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EC number: 246-467-6
CAS number: 24801-88-5
concentration = 5x10-4 M (~110 mg/L). The
concentration was not directly measured; rate constants were extracted
from changes in analytical response for each component.
Table 1. Observed
rate constants for hydrolysis reactions of APTES
T / °C
k1* 104. s-1(uncertainty)
k2* 104. s-1
k3* 104. s-1
Effect of pH on the hydrolysis
For an acid-base catalysed
hydrolysis reaction in aqueous buffered solution, the measured rate
constant kobs is described by the general equation:
kobs = k0+ kH3O+[H3O+]
+ kOH-[OH-] + ka[acid] + kb[base]
where k0 refers to the
spontaneous reaction with water and the latter two terms provide for
possible catalysis by the conjugate acid and base of the particular
buffer. kH3O+ and kOH- are the acid and base
catalysed rate constants.
In order to understand the effect
of pH on the stepwise hydrolysis of the test substance, a series of
kinetic runs were conducted over a range of pH using acetate and Tris
buffers of varying concentration. Varying the concentration of the
buffer allows its catalytic effect to be elucidated; this is mainly
interesting in terms of its impact on the investigation of pH effects.
Non-linear regression analysis was used (as discussed in the methods
section) to determine values of k1 and k2 and, if
possible, k3 for each experiment corresponding to a
particular pH and buffer composition. The results are shown in Table 1
Multiple linear regression
analysis was then used to model the effect of hydronium or hydroxide ion
concentration and buffer concentration on the observed rates of
hydrolysis. The results are shown in Table 2 (for pH 4.7 -5.9, dominated
by hydronium ion catalysis) and Table 3 (for pH , dominated by hydroxide
Table 2: Results of multiple
linear regression analysis of APTES kinetic experiments in the pH range
4.7 -5.8 at 24.7°C
Table 3: Results of multiple
linear regression analysis of APTES kinetic experiments in the pH range
7.0 -9.0 at 24.7°C
pH 9.0 data not included in the model for k2 due to poor
initial fit with a large standardized residual for this observation.
b Final hydrolysis step
was too rapid to measure quantitatively
It can be seen from the above
tables that [H3O+] and [OH-] are very
significant (P<0.01) in all cases. The coefficients are the second order
catalytic constants, kH3O+ and kOH-, for the
first, second and (for kH3O+) third hydrolysis steps. At the
higher pH, the third hydrolysis step was too rapid to measure
quantitatively in all cases. The adjusted r2 for the final
model is >0.98 in all cases.
The buffer concentrations,
described by [HOAc] and [Tris] were found to be significant, indicating
that buffer catalysis is occurring.
kH3O+ and kOH-
both increase for successive hydrolysis steps, with kOH-
increasing to a much greater extent.
A statistically significant
intercept term (P<0.1) was obtained for the intercept of k1
in the higher pH experiments. This represents k0 for the
first hydrolysis step.
There was good agreement between
measure values of k1, k2 and k3 and
those predicted based on the linear regression analyses from the two
catalytic regimes. This indicates that the model results accurately
represent the experimental data and that the chosen variables account
for most of the variance in the data.
Effect of temperature on the
To determine the effect of
temperature on the rate of hydrolysis of the parent silane and the
intermediate hydrolysis products, additional kinetic runs were made at
10 and 35°C
and pH 4.7 and 9.0, using the lowest buffer concentrations from the
runs. Under these conditions, the change in the observed rate constant
with temperature should relate predominantly to the specific acid and
base catalysed mechanisms.
The results are shown in Table 1 above. These rate constants were used
to construct a series of three-point Arrhenius plots from which
pre-exponential factors (A) and activation energies (Ea) were estimated
for the specific acid and base catalysed reactions. The results are
given in Table 4 below.
4: Arrhenius parameters for the Hydronium and Hydroxide Ion Catalyzed
Hydrolysis Reactions of APTES
For each plot r2 was
found to exceed 0.99, suggesting that a single dominant reaction pathway
(ie specific acid or specific base catalysis) is being observed at each
extreme of pH. There is not enough data to draw conclusions on the
significance of the variation in Ea among the stepwise
reactions, although they do appear to be very similar. However, it is
clear that the activation energies are approximately a factor of 2
larger for the hydroxide catalysed reaction.
