2,11-Dioxa-3,10-disiladodecane, 3,3,10,10-tetramethoxy-

Hydrolysis

Description of key information

Hydrolysis half-life: 5.2 h at pH 7, 0.08 h at pH 5, 0.15 h at pH 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:

5.2 h

at the temperature of:

25.1 °C

Additional information

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

The hydrolysis half-lives of the substance are: 0.08 h at pH 5 and 25°C, 5.2 h at pH 7 at 25.1°C and 0.15 h at pH 9 at 25°C. At 35°C, the half-lives are: 0.05 h at pH 5, 2.6 h at pH 7, and 0.06 h at pH 9. The results are considered to be reliable.

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⁺]

 

The half-life at pH 2 may be estimated from t_1/2(pH 2) = t_1/2(pH 5) / 1000 = 0.00008 h (0.3 seconds). However, it is not appropriate or necessary to attempt to predict accurately when the half-life is less than 5-10. As a worst-case it can therefore be considered that the half-life for the substance 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) x e^(0.08.(T-X))

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

Thus, for 1,6-bis(trimethoxysilyl)hexane, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is 1.9 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 products of hydrolysis are 1,6-bis(trihydroxysilyl)hexane and methanol.