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Description of key information

The substance is a multi-constituent substance with two main constituents, N-[3-(trimethoxysilyl)propyl]-1,3-benzenedimethanamine (Constituent 1) and 3-[(2,2-dimethoxy-1,2-azasilolidin-1-yl)benzenedimethanamine (Constituent 2).

Since the substance is a multi-constituent substance, two aspects need to be considered in relation to physicochemical properties:

1) In the context of raw material handling and safe use it is important to understand the properties of the product as a whole

The substance is a liquid at room temperature and pressure, with a freezing temperature of <-80°C. No measured boiling point can be determined because the substance undergoes thermal decomposition from about 275°C. It has a measured relative density of 1.09 at 20°C. The vapour pressure of the whole substance was determined to be 3.6E-03 Pa at 20°C and 6.1E-03 Pa at 25°C.

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008, on the basis of a measured flash point of 153±2°C. It has a measured auto-ignition temperature of 320°C at 1014.3 - 1022 hPa and is not explosive and not oxidising on the basis of chemical structure. Also, none of the constituents of the substance are expected to be surface active.

2) For the purposes of assessing environmental fate and behaviour, individual constituents may behave independently of each other. It is therefore essential to consider the properties of individual constituents, and whole property data may have no real scientific meaning or significance.

The predicted key physicochemical properties of the individual constituents are shown in Table 1.1

Table 1.1: Predicted key physicochemical properties of constituents of the substance

Constituent number

Constituent name

SMILES

Molecular weight (g/mol)

Vapour pressure at 25°C (Pa)

log Kow at 20°C

Water solubility at 20°C (mg/L)

pKa

1

N-[3-(trimethoxysilyl)propyl]-1,3-benzenedimethanamine

NCc1cccc(c1)CNCCC[Si](OC)(OC)OC

298.46

1.3E-03

1.5 for the unionised form

For the ionised form; using pKa2: -1.3 at pH 7, -4 at pH 2, -4 at pH 4, 0.63 at pH 9 and 20°C

4.1E+04

pKa1 = 8.6

 

pKa2 = 9.8

2

3-[(2,2-Dimethoxy-1,2-azasilolidin-1-yl)benzenedimethanamine

NCc1cccc(c1)CN1CCC[Si]1(OC)OC

266.42

9.7E-03

Not determined, due to very rapid hydrolysis

Not determined, due to very rapid hydrolysis

Not determined, due to very rapid hydrolysis

Final silanol HP

N-[3-(trihydroxysilyl)propyl]-1,3-benzenedimethanamine

NCc1cccc(c1)CNCCC[Si](O)(O)O

256.38

<1.0E-03

-1.6 for the unionised form

For the ionised form; using pKa2: -4 at pH 7, -4 at pH 2, -4 at pH 4, -3 at pH 9 and 20°C

 

 1.0E+06; limited by condensation at about 1000 mg/L

pKa1 = 8.8

 

pKa2 = 10.4

 

pKa3 = 12.1

 

Both constituents contain methoxysilane groups (Si-OMe) which are susceptible to hydrolysis in contact with water. In addition to the Si-OMe group in both constituents, Constituent 2 also contains a silyl amine group (Si-N) which reacts very rapidly in contact with water.

The constituents undergo hydrolysis reactions as shown in Figure 1 (attached in Section 13). Each constituent undergoes three consecutive hydrolysis reactions. The first hydrolysis step for Constituent 1 is loss of a methoxy group to give Intermediate 1 (N-[3-(hydroxy(dimethoxy)silyl)propyl]-1,3-benzenedimethanamine) and methanol according to the following equation:

C11H17N2Si(OCH3)3 + H2O → C11H17N2Si(OCH3)2OH + CH3OH

Predicted hydrolysis half-lives for this reaction are 0.3 h at pH 4, 4.8 h at pH 7, 0.1 h at pH 9 and 20-25C.

 

The first hydrolysis step for Constituent 2 is ring-opening at the Si-N bond. This also results in Intermediate 1, N-[3-(hydroxy(dimethoxy)silyl)propyl]-1,3-benzenedimethanamine (but not methanol) according to the following equation:

C11H16N2Si(OCH3)2 + H2O → C11H17N2Si(OCH3)2OH

The silyl amine (Si-N) bond is highly susceptible to hydrolysis by both acid-catalysed and base-catalysed mechanisms. Therefore, the Si-N bond present in Constituent 2 will undergo very rapid hydrolysis on contact with water. The half-life is estimated as <1 minute at pH 4, 7 and 9.

 

Intermediate 1 undergoes two further hydrolysis steps involving loss of its two methoxy groups to give Intermediate 2 (N-[3-(diihydroxy(methoxy)silyl)propyl]-1,3-benzenedimethanamine) and the final hydrolysis product (N-[3-(trihydroxysilyl)propyl]-1,3-benzenedimethanamine) according to the following equations:

C11H17N2Si(OCH3)2OH + H2O → C11H17N2Si(OCH3)(OH)2+ CH3OH

C11H17N2Si(OCH3)(OH)2→ C11H17N2Si(OH)3+ CH3OH

The predicted half-lives for these reactions are each <0.3 h at pH 4, <4.8 h at pH 7 and <0.1 h at pH 9.

 

Thus, the intermediate and final silicon-containing hydrolysis products are the same for Constituent 1 and Constituent 2. The final silicon-containing hydrolysis product is N-[3-(trihydroxysilyl)propyl]-1,3-benzenedimethanamine (1 mole) for both constituents. The co-product of hydrolysis is methanol for both constituents; 3 moles for Constituent 1 and 2 moles for Constituent 2.

Therefore, the requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the hydrolysis products, N-[3-(trihydroxysilyl)propyl]-1,3-benzenedimethanamine and methanol are assessed instead.

The saturation concentration in water of the silanol hydrolysis products, N-[3-(trihydroxysilyl)propyl]-1,3-benzenedimethanamine, is limited by condensation reactions that can occur over time at loadings above approximately 1000 mg/L. However, it is very hydrophilic (calculated solubility is 1E+06 mg/L at 20°C using a QSAR method). The unionised form of the silanol hydrolysis product has a predicted log Kow of -1.6. In solution, the amine groups will protonate; therefore, the estimated log Kow was corrected for ionisation based on the second the pKa (which will generate the unionised form) using the equation CORR = 1/1 +10A(pH-pKa) [where A = 1 for acids, -1 for bases; pH = pH-value of the environment; pKa = acid/base dissociation constant]. Log Kow values and vapour pressure of the silanol hydrolysis product are reported in Table 1.1. The silanol hydrolysis product is not surface active.

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