The Arrhenius parameters can be
used with the previously discussed catalytic constants to predict t1/2
for the disappearance of the test substance as a function of pH and
temperature at zero buffer concentration. This is shown in Figure 5
(attached) for the three temperatures examined during the study. It
should be noted that k0 is only included in the 25°C curve as
the temperature dependence of this reaction pathway has not been
determined. Therefore, the other curves represent conservative estimates
of half-life particularly in the pH region where the rate is near
The kinetics of the hydrolysis
reactions of 3-aminopropyltriethoxysilane in dilute aqueous solution
were characterized over a range of environmentally relevant pH and
temperature. The results are consistent with a series of consecutive
pseudo-first order reactions having an increasing rate for each
subsequent hydrolysis step (k1<k2<k3). Reaction rates are strongly
influenced by pH, with catalysis by hydroxide ion being 5 times more
effective than hydronium ion at promoting hydrolysis of the parent
trialkoxysilane; this discrepancy increases for the subsequent reactions
leading to formation of the silanetriol. In addition, the contribution
of the solvent catalysed reaction, k0, is significant to the
overall rate of hydrolysis of ATPES extrapolated to zero buffer
concentration. Given that the first hydrolysis reaction, k1,
is rate limiting and using 10 half-lives as the definition of
"complete", this study indicates that the trialkoxysilane will be
exhaustively hydrolysed to the silanetriol in ≤4.5
days at 25°C.
-0.0858x + 3.99
-0.182x + 3.87
-0.276x + 4.00
Correlation factor [r2]
Reaction rate constant kobs [1/min]
8.58 x 10-2
1.82 x 10-1
2.76 x 10-1
Half life T½ [min]
Confidence interval of half lifeT½ [min]
7.42 to 8.71
3.40 to 4.21
2.18 to 2.85
-0.0574x + 4.04
-0.142x + 3.97
-0.322x + 4.06
5.74 x 10-2
1.42 x 10-1
3.22 x 10-1
Half lifeT½ [min]
11.9 to 12.3
4.41 to 5.34
2.00 to 2.30
-0.120x + 3.96
-0.360x + 3.76
-0.0156x + 4.06
1.20 x 10-1
3.60 x 10-1
2.60 x 10-4
5.30 to 6.25
1.73 to 2.12
0.72 to 0.76
hydrolysis of 2,2,4(or 2,4,4)-trimethylhexane-1,6-diisocyanate was
studied according to OECD Test Guideline 111 (2004) and Council
Regulation (EC) No. 440/2008, Method C.7 with a test item concentration
of 100 µg/L in buffer solutions of pH 4, 7 and 9 at temperatures of 10,
20 and 30 °C. Rapid hydrolysis following pseudo-first order kinetics
with the following half-lives was observed:
4: 8.08 min (10°C), 3.81 min (20°C), 2.51 min (30°C)
7: 12.1 min (10°C), 4.88 min (20°C), 2.15 min (30°C)
9: 5.78 min (10°C), 1.93 min (20°C), 0.74 min (30°C).
the base catalyzed hydrolysis is the fastest hydrolysis process.
Hydrolysis half-life (isocyanate
group): 3.81 min at pH 4, 4.88 min at pH 7 and 1.93 min at pH 9 and 20°C
(OECD 111) based on read-across from 2,2,4(or
(triethoxysilane group): 0.8 h at pH 5, 8.5 h at pH 7, and 0.15 h at pH
9 and 24.7°C (OECD 111)
There are no reliable measured
hydrolysis data for the submission substance. The substance,
triethoxy(3-isocyanatopropyl)silane has two types of hydrolysable
groups, triethoxy (-OCH3CH2) and isocyanate
(-N=C=O). The isocyanate group is expected to hydrolyse very rapidly in
contact with water, for example the hydrolysis half-lives of 2,2,4(or
2,4,4)-trimethylhexane-1,6-diisocyanate were measured in accordance with
OECD Test Guideline 111 and in compliance with GLP (Lange 2013). Very
rapid hydrolysis following pseudo-first order kinetics with the
following half-lives was determined:
pH 4 - 8.08 min at 10°C, 3.81 min
at 20°C and 2.51 min at 30°C
pH 7 - 12.1 min at 10°C, 4.88 min
at 20°C and 2.15 min at 30°C
pH 9 - 5.78 min at 10°C, 1.93 min
at 20°C and 0.74 min at 30°C
isocyanate (CAS 111-36-4) was reported to undergo complete hydrolysis in
water within a few minutes at 20°C (OECD 2005).
For the registration substance,
this means very rapid hydrolysis to form 3-aminopropyltriethoxysilane
(CAS 919-30-2), as an intermediate hydrolysis product, and carbon
dioxide. The hydrolysis half-lives of 3-aminopropyltriethoxysilane have
been measured in accordance with OECD 111 to be 0.8 h at pH 5, 8.5 h at
pH 7, and 0.15 h at pH 9 and 24.7°C (Dow
Corning Corporation 2001a). The quoted results relate to
disappearance of parent substance. The measured result is supported by
predicted hydrolysis half-lives of 0.4 h at pH 4, 0.4 h at pH 5 and 0.1
h at pH 9 and 20-25°C using a validated QSAR estimation method. In Beari et
al (2001), a hydrolysis half-life of <1 hour at pH 6 was reported
for the substance.
In the measured study conducted
for the intermediate hydrolysis product, two intermediates and the final
hydrolyis product were observed and quantified. The substance was found
to hydrolyse according to the following reaction scheme:
RSi(OEt)3 → RSi(OEt)2(OH)
→ RSi(OEt)(OH)2 → RSi(OH)3
One mole of ethanol is released at
each hydrolysis step.
Estimates for the rate constants
for the first, second and third reaction steps (k1, k2 and
k3 respectively) were obtained; over the range of pH and
temperature investigated the intermediate silanol products were found to
hydrolyse more rapidly than the original trialkoxysilane. Consequently,
these intermediates can be considered transient. The
rate constant values obtained for the hydroxonium ion catalysed reaction
are: k1 = 23.1 M-1s-1, k2
= 71.1 M-1s-1, k3 =
132 M-1s-1. The values obtained for the hydroxide
ion catalysed reaction are: k1 = 125 M-1s-1,
k2 = 1130 M-1s-1, k3 =
not measured (as the reaction was too fast).
The concentration of each
hydrolysis product has been plotted against time (expressed as number of
half-lives for degradation of parent substance) for the acid catalysed
reaction. The parent
compound dominates during the time span <1 half-life of the parent
compound. The final hydrolysis product starts to dominate after
approximately 1.5 half-lives of the parent compound have passed. After
approximately 4 half-lives, the final hydrolysis product represents 90%
of the compound present. Under basic or neutral conditions, the
concentrations of the intermediate hydrolysis products reach lower
maxima and begin to decrease more quickly because the ratios of k2
and k3 to k1 are greater than for
the acid catalysed reaction.
The pH dependence of the
hydrolysis kinetics was investigated, by carrying out experiments at a
range of pH values between 4.7 and 9.0. The reaction rate was found to
be slowest at pH 6.5 - 7 and increase as the pH was raised or lowered.
Estimates of kH3O+ (the hydroxonium ion catalysed rate
constant) for the first, second and third reaction steps; kOH- (the
hydroxide ion catalysed rate constant) for the first and second reaction
steps; and k0 (the solvent catalysed rate constant) for the
first reaction step were obtained. These can be used to estimate the
reaction rate at any pH for zero buffer concentration.
The temperature dependence of the
hydrolysis kinetic was investigated by carrying out experiments at 10,
25 and 35°C.
The reaction rate was found to increase with temperature, to a greater
extent for the hydroxide catalysed reaction than the hydronium ion
catalysed reaction. Arrhenius parameters were calculated for the
hydronium and hydroxide calculated reactions and these can be used to
estimate the reaction rate at any temperature.
The authors of this summary have
used the rate constants and Arrhenius parameters quoted in the study
report to calculate the half-lives in the table below for a range of
relevant temperature and pH values.
Table1: Estimated half-lives at
a range of temperature and pH values.
calculated value is 2 s. However,
it is not appropriate or necessary to attempt to predict accurately when
the half-life is less than 5-10 seconds. Therefore, the value is
reported as 5 s.
The measurements in the study were
taken at pH 4.7 - 9.0 and 10 - 35°C;
therefore, the estimates at 37.5°C
and pH 2 represent extrapolations. The difference between 35°C
is very small, so this extrapolation is not considered to add
significant uncertainty to the results. The estimated result at pH 2
does represent an extrapolation significantly outside the range of pH
values studied (4.7 - 9.0). However, the study results are strongly
supportive of a hydroxonium ion catalysed reaction being dominant in the
acid pH range (4.7-5.9) and the reaction rate increasing as the
hydroxonium ion concentration increases (pH decreases). Therefore, it is
extremely unlikely that the hydrolysis at pH 2 is slower than that at pH
The ultimate product of the
hydrolysis reaction under dilute condition is 3-aminopropylsilanetriol.
The other hydrolysis products are ethanol and carbon dioxide.
The hydrolysis of other substances
used for read-across in other sections are discussed below.
Hydrolysis of the
read-across substance 3-aminopropyltriethoxysilane (CAS 919-30-2)
919-30-2) is the intermediate hydrolysis product of
triethoxy(3-isocyanatopropyl)silane. Therefore, available data for
3-aminopropyltriethoxysilane (CAS 919-30-2) are read across to the
submission substance triethoxy(3-isocyanaotopropyl)silane for the
following endpoints, short-term toxicity to fish, short-term toxicity to
aquatic invertebrates, toxicity to aquatic algae, repeated dose
toxicity: oral, toxicity to reproductive toxicity and developmental
toxicity. Following the very rapid hydrolysis of the isocyanate group in
triethoxy(3-isocyanatopropyl)silane, the read across substance;
3-aminopropyltriethoxysilane is the intermediate hydrolysis product.
Further information on the
hydrolysis half-lives of 3-aminopropyltriethoxysilane are discussed as
part of the hydrolysis of the submission substance above.
Hydrolysis of the
read-across substance, N-[3-(trimethoxysilyl)propyl]ethylenediamine (CAS
Data for the substance,
N-[3-(trimethoxysilyl)propyl]ethylenediamine (CAS 1760-24-3) are
read-across to the submission substance, triethoxy(3-isocyanatopropyl)silane
for the long-term toxicity to aquatic invertebrates endpoint. The
hydrolysis half-lives and the silanol hydrolysis products of the two
substances are relevant to this read-across, as discussed in the
appropriate section for the endpoint.
hydrolysis half-lives at 24.7°C of 0.1 h at pH 4, 0.32 h at pH 5, and
0.025 h at pH 7 were determined in accordance with OECD 111 (Dow Corning
Corporation 2001b). At
pH >7, the half-life became too rapid (<90 s) to measure using the
methodology of this study. In
other secondary sources to which reliability could not be assigned, a
hydrolysis half-life of 0.016 h at pH 7 at 24.7°C and a hydrolysis
half-life of 24.1 h at 25°C (information on pH was not stated) were
complete hydrolysis of CAS 1760-24-3 involves consecutive removal of the
three methoxy groups; it is therefore a three-step process. The quoted
half-lives refer to degradation of parent substance. In addition,
separate rate constants for the threeconsecutive
hydrolysis reactions have been measured. For the acid catalysed rate
constants, the second and third reaction steps were found to be
approximately twice as fast as the previous step (k1<k2<k3).
For the base catalysed rate constants, the second step was found to be
approximately 1.5-fold slower than the first step which was about the
same as the third step (k2<k1≈k3).
Therefore, rapid formation of the final product is expected across the
hydrolysis products areN-(3-(trihydroxysilyl)propyl)ethylenediamine
(1 mole) and methanol (3 moles).
Hydrolysis of the
read-across substance 3-(trimethoxysilyl)propyl isocyanate (CAS
for the substance, 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6)
are read-across to the submission substance,
triethoxy(3-isocyanatopropyl)silane for the following endpoints;
short-term toxicity to fish and short-term toxicity to aquatic
invertebrates. The silanol hydrolysis product and the rate of hydrolysis
of the two substances are relevant to this read-across, as discussed in
the appropriate sections for each endpoint.
isocyanate group is expected to hydrolyse very rapidly to form 3-aminopropyltrimethoxysilane
(CAS 13822-56-6) as an intermediate hydrolysis product and carbon
dioxide. The hydrolysis half-lives of 3-aminopropyltrimethoxysilane have
been predicted using a validated QSAR estimation method to be 0.2 h at
pH 4, 0.3 h at pH 5, 2.6 h at pH 7, and 0.1 h at pH 9 and 20-25°C. Also, hydrolysis
half-life of (3-isocyanatopropyl)trimethoxysilane was found to be much
less than 2.4 h at pH 4, pH 7 and pH 9 and 50°C. Therefore, the
half-lives at 25°C and pH 4, pH 7 and pH 9, were estimated to be <1 day,
which is consistent with the expected behaviour. The study was conducted
according to OECD Test Guideline 111 (1981) and in compliance with GLP;
the results are considered reliable. However, according to the most
recent version of the test guideline, a higher-tier test should be
carried out if a substance is found to be unstable in the preliminary
test and this was not done in this study. However, in view of the
extreme instability of the isocyanate group, in practice it may not be
technically feasible to do so and obtaining a more accurate half-life
value would be of limited use.
hydrolysis products are 3-aminopropylsilanetriol,
methanol and carbon dioxide.
The table below summarises
all relevant hydrolysis half-lives used in this chemical safety
Table: Summary of relevant hydrolysis half-lives
Half-lives at 20-25°C
Half-lives at 37.5°C
2,2,4 (or 2,4,4)-Trimethylhexane-1,6-diisocyanate
0.1 – 1 hour
0.15 – 3 hour
Lange (2013).Lange, J.
(2013). Vestanat TMDI - hydrolysis as a function of pH. Dr. U.
Noack-Laboratorien, Sarstedt (Germany). Test report. Testing laboratory:
Dr. U. Noack-Laboratorien, Sarstedt (Germany). Report no.: CPH14411.
Owner company: Evonik lndustries AG. Report date: 2013-01-21.
OECD (2005). SIDS Initial
Assessment Report for SIAM 21, Washington, 18-21 October 2005, n-Butyl
isocyanate, CAS 111-36-4.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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