3,3'-(1,1,3,3-tetramethyldisiloxane-1,3-diyl)bispropylamine

Hydrolysis

Description of key information

Hydrolysis half-life: 63.0 h at pH 7, 1.0 h at pH 4, 0.8 h at pH 9 and 20-25°C (QSAR).

Key value for chemical safety assessment

Half-life for hydrolysis:

63 h

at the temperature of:

25 °C

Additional information

Hydrolysis half-lives of 1.0 h at pH 4, 63.0 h at pH 7 and 0.8 h at pH 9 and 20-25°C were determined for the substance using a validated QSAR estimation method. The result is considered to be reliable and was selected as key study.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around 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 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.

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 is therefore 0.01 hours (approximately 40 seconds) at 20 -25°C. 

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 registration substance; the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 23 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 0.0037 hours (approximately 13 seconds).

 

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 and pH 7.

The ultimate product of hydrolysis is 3-aminopropyldimethysilanol. Two moles of the hydrolysis product are produced for every mole of parent substance.