Registration Dossier
Registration Dossier
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EC number: 226-956-0 | CAS number: 5578-42-7
Endpoint summary
Administrative data
Description of key information
The substance, dichloro(cyclohexyl)methylsilane is not stable in water, which affects the approach to the determination of physicochemical properties. The significance of this for read-across is discussed in relevant sections.
Dichloro(cyclohexyl)methylsilane is a liquid at standard temperature and pressure, with a measured melting point of -45.7°C and a measured boiling point of 206°C. It has a measured relative density of 1.095 at 20°C and a measured kinematic viscosity of 1.85 mm2/s at 20°C. The substance has a measured vapour pressure of 3 Pa at 20°C, 6 Pa at 25°C and 97 Pa at 50°C.
The substance is not classified for flammability on the basis of a measured flash point of 76°C and the boiling point of 206°C. It is not flammable in contact with water according to EU Method A.12. It has a measured auto-ignition temperature of 249°C at 954.8 to 965.5 hPa. The substance is not oxidising on the basis of structural examination. Similarly, dichloro(cyclohexyl)methylsilane is not explosive based on structural examination.
In contact with water dichloro(cyclohexyl)methylsilane reacts 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:
C6H11(Me)SiCl2 + 2H2O → C6H11(Me)Si(OH)2+ 2HCl
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, cyclohexyl(methyl)silanediol are assessed instead.
The silanol hydrolysis product, cyclohexyl(methyl)silanediol, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and a dynamic equilibrium is established.
The overall rate and extent 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 may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. The reactions are reversible as long as the concentrations are low and all species remain soluble; thus, a dynamic equilibrium is established. At higher concentrations one or more of the oligomers may exceed their solubility limit and form a separate phase, driving the equilibrium towards the oligomer/polymers. At loadings below 1000 mg/L of cyclohexyl(methyl)silanediol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above about 30 mg/L (or lower under some circumstances) causing the formation of insoluble polymeric particles and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.
The saturation concentration in water of the silanol hydrolysis product, cyclohexyl(methyl)silanediol, is therefore limited by condensation reactions to approximately 30 mg/L. However, it is very hydrophilic (calculated solubility is 3.2E+04 mg/L at 20°C using a QSAR method) with a predicted low log Kow of 1.9. It is not surface active and the silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure 8.7E-3 Pa at 25°C).The hydrolysis product (cyclohexyl(methyl)silanediol) is not expected to dissociate within the relevant environmental range.
Reference:
PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003
Additional information
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