Tris(2-methoxyethoxy)vinylsilane

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2004-09-07 to 2004-02-09

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:

yes

Details on sampling:

Individual kinetic experiments were carried out by staggered starts of multiple reaction aliquots, one for each reaction time to be sampled. At a predetermined time, the hydrolysis reaction was effectively stopped by addition of the extraction solution (5 ml of the target 100µg/g tetradecane in toluene). This approach was used to minimize the time required to complete an experiment and achieve convenient sampling intervals to collect quantitative hydrolysis data spanning 2-3 half-lives of the substance. On average, data was collected at 10 discrete time intervals for each kinetic experiment.

Buffers:

Buffer solutions of known pH (targets were 4.00, 7.00, and 9.00) and concentration (0.05 M) were prepared by titration of formic acid, sodium phosphate monobasic, and boric acid solution for pH 4, 7, and 9, respectively, with sodium hydroxide solution.  

Details on test conditions:

The hydrolysis reaction was followed kinetically by measuring the concentration of vinyl-tris(2-methoxyethoxy)-silane in the extract at various times through 2-3 half-lives.  The reactions employed an initial analyte concentration in buffer of 7x10-4 M (~200 mg/L).  The test system consisted of aqueous buffers made using deionized water from a Millipore Milli-Q system.  Due to the hydrolytically unstable nature of vinyl-tris(2-methoxyethoxy)-silane, the water miscible solvent acetonitrile was used for application and distribution of the test substance in the test system. Co-solvent:  <1% acetonitrile.  Constant ionic strength of 0.25 M was maintained for buffers by the addition of sodium chloride. 

Number of replicates:

The study at 25°C and pH 7 was carried out in duplicate.

Statistical methods:

6-9 data points were quantitatively used for regression analysis except for pH 9 at 35°C where only 5 data points were used due to the fast reaction time. This was sufficient to determine the rate constants for the reactions. The natural logarithm of the concentration was plotted as a function of reaction time. The observed rate constant, k, for the hydrolysis reaction was determined as the slope of the first-order regression line fitted to the data. The half-life, t1/2, of the hydrolysis reaction was calculated from t1/2 = ln2/k.
The observed rate constants as a function of temperature, T, at the two extremes of pH were used to construct an Arrhenius diagram for each catalytic condition by plotting ln k against 1/T for constant pH. This allows the rate constant to be calculated for any temperature.
Descriptive statistics such as average, standard deviation, relative standard deviation and linear regression analysis were performed using Microsoft Excel spreadsheets.

Preliminary study:

The OECD guideline preliminary test at 50°C was not conducted since the test substance was expected to be hydrolytically unstable (t1/2 < 1 year).

Transformation products:

yes

No.:

#1

No.:

#2

pH:

4

Temp.:

10 °C

Hydrolysis rate constant:

0.21 min-1

DT50:

3.3 min

Type:

(pseudo-)first order (= half-life)

Key result

pH:

4

Temp.:

25 °C

Hydrolysis rate constant:

0.43 min-1

DT50:

1.6 min

Type:

(pseudo-)first order (= half-life)

pH:

4

Temp.:

35 °C

Hydrolysis rate constant:

0.72 min-1

DT50:

0.96 min

Type:

(pseudo-)first order (= half-life)

Key result

pH:

7

Temp.:

25 °C

Hydrolysis rate constant:

0.011 min-1

DT50:

61.5 min

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

10 °C

Hydrolysis rate constant:

0.19 min-1

DT50:

3.7 min

Type:

(pseudo-)first order (= half-life)

Key result

pH:

9

Temp.:

25 °C

Hydrolysis rate constant:

0.73 min-1

DT50:

0.94 min

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

35 °C

Hydrolysis rate constant:

1.6 min-1

DT50:

0.43 min

Type:

(pseudo-)first order (= half-life)

The hydrolysis of rate of vinyltris(2-methoxyethoxy)silane was observed to followpseudo first-order kinetics, was pH dependent and increased as a function of temperature. The catalytic constants, kH30+ = 4160 M-1min-1 and kOH- = 73400 M-1min-1 demonstrate that base catalysis is faster than acid catalysis. Average % recovery of vinyltris(2-methoxyethoxy)silane in the solvent extract from spiked buffer blanks thermostated at 25°C ranged from 59% for pH 9, 76.2% for pH 4 to 93.4% for pH 7. The low recoveries observed at pH 4 and pH 9 at 25°C are easily understood due to the rapid hydrolysis rate at the extreme ends of the pH range tested. Duplicate analyses were conducted at pH 7 and 25°C with difference of 2.5%. Additional experiments were conducted at 10°C and 35°C. These experiments were conducted at both pH 4 and pH 9 using 0.05 M buffer concentration employed for the 25°C experiments. The information for each pH value was used to construct a linear Arrhenius plot from which Arrhenius constant (A) and activation energy (Ea) were determined for the specific acid and base catalysed reactions. The results show that activation energy is nearly 2-fold greater for the hydroxide ion catalysed hydrolysis reaction than the hydronium ion catalysed hydrolysis reaction, although a much greater preexponential factor, A, is associated with the hydroxide ion catalysed hydrolysis reaction.

 Table 1: Results

pH	Temp (°C)	Rate constant, k (min^-1)	t1/2(min)
4	10.0±0.1	0.208	3.3
4	25.0±0.1	0.426	1.6
4	35.0±0.1	0.721	0.96
7	25.0±0.1	0.0113	62^a
9	10.0±0.1	0.186	3.7
9	25.0±0.1	0.734	0.94
9	35.0±0.1	1.60	0.43

^aaverage of duplicate samples

Validity criteria fulfilled:

yes

Conclusions:

Hydrolysis half-lives of 1.6 min at pH 4, 61.5 min at pH 7 and 0.94 min at pH 9 and 25°C were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Executive summary:

According to the definition put forth in the test guidelines, the test substance was observed to be hydrolytically unstable (t_1/2 <1 year) over a range of environmentally relevant pH conditions at 10.0, 25.0, and 35.0°C.

Description of key information

Hydrolysis half-lives: 1.6 minutes at pH 4, 61.5 minutes at pH 7 and 0.94 minutes at pH 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:

61.5 min

at the temperature of:

25 °C

Additional information

Measured hydrolysis half-lives of 1.6 minutes at pH 4, 61.5 minutes at pH 7 and 0.94 minutes at pH 9 and 25°C were obtained for the submission substance in accordance with OECD 111 and in compliance with GLP. In order to determine the effect of temperature on hydrolysis rate of the submission substance, additional experiments were conducted at both pH 4 and pH 9 and 10°C and 35°C. At 10°C, hydrolysis half-life values of 3.3 minutes at pH 4 and 3.7 minutes at pH 9 were obtained. Similarly, at 35°C, hydrolysis half-lives of 0.96 minutes at pH 4 and 0.43 minutes at pH 9 were obtained. The results are considered to be reliable and selected 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.

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 tris(2-methoxyethoxy)vinylsilane at pH 2 and 25°C is therefore 0.00027 hours (approximately 1 second). 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 tris(2-methoxyethoxy)vinylsilane at pH 2 and 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) * e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for tris(2-methoxyethoxy)vinylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 0.38 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 are vinylsilanetriol (1 mole) and 2-methoxyethanol (3 moles).

Hydrolysis of the read-across substance trimethoxy(vinyl)silane (CAS 2768-02-7)

Data for the substance, trimethoxy(vinyl)silane (CAS 2768-02-7) are read-across to the submission substance tris(2-methoxyethoxy)vinylsilane for long-term toxicity to aquatic invertebrates endpoint. The hydrolysis half-lives and the hydrolysis products of the two substances are relevant for this read-across as discussed in the relevant sections for each endpoint.

Trimethoxy(vinyl)silane has measured half-lives of <10 minutes, <2.4 h and <10 minutes at pH 4, pH 7 and pH 9 and 50°C in a preliminary study according to OECD 111. Also, it has predicted half-lives at 20-25ºC of 0.04 h (2 minutes) at pH 4, 0.1 h at pH 5 and 7, and 0.004 h (14 seconds) at pH 9. In a secondary source to which reliability could not be assigned, half-lives of 0.24 h at pH 7 and 0.58 h at pH 4 and 23°C were reported for trimethoxy(vinyl)silane.

The hydrolysis products are vinylsilanetriol (1 mole) and methanol (3 moles).

Hydrolysis of the read-across substance trichloro(vinyl)silane (CAS 75-94-5)

Data for the substance, trichloro(vinyl)silane (CAS 75-94-5) are read-across to the submission substance tris(2-methoxyethoxy)vinylsilane for the toxicity to microorganisms endpoint. The silanol hydrolysis products of the substances are relevant to this read-across, as discussed in the relevant section for the endpoint.

 

For trichloro(vinyl)silane, hydrolysis half-lives of <1 minute at pH 4, pH 7 and pH 9 at 1.5°C were read-across from trichloro(methyl)silane which has been tested in accordance with OECD 111 (Dow Corning Corporation 2001).

 

The hydrolysis products are vinylsilanetriol (1 mole) and hydrochloric acid (3 moles).