Chemistry Letters Vol.34, No.4 (2005)
461
Table 1. Rate constants of reaction between Ti–OH groups and
Si–H groups in the presence of molecules containing an OH group
at 323 K
2
1
0
.5
(
b)
2
Molecule containing
an OH group
2-Butoxyethyl Propyl Isopropyl t-Butyl
alcohol
Water
alcohol alcohol alcohol
.5
1
2
ꢀ1
ꢀ1
(
a)
kapp ꢁ 10 /min
0.16
1.8
0.25
2.7
0.34
3.8
0.8
8.9
0.83
9.2
k/gꢃmolꢀ ꢃmin
1
pKSH
14.0
19.4
20.3
.5
0
(c)
kapp: Apparent rate constant of reaction in the presence of alcohol, obtained
from the slope of the plot of lnð1=ð1 ꢀ xÞ against time.
k: Rate constant of reaction, calculated by dividing kapp by the initial con-
centration of Si–H groups.
0
100
200
300
400
500
600
Time / min
1
2
KSH: Self dissociation constant.
Figure 2. Plots of lnð1=ð1 ꢀ xÞÞ against reaction time at 323 K for
a) 10 wt % water-containing TiO2 without pretreatment, (b)
wt % water-containing TiO2 preheated at 393 K for 12 h, and
c) 10 wt % water-containing TiO2 in solvent with added water.
Figure 2a. From the slopes in the fast and slow reaction regions
ꢀ1
(
2
(
ꢀ1
shown in Figure 2, values of k( free) ¼ 133:3 gꢃmol ꢃmin and
ꢀ1
ꢀ1
k(H O) ¼ 1:8 gꢃmol ꢃmin are obtained at 323 K. Thus, k( free)
2
is two orders of magnitude larger than k(H O).
2
with BTMS (5.0 g, Si{H ¼ 22:5 mmol) in toluene (16.8 g) at
23 K. The plot of lnð1=ð1 ꢀ xÞÞ against time for this reaction
Figure 2a) indicates the existence of two distinct reaction re-
gions; an initial reaction region with relatively high reaction rate,
and a subsequent slow reaction region.
This reaction mechanism was further investigated by exam-
ining the effect of alcohol, which also contains OH, on the reac-
tion rate. Various alcohols were used as solvents in place of tol-
uene. From the plot of lnð1=ð1 ꢀ xÞÞ against time (not shown),
the rate constants of the reaction between alcohol-solvated Ti–
OH groups and Si–H groups were obtained, as listed in
Table 1. The rate constants are in agreement with the acid
strengths (electrophilicity) of alcohol. Strong solvation of the
Ti–OH groups by alcohol results in a substantial decrease in
the nucleophilicity of the Ti–OH groups and a corresponding
suppression of the reaction rate. This effect is considered to be
due to restriction of the reactivity of Ti–OH groups as a result
of hydrogen bonding with the alcohol.
In conclusion, kinetic study of the solid–liquid reaction be-
tween TiO2 and H–siloxane revealed that the dehydrogenation
condensation between Ti–OH groups and Si–H groups involves
two dehydrogenation condensations; one between free Ti–OH
groups and Si–H groups, and another between hydrated Ti–OH
groups and Si–H groups. The reaction was found to be strongly
inhibited by the presence of molecules containing OH, with the
degree of inhibition dependent on the acid strength of the mole-
cule, that is, the strength of the hydrogen bond with the Ti–OH
group.
3
(
Thermogravimetric/differential thermal analysis (TG-DTA;
DTG-60A/60AH, Shimadzu) of the TiO2 powder revealed that
the powder contained about 10 wt % physically adsorbed water.
The influence of this physically adsorbed water on the reactivity
of the TiO2 powder with BTMS was therefore examined using
1
1
preheated TiO2 powder containing about 2 wt % physically ad-
sorbed water. It was observed that use of the preheated TiO2
powder clearly increased the rate of reaction in the fast initial re-
action region (Figure 2b). In contrast, the rate of reaction in the
fast initial reaction region was substantially suppressed when
water (0.1 g) was simply added before mixing the TiO2 powder
with toluene and BTMS (Figure 2c), presumablly resulting from
the increased proportion of hydrated TiOH groups.
A tentative reaction mechanism between TiO2 and BTMS
including this water suppression effect is postulated as shown
in Scheme 