tert-butylchlorodimethylsilane

Endpoint summary

Description of key information

The substance, tert-butyl(chloro)dimethylsilane, is not stable in water, which affects the approach to the determination of physicochemical properties.

tert-Butyl(chloro)dimethylsilane is a solid at standard temperature and pressure, with a measured melting temperature range of 90.1 to 93.4°C (363.3 to 366.6 K), and a measured boiling point of 126°C. It has a predicted density of 0.94 g/cm³ at 20°C and a predicted vapour pressure of 710 Pa at 25°C.

 

tert-Butyl(chloro)dimethylsilane is classified as a flammable solid on the basis of a measured flammability of solids test in accordance with EU Method A.10. The self-ignition temperature is waived based on a melting point of <160°C. The substance is not explosive and not oxidising on the basis of chemical structure.

 

In contact with water, tert-butyl(chloro)dimethylsilane is expected to react very rapidly (based on read-across from related substance: half-life <1 minute at 25°C and pH 4, pH 7 and pH 9), according to the following equation:

C₆H₁₅ClSi + H₂O → C₆H₁₆OSi + HCl

Therefore, requirements for testing of water-based physicochemical properties for the submission substance are waived on the basis of instability in water. The properties of the silanol hydrolysis product, tert-butyl(dimethyl)silanol, are assessed instead.

The silanol hydrolysis product, tert-butyl(dimethyl)silanol, may undergo condensation reactions in solution to give the siloxane dimer and a dynamic equilibrium is established. The overall rate of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution.

The condensation reactions of monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. For tert-butyl(dimethyl)silanol, a solution at 100 mg/L (the highest concentration often used in ecotoxicity tests) is predicted to contain >99.9% monomer. At loadings above about 80 to 100 mg/L the concentration of the dimer is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the dimer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The saturation concentration in water of the silanol hydrolysis product, tert-butyl(dimethyl)silanol is limited by condensation reactions that can occur over time at loadings above approximately 80 to 100 mg/L. However, it has a predicted water solubility of 970 mg/l at 20°C using a validated QSAR method and a log K_ow of 2.5. It is not expected to be surface active and it is much less volatile than the parent substance (vapour pressure = 60 Pa at 25°C, predicted).

Reference:

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003