[3-(trimethoxysilyl)propyl]urea

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

(Q)SAR

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification

Principles of method if other than guideline:

The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details).

Transformation products:

yes

No.:

#1

No.:

#2

pH:

7

DT50:

2.8 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: 20-25 °C

pH:

4

DT50:

0.2 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: 20-25 °C

pH:

5

DT50:

0.3 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: 20-25 °C

pH:

9

DT50:

0.1 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: 20-25 °C

Conclusions:

A hydrolysis half-life of approximately 2.8 h at 20-25°C and pH 7 was calculated for [3 -(trimethoxysilyl)propyl]urea.

Description of key information

DT50 = 2.8 h (20 -25°C and pH 7, QSAR)

Key value for chemical safety assessment

Half-life for hydrolysis:

2.8 h

Additional information

A hydrolysis half-life of approximately 2.8 h at 20-25°C and pH 7 was obtained for the substance using an appropriate calculation method. The result is considered to be reliable and has been assigned as key study.

A QSAR that was developed (Peter Fisk Associates, 2014) predicts half-lives at 20-25 °C of 0.2 h at pH 4, 0.3 h at pH 5 and 0.1 h at pH 9. 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.

 

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

 

At extreme pH values 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 kH3O+and kOH-from the experimental results at pH 4 and 9, respectively, resulted in reasonable estimates of the half-life at pH 7 (Peter Fisk Associates, 2014).

Therefore, at low pH:

kobs˜kH3O+[H3O+]

 

At pH 4 [H3O+]=10^-4 mol dm^-3 and at pH2 [H3O+]=10^-2 mol dm^-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

 

t1/2 (pH 2) = t1/2 (pH 4) / 100

The reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions and the half-lives at 37.5 ºC (relevant for in vivo studies) are expected to be faster than those at 20-25°C. 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.

the hydrolysis half-life at 37.5°C and pH 2 (relevant for oral exposure) is < 5 s.

 

The initial hydrolysis products of [3-(trimethoxysilyl)propyl]urea (CAS23843-64-3) are [3-(trihydroxysilyl)propyl]urea and methanol.