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EC number: 232-136-3 | CAS number: 7787-93-1
Phototransformation in air
Administrative data
Link to relevant study record(s)
Description of key information
Key value for chemical safety assessment
Additional information
Dichloro(3-chloropropyl)methylsilane and its hydrolysis product (3-chloropropyl)methylsilanediol contain no chromophores that would absorb visible or UV radiation, so direct photolysis is not likely to be significant. Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur.
The SRC AOPWIN program (v1.92) has been used to obtain values of the rate constant kOHfor reaction of dichloro(3-chloropropyl)methylsilane and (3-chloropropyl)methylsilanediol with hydroxyl radicals. This prediction method has not been validated to assess applicability to siloxanes/silanes/silanols; therefore, there is uncertainty associated with the calculated values obtained.
The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:
kdegair(d-1) = kOH(cm3/molecule.sec) x OH Concair(molecules/cm3) x 24 x 3600
DT50(d) = ln 2/ kdegair(d-1)
Where:
kdegair= total rate constant for degradation in air
kOH= rate constant for reaction with hydroxyl radicals
OH Concair= concentration of hydroxyl radicals in air =5 x 105OH molecules/ cm3
DT50= half-life
The concentration of hydroxyl radicals in air of 5 x 105OH molecules/ cm3, and the 24 hour photoperiod, are the values specified in the Technical Guidance Document (EC, 2003a).
The results are given inTable4.1.3.
Table4.1.3: Results of photodegradation in air calculations
Parameter |
Result, dichloro(3-chloropropyl)methylsilane |
Result, (3-chloropropyl)methylsilanediol |
kOH(cm3/ molecule.sec) |
2.3 x 10-12 |
9.2 x 10-12 |
kdegair(d-1) |
0.1 |
0.4 |
DT50(days) |
7.1 |
1.7 |
Dichloro(3-chloropropyl)methylsilane is very hydrolytically unstable. Therefore, reaction with water vapour, rather than photodegradation, is expected to be the primary degradation process in air.
This interpretation is supported by a simulated nose-only exposure study (Dow Corning Corporation 2013) with a chlorosilane substance. The study was conducted to determine the hydrolytic stability of dichlorodimethylsilane (CAS 75-78-5) under conditions typical of nose-only vapour inhalation exposures. The estimated hydrolysis half-life at pH 7, 22°C and 57% RH was in the range 3-11 seconds (half-life calculated from the data by the reviewer of the report for REACH technical dossier). 57% RH is within the typical range of RH of indoor/outdoor air. This half-life is similar to the measured half-life for dichlorodimethylsilane in water: 1-5 seconds at 22°C (result extrapolated to 22°C from 1.5°C by the reviewer of the report for REACH technical dossier).
Reference: Dow Corning Corporation (2013). Hydrolytic stability of Dimethyldichlorosilane Under Conditions Typical of Nose-Only Vapor Inhalation Exposures. Dow Corning Corporation Health and Environmental Sciences Technical Report. Study Number 12356-116
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