
4.1191	4.3717	12.67
2.8151	5.7787	25.71
1.8272	7.0574	40.94
1.2529	8.027	59.15
0	8.9155	124.11

Validity criteria fulfilled:

yes

Conclusions:

The half-life of test item propyltriacetoxysilane was determined to be < 37.5 seconds at pH 1.2, 4, 7 and 9. The stated mechanism of hydrolysis of propyltriacetoxysilane was free acetic acid as the main hydrolysis product and the fast kinetic polycondensation reaction of the forming silanols (trihydroxypropylsilane).

Executive summary:

The abiotic degradation of the test item propyltriacetoxysilane was determined according to OECD-Guideline 111 and in accordance with European Commission Council Regulation (EC) No 440/2008, Annex, Part C, method C.7. Due to the fast kinetics of the hydrolysis of the test item, half life times and hydrolysis rate constants could not be determined properly with common analytical methods.

While the kinetics of the hydrolysis reaction was too fast to be observed, the mechanism of the reaction could be proved by a stepwise NMR experiment. For this experiment, increasing amounts of water (D2O) were added to solutions of the test item in a dry organic solvent (acetone-d6). The stepwise hydrolysis with formation of acetic acid as the main hydrolysis product could be shown by this set up. It could also be shown the presence as well as the fast kinetics of polycondensation reactions of the forming silanols. In addition to this, the degradation in buffered aqueous solutions at different pH values (1.2, 4, 7 and 9) was investigated. All tests were performed at a temperature of 22°C (± 2°C) using 1H-NMR experiments. Measurements were performed 150 seconds after the initial contact of the test item with the aqueous phases. Independent from pH, after a reaction time of 150 seconds, none of the initially present test item could be measured in the aqueous solutions, thus total hydrolysis is completed at least 150 seconds after the initial contact of the test item with water. With respect to the detection limits of the 1H-NMR method, a minimum of four completed half life cycles can be estimated for the test item in a reaction time of 150 seconds. Therefore a maximum half life time of <37.5 seconds can be estimated for test item propyltriacetoxysilane. The half life times of the test item – with respect to the minimum time scale of the analysis – are independent from pH.

Reference 2

Endpoint:

hydrolysis

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH
The analogue Propyltriacetoxysilane which shares the same functional group with Diacetoxydi-tert-butoxysilane, also has comparable values for the relevant molecular properties for abiotic degradation.
See attached the reporting format.

Reason / purpose for cross-reference:

read-across source

Preliminary study:

The preliminary test on analogue propyltriacetoxysilane was not performed due to the known instability of the test item.

Transformation products:

yes

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

Based on the experimental results with analogue propyltriacetoxysilane and applying the read-across approach, it was determined that diacetoxydi-tert-butoxysilane hydrolyses into free acetic acid as the main hydrolysis product and silanols with fast kinetics of polycondensation reactions.

Key result

pH:

1.2

Temp.:

22 °C

DT50:

< 0.625 min

Remarks on result:

other: (results obtained with analogue substance propyltriacetoxysilane)

Key result

pH:

4

Temp.:

22 °C

DT50:

< 0.625 min

Remarks on result:

other: (results obtained with analogue substance propyltriacetoxysilane)

Key result

pH:

7

Temp.:

22 °C

DT50:

< 0.625 min

Remarks on result:

other: (results obtained with analogue substance propyltriacetoxysilane)

Key result

pH:

9

Temp.:

22 °C

DT50:

< 0.625 min

Remarks on result:

other: (results obtained with analogue substance propyltriacetoxysilane)

Other kinetic parameters:

Due to the extremely fast hydrolysis reaction of analogue propyltriacetoxysilane, a detailed determination of rate constants and half life times was technically not possible.

Details on results:

Based on experimental results obtained with analogue propyltriacetoxysilane where hydrolysis was finished at least 150 seconds after the initial contact of the test item with water and the maximum half life time of test item was determined to be 37.5 seconds at pH 1.2, 4, 7 and 9, the read-across approach was applied and diacetoxydi-tert-butoxysilane was determined to be hydrolytically unstable, with a fast hydrolysis and low half-life regardless of pH.

Validity criteria fulfilled:

not applicable

Conclusions:

Based on the read-across approach from experimental results on analogue propyltriacetoxysilane, diacetoxydi-tert-butoxysilane was determined to be hydrolytically unstable, with a fast hydrolysis (very low half-life time) into acetic acid and silanols regardless of pH.

Executive summary:

The abiotic degradation of analogue substance propyltriacetoxysilane was performed according to OECD Guideline 111. The stepwise hydrolysis with formation of acetic acid as the main hydrolysis product and the presence as well as the fast kinetics of polycondensation reactions of the forming silanols (trihydroxypropylsilane) could be observed. The hydrolysis of analogue propyltriacetoxysilane was completed at least 150 seconds at pH 1.2, 4, 7 and 9 after the initial contact of the test item with water. A minimum of four completed half life cycles were estimated for the test item in a reaction time of 150 seconds and therefore the maximum half life time of <37.5 seconds was estimated for propyltriacetoxysilane. Based on these results, the read-across approach was applied and diacetoxydi-tert-butoxysilane is determined to be rapidly hydrolyzed into acetic acid and corresponding silanols in contact with water, resulting on a very low half-life time, regardless of pH.

Description of key information

Key study: Based on read-across approach from experimental results (OECD Guideline 111 and GLP) on analogue propyltriacetoxysiloxane,  diacetoxydi-tert-butoxysilane was determined to be hydrolytically unstable, with a fast hydrolysis (very low half-life time) into acetic acid and the corresponding silanols, regardless of pH.

Additional information

Key study: Read-across from experimental results on analogue propyltriacetoxysilane.

An abiotic degradation study was performed on propyltriacetoxysilane according to OECD Guideline 111 (GLP study). The half-life at pH 1.2, 4, 7 and 9 of propyltriacetoxysilane was determined to be < 37.5 seconds since the test item was completely hydrolysed at 150 seconds after the initial contact of the test item with water and four completed half life cycles could be estimated in a reaction time of 150 seconds. The stated mechanism of hydrolysis of propyltriacetoxysilane was free acetic acid as the main hydrolysis product and the fast kinetic polycondensation reaction of the forming silanols (trihydroxypropylsilane). Based on these results, the read-across approach was applied and Diacetoxydi-tert-butoxysilane was determined to rapidly hydrolyzed into acetic acid and corresponding silanols in contact with water, resulting on a very low half-life time, regardless of pH.