Tetrapropyl orthosilicate

Hydrolysis

Description of key information

Hydrolysis half-life: 0.22 h, 6.7 h and 0.44 at pH 4, 7 and 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:

6.7 h

at the temperature of:

25 °C

Additional information

Measured hydrolysis half-lives of 0.22 h, 6.7 h and 0.44 h at pH 4, 7 and 9 respectively were determined at 25°C for the substance in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable with restrictions and is selected as key study. A 0.1 M buffer was used in the pH 7 experiments, which can result in buffer catalysis. The measured half-life at pH 7 is faster than that calculated by extrapolation from the pH 4 and 9 results (around 40 hours at pH 7 and 25°C). However, the measured half-life of 6.7 h at pH 7 and 25°C is considered suitable for the purposes of environmental risk assessment.

In a non-guideline study, hydrolysis half-life of >10 h, >>10 h and >10 h at pH 4, 7 and 9 respectively were determined for the substance. However, the study was disregarded because the test substance was overloaded above the limit of solubility. The substance has a predicted water solubility of 78 mg/l at 20°C.

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.

 

Therefore, at low pH:

k_obs≈k_H3O+[H₃O⁺]

 

At pH 4 [H₃O⁺]=10^-4mol dm^-3and 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 tetrapropyl orthosilicate at pH 2 is therefore 0.002 hours (8 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 20-25°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.

At 40°C, a measured half-life of 2.7 h was determined for the substance. Thus, for tetrapropyl orthosilicate the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is about 2.5 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 silicic acid and propanol.