Triethylsilane

Hydrolysis

Description of key information

Hydrolysis half-life (OECD 111)

53.8 hours at pH 4 (time period 0-53.8 hours)

377 hours at pH 7

56.8 hours at pH 9 and 20°C

Key value for chemical safety assessment

Half-life for hydrolysis:

377 h

at the temperature of:

20 °C

Additional information

The hydrolysis half-lives of the substance have been measured in accordance with OECD Test Guideline 111 and in compliance with GLP.

The hydrolysis half-lives of triethylsilane are:

pH 4.0 = 53.8 hours (at time period 0 -53.8 hours), 250 hours (at time period 0 -269 hours) at 20°C; 26.3 hours (at time period 0 -30.9 hours), 199 hours (at time period 0 -245 hours) at 30°C and 16.6 hours (at time period 0 -30 hours), 94.8 hours (at time period 0 -221 hours) at 50°C; 218 hours at 25°C.

pH 7.0 = 377 hours at 20°C, 314 hours at 25°C, 279 hours at 30°C and 60.7 hours at 50°C

pH 9.0 = 56.8 hours at 20°C, 39.9 hours at 30°C and 8.67 hours at 50°C

The ln concentration vs. time plots indicate a non-linear correlation, consequently a higher order reaction kinetics could be assumed. The course of the ln concentration over time indicates that the half-life depends on the test item concentration and the strongest effect was observed at the pH 4 test condition. Nevertheless, as the goodness of fit for the pH 7 and pH 9 test conditions was deemed to be acceptable, pseudo first order reaction kinetics were used for evaluation of the hydrolysis data. In order provide a more sound half-life value for the pH 4 test condition an additional evaluation was performed, where only the data of the linear part of the ln concentration vs. time plots were computed (see results above). The obtained kinetics data are only valid for the tested concentration conditions and time frames. The result is considered to be reliable and is used for assessment purposes.

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 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 the substance at pH 2 (using the half-life for the time period 0 -53.8 hours) is therefore 0.538 hours (32.3 minutes). 

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 the substance, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 150 hours (extrapolated from the measured value at 30°C). At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 0.14 hours. At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 4 (14 hours) and pH 7 (150 hours).

The ultimate products of hydrolysis are triethylsilanol and hydrogen.

The hydrolysis half-lives of substances used for read-across are discussed below:

Hydrolysis of the read-across substance trimethylsilane (CAS No. 993-07-7)

Data for the substance, trimethylsilane (CAS No. 993-07-7) are read-across to the submission substance triethylsilane for appropriate endpoints (see Section 1.4 of the CSR). The hydrolysis half-life and the silanol hydrolysis products of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

The stability in water study with trimethylsilane was carried out using a non-standard method. The study was performed using well water at a test substance concentration of 100 µg/l under static conditions for a period of 14 days. The sample analysis was performed using Gas Chromatography (GC) and the reaction product, trimethylsilanol, was monitored using GC-MS. The studies were performed in triplicate and the disappearance of the parent trimethylsilane was monitored as well as the formation of the reaction product, trimethylsilanol. A half-life of 4.2 d was reported by the study author. However, a further review of the study indicates that the rate increased during the study, with a half-life of around 3 days obtained towards the end of the study. This value is therefore used for the chemical safety assessment. There was no information on the statistical analysis of the degradation rate included in the study report. On the other hand, a mass balance of 100% was achieved for the degradation of the parent trimethylsilane and formation of the hydrolysis product, trimethylsilanol.

The hydrolysis products are trimethylsilanol and hydrogen.

 

Hydrolysis of the read-across substance tetramethylsilane (CAS No: 75-76-3)

Data for the substance, tetramethylsilane (CAS No: 75-76-3) are read-across to the submission substance triethylsilane for appropriate endpoints. The structural similarities of the two substances are relevant to this read-across as discussed in appropriate sections for each endpoint.

Tetramethylsilane is a silane with four methyl groups attached to the silicon atom. Tetramethylsilane contains only Si-C and C-H bonds which are stable to hydrolysis. Triethylsilane is a structurally related silane, with three ethyl groups (Si-C) and one hydrogen atom (Si-H) attached to the silicon atom.