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3-[Dimethoxy(methyl)silyl]propanethiol is a liquid at standard temperature and pressure, with a measured melting point of <-80°C, and a predicted boiling point of 190°C. It has a predicted density of 1.0 g/cm³ at 20°C and a predicted vapour pressure of 23 Pa at 25°C.

The substance is not classified for flammability in accordance with Regulation (EC) No 1272/2008; this is on the basis of a measured flash point of 76°C and predicted boiling point of 190°C. It has a measured auto-ignition temperature of 235°C and is not explosive and not oxidising on the basis of structural examination.

In contact with water, 3-[dimethoxy(methyl)silyl]propanethiol reacts rapidly (half-life 1.4 h at 25°C and pH 7, 0.1 h at 25°C and pH 4, and 0.03 h at 25°C and pH 9) to produce 3-(dihydroxymethylsilyl)propanethiol and methanol according to the following equation:

Si(CH3)(OCH3)2C3H6SH + 2H2O → Si(CH3)(OH)2C3H6SH + 2CH3OH

Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the silanol hydrolysis product, 3-(dihydroxymethylsilyl)propanethiol and methanol are assessed instead.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12,790 Pa at 20°C) (OECD 2004).

The saturation concentration in water of the silanol hydrolysis product, 3-(dihydroxymethylsilyl)propanethiol, is limited by condensation reactions to approximately 45 mg/l. However, it is very hydrophilic (calculated solubility is 47000 mg/l at 20°C using a QSAR method) with predicted low log Kow of 0.5 at 20°C. It is not surface active. The hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 0.0092 Pa at 25°C).

The molecular weight of 3-[dimethoxy(methyl)silyl]propanethiol is 180.34 g/mol. The molecular weight of 3-(dihydroxymethylsilyl)propanethiol is 152.28. One mole of the parent substance generates one mole of the silanol hydrolysis product plus two moles of methanol.

Reference:

OECD (2004): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.

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