Dibutoxy(dimethyl)silane

Hydrolysis

Description of key information

Hydrolysis half-lives: < 8 minutes at pH 4 and 19.6°C, 1.4 h at pH 7 and 25°C and 0.59 h at pH 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:

1.4 h

at the temperature of:

25 °C

Additional information

The hydrolysis rates of the substance have been measured in accordance with OECD 111 and in compliance with GLP. Hydrolysis half-lives obtained for the substance are: <8 minutes at pH 4 and 19.6°C, 4.1 h at 15°C, 1.4 h at 25°C and 0.65 h at 35°C and pH 7. At pH 9, the half-lives are 1.6 h at 15°C, 0.59 h at 25°C and 0.26 h at 35°C. The results are considered to be reliable and are used for the chemical safety assessment.

Hydrolysis reactions of alkoxysilanes can be catalysed by both acid and base. The rate of hydrolysis is slowest close to pH 7 and increases 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 uncatalysed 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 dibutoxy(dimethyl)silane at pH 2 based on measured half-life at pH 4 and 19.6°C is therefore <0.0013 h (approximately 5 seconds). However, it is likely that factors such as diffusion become rate-determining when the half-life is less than 5-10 seconds. As a worst-case it can therefore be considered that the half-life of the substance at pH 2 and 19.6°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 dibutoxy(dimethyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 0.52 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 and the hydrolysis half -life is therefore approximately 5 seconds.

The product of the hydrolysis reaction under dilute condition is dimethylsilanediol. The other hydrolysis product is n-butanol.

The hydrolysis data for substances used in this dossier for read-across purposes for other endpoints are now discussed.

Hydrolysis of the read-across substance dichloro(methyl)(vinyl)silane (CAS No. 124-70-9)

Data for the substance dichloro(methyl)(vinyl)silane (CAS No. 124-70-9) are read-across to the submission substance dibutoxy(dimethyl)silane for appropriate endpoints. The silanol hydrolysis products of the two substances are relevant to this read-across.

 

For dichloro(methyl)(vinyl)silane, hydrolysis half-lives are read-across from related dichlorosilane analogue, dichloro(dimethyl)silane (CAS No. 75-78-5). Half-lives of approximately 0.2 minutes at pH 4, approximately 0.3 minutes at pH 7 and approximately 0.1 minutes at pH 9 and 1.5°C were obtained in accordance with a relevant test method (Dow Corning Corporation, 2001).

These half-lives relate to degradation of the parent substance to give dimethylsilanediol and hydrochloric acid.