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EC number: 257-473-3
CAS number: 51851-37-7
Hydrolysis half-life: 1.44 h at 20°C, 0.9 h at 30°C, 0.474 h at 40°C and pH 4, 12.5 h at 20°C, 5.25 h at 30°C, 2.74 h at 40°C and pH 7, 5.18 h at 20°C, 2.61 h at 30°C, 1.38 h at 40°C and pH 9 (OECD 111)
hydrolysis half-lives of the substance have been measured in accordance
with OECD Test Guideline 111 and in compliance with GLP.
The hydrolysis half-lives of the
pH 4.0 = 1.44 h at 20°C, 0.9 h at
30°C, 0.474 h at 40°C
pH 7.0 = 12.5 h at 20°C, 5.25 h at
30°C, 2.74 h at 40°C
pH 9.0 = 5.18 h at 20°C, 2.61 h at
30°C, 1.38 h at 40°C
the hydrolysis reaction may be acid or base-catalysed, the rate of
reaction is expected to be slowest at around pH 7 and increase as the pH
is raised or lowered. For an acid-base catalysed reaction in buffered
solution, the measured rate constant is a linear combination of terms
describing contributions from the uncatalysed reaction as well as
catalysis by hydronium, hydroxide, and general acids or bases.
k0+ kH3O+[H3O+] + kOH-[OH-]
+ ka[acid] + kb[base]
extremes of pH and under standard hydrolysis test conditions, it is
reasonable to suggest that the rate of hydrolysis is dominated by either
the hydronium or hydroxide catalysed mechanism.
at low pH:
pH 4 [H3O+] = 10-4 mol dm-3 and
at pH 2 [H3O+] = 10 -2 mol dm-3;
therefore, kobs at pH 2 should be approximately 100 times
greater than kobs at pH 4.
half-life of a substance at pH 2 is calculated based on:
2) = t1/2(pH 4) / 100
calculated half-life of the substance at pH 2 is therefore 0.0144 hours
(approximately 52 seconds) at 20°C.
rate increases with temperature therefore hydrolysis will be faster at
physiologically relevant temperatures compared to standard laboratory
conditions. Under ideal conditions, hydrolysis rate can be recalculated
according to the equation:
= DT50(T°C) * e(0.08.(T-X))
T = temperature for which data are available and X = target temperature.
[2-(perfluorohexyl)ethyl]triethoxysilane the hydrolysis half-life at
37.5ºC and pH 7 (relevant for lungs and blood) is approximately 3.1
hours. At pH 7 and 40°C, measured half-life is 2.7 hours. At
37.5ºC and pH 2 (relevant for conditions in the stomach following oral
exposure), the half-life is 0.0036 hours (approximately 13 seconds).
hydrolysis products in this case are [2-(perfluorohexyl)ethyl]silanetriol
The hydrolysis half-lives of
substances used for read-across in other areas are discussed below:
Hydrolysis of the
read-across substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane (CAS
Data for the substance
No 73609-36-6) are read-across to the submission substance
[2-(perfluorohexyl)ethyl]triethoxysilane (CAS No 51851-37-7) for
appropriate endpoints (see
Section 1.4 of the CSR). The hydrolysis half-life and the silanol
hydrolysis product of the two substances are relevant to this
read-across, as discussed in the appropriate Sections of the CSR for
[2-(perfluorohexyl)ethyl]dichloro(methyl)silane, hydrolysis half-lives
are read-across from analogous dichlorosilane substances. Hydrolysis
half-lives of 10 seconds at pH 4, 17 seconds at pH 7 and 7 seconds at pH
9 and 1.5°C were determined for dichloro(dimethyl)silane in accordance
with OECD 111 (Dow Corning Corporation 2001).
half-lives of 8 seconds at pH 4, 12 seconds at pH 7 and 9 seconds at pH
9 and 1.5°C were determined for
dichloromethyl(3,3,3-trifluoropropyl)silane in accordance with OECD 111
(Dow Corning Corporation 2001).
half-lives of 6 seconds at pH 4, 10 seconds at pH 7 and 8 seconds at pH
9 and 1.5°C were determined for dichloro(diphenyl)silane in accordance
with OECD 111 (Dow Corning Corporation 2001). Hydrolysis half-lives of
<1 minute at pH 4, pH 7 and pH 9 and 1.5°C were determined in accordance
with OECD 111 (Dow Corning Corporation 2001 ).
Measured hydrolysis half-lives of
<<27 mins at pH 4, pH 7 and pH 9 and 27°C were determined for
dichlorocyclohexylmethylsilane in a study conducted according to
generally acceptable scientific principles (Haas 2012). Only a
preliminary study was carried out and a more precise knowledge of the
half-life is needed for use in the chemical safety assessment.
Given the very rapid hydrolysis
rates in water ( ≤17 seconds at 1.5°C and pH 4, 7 and 9) observed for
all tested dichlorosilanes, and the lack of significant variation in the
half-lives for the different substances, it is considered appropriate to
read-across this result to
rate of hydrolysis is faster at increased temperature, at ambient
conditions (20 - 25°C),
relevant to the environment, the hydrolysis half-lives are expected to
be faster. Therefore, half-lives at pH 2 and 25°C, at pH 7 and 37.5°C
and at pH 2 and 37.5°C, as a worst case considered to be approximately 5
The hydrolysis products are
[2-(perfluorohexyl)ethyl]methylsilanediol and hydrochloric acid.
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