1,1,1,3,3,3-hexamethyldisilazane

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2005-10-26 to 2006-05-05

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

GLP compliance:

yes

Analytical monitoring:

no

Details on sampling:

Not applicable.

Buffers:

Buffer solutions were prepared to known final volumes in polypropylene volumetric flasks with Milli-Q water. If necessary, final pH adjustments were made by dropwise addition of 1M lithium hydroxide solution. Prior to use, all buffer solutions were sparged with helium gas for a minimum of 5 min to exclude oxygen and were sterilized using a Nalgene sterile filtration unit with a 0.20 µM cellulose nitrate (CN) membrane.

- pH: Target: 4.0; Measured: 4.01
- Type and final molarity of buffer: Acetic Acid/Lithium Hydroxide, 0.05M
- Composition of buffer: 50mL 1 M Acetic Acid solution, 7.47 mL 1M Lithium Hydroxide solution. Total volume 1000 mL.

- pH: Target: 7.0; Measured: 7.01
- Type and final molarity of buffer: Citric acid/Lithium Hydroxide, 0.05M
- Composition of buffer: 50mL 1 M Citric acid solution, 140 mL 1M Lithium Hydroxide solution. Total volume 1000 mL.

- pH: Target: 9.0; Measured: 8.99
- Type and final molarity of buffer: Boric acid/Lithium hydroxide, 0.30M
- Composition of buffer: 125 mL 0.1 M boric acid solution, 6.8 mL 1M Lithium Hydroxide solution. Total volume 1000 mL.

Estimation method (if used):

Not applicable.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: The test vessels were uniquely labelled 100 mL clear glass beakers. Parafilm was used to maintain a closed system.
- Sterilisation method: Sterility tests were not conducted since they were deemed unnecessary due to the rapid hydrolysis of the test substance. Similarly, non-sterilized labware were used.
- Measures taken to avoid photolytic effects: Not applicable.
- Measures to exclude oxygen: Not applicable.
- Details on test procedure for unstable compounds: The test substance is unstable with respect to moisture. To prevent exposure of the test substance to moisture in air, the test substance solution was prepared in a nitrogen purged glove bag. The test substance solution was contained in 2 x 20 mL glass vials having septum lined open-top caps. The test substance containers were stored over Drierite in a secondary air tight container in a freezer when not in use.
- Details of traps for volatile, if any: None
- If no traps were used, is the test system closed/open: Closed
- Is there any indication of the test material adsorbing to the walls of the test apparatus? No

TEST MEDIUM
- Volume used/treatment: 0.45 mL of the test substance solution in THF was added to 50 mL of the buffer solution at the start of the hydrolysis test.
- Kind and purity of water: Milli-Q water
- Preparation of test medium: A 0.003 M (590 µg/g) test substance solution in tetrahydrofuran was prepared gravimetrically using a verified and calibrated analytical balance. The test substance solution composition was chosen to target a final spiked buffer concentration of 0.00003 M (~5 ppm)test substance.
- Renewal of test solution: Not required.
- Identity and concentration of co-solvent: Tetrahydrofuran (THF), <1% v/v

OTHER TEST CONDITIONS
- Adjustment of pH: No pH adjustment was carried out during the test.
- Dissolved oxygen: No details given.

Duration:

2.2 min

pH:

4

Initial conc. measured:

30 µmol/L

Duration:

3 min

pH:

4

Initial conc. measured:

30 µmol/L

Duration:

2.5 min

pH:

7

Initial conc. measured:

30 µmol/L

Duration:

7.5 min

pH:

7

Initial conc. measured:

30 µmol/L

Duration:

1.3 min

pH:

9

Initial conc. measured:

30 µmol/L

Duration:

3 min

pH:

9

Initial conc. measured:

30 µmol/L

Number of replicates:

2 at each pH (4, 7 and 9)

Positive controls:

not specified

Negative controls:

not specified

Statistical methods:

Since the hydrolysis was so rapid, there was insufficient data to use statistical methods to interpret the results.

Preliminary study:

The substance is known to be unstable at environmentally relevant temperatures, therefore, no preliminary study was required.

Transformation products:

yes

No.:

#1

No.:

#2

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

The hydrolysis of  hexamethyldisilazane to form ammonium ion and trimethylsilanol is well established. Hydrolysis of the Si-N bond results in the formation of amines/ammonia and silanols as co-products. (see Bazant, V.; Chvalovsky, V. In Chemistry of Organosilicon Compounds;  Organosilicon Compounds Volume 1; Academic Press Inc.: New York,  1965;  pp. 85-86.).

The analytical method used in this study  measured formation of one of the co-products, ammonia, as ammonium ion. 

Key result

pH:

4

Temp.:

1.5 °C

DT50:

<= 0.04 min

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material (based on ammonium ion concentration).

Key result

pH:

7

Temp.:

1.5 °C

DT50:

<= 0.5 min

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material (based on ammonium ion concentration).

Key result

pH:

9

Temp.:

1.5 °C

DT50:

<= 0.1 min

Remarks on result:

other: This value represents an estimated upper limit of the hydrolysis half-life. It refers to disappearance of the test material (based on ammonium ion concentration).

Other kinetic parameters:

None determined.

Details on results:

TEST CONDITIONS
- pH, sterility, temperature, and other experimental conditions maintained throughout the study: Yes

MAJOR TRANSFORMATION PRODUCTS
The ammonium ion concentration was measured over the course of the hydrolysis. The ratios of measured to theoretical (based on complete hydrolysis) ammonium ion concentration for duplicate measurements at pH 4, 7 and 9 were 109-136%, 96-98% and 96-104%, respectively.
No other transformation products were identified during the test.

PATHWAYS OF HYDROLYSIS
The proposed three step reaction scheme is shown below:
- Step 1:   (CH3)3SiNHSi(CH3)3  +  H2O → (CH3)3SiNH2  +  (CH3)3SiOH
- Step 2:   (CH3)3SiNH2  +  H2O  +  (CH3)3SiOH → NH3  + 2 (CH3)3SiOH
- Step 3:   NH3      +       H2O → NH4+OH-
(see Bazant, V.; Chvalovsky, V. In Chemistry of Organosilicon Compounds;  Organosilicon Compounds Volume 1; Academic Press Inc.: New York,  1965;  pp. 85-86.).

Since the hydrolysis was so rapid, there was insufficient data to determine rate constants for the hydrolysis reactions by regression modelling.

First order or pseudo-first order behaviour could not be confirmed because: (a) sparse nature of the data during the critical portion of the process, b) the inherent limitation associated with measuring co-product concentration for consecutive reactions. Although rate constants and half-lives could not be determined quantitatively, the data was adequate for estimating the upper limit of t_1/2.

Tables 1 shows key results for each hydrolysis test.

Table 1. Results

pH		Replicate	Duration of test	% Theoretical [Cl-] at this temperature	Time used for t1/2 determination	% Theoretical [Cl-] at this time	t1/2 / s
4		A	130	114	20	116	2
4		B	180	119	30	136	3
7		A	150	98	130	98	13
7		B	450	96	440	96	44
9		A	80	104	60	99	6
9		B	180	116	60	98	6

* The time at which hydrolysis was deemed complete because two ammonium ion concentration readings were at least within 5% of the expected ammonium ion concentration.

The half-life at each pH is the average of the two replicates: 2.5 s, 28.5 s, and 6 s at pH 4, 7, and 9, respectively.

Conclusions:

A hydrolysis half-life of ≤0.5 minutes at pH 7 was determined for the substance in a reliable study conducted according to an appropriate test protocol. The result is considered to be reliable.

Description of key information

Hydrolysis half-life: << 1 minute at 25°C and pH 4, pH 7 and pH 9 (OECD 111)

Additional information

Measured hydrolysis half-lives of ≤0.04 min at pH 4, ≤0.5 min at pH 7 and ≤0.1 at pH 9 and 1.5°C were determined for the submission substance in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable and is selected as key study.

The key study is supported by another measured half-life of <150 minutes at pH 4, pH 7 and pH 9 and 50°C determined for the substance using a relevant test method. The supporting study is in agreement that the substance reacts very rapidly.

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.

k_obs = k₀ + k_H3O+[H₃O⁺] + k_OH-[OH^-] + k_a[acid] + k_b[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 k_H3O+ and k_OH- 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:

k_obs≈ k_H3O+[H₃O⁺]

At pH 4 [H₃O⁺] = 10^-4 mol dm^-3 and at pH 2 [H₃O⁺] = 10^-2 mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at pH 4.

 

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

t_1/2(pH 2) = t_1/2(pH 4)/100

 

The calculated half-life of 1,1,1,3,3,3-hexamethyldisilazane at pH 2 and 1.5°C is therefore <0.0004 minutes (0.02 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 the substance 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 the submission substance, the predicted hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is <2 seconds. As discussed above, it is appropriate to consider that the half-life at pH 7 and 37.5 ºC is <5 seconds. 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 <5 seconds.

The hydrolysis products for the submission substance are trimethylsilanol and ammonia.

Ammonia (NH₃) exists in solution in equilibrium with the ammonium ion. (NH₄⁺). Under normal environmental conditions, ammonium ions (NH₄ ⁺) predominate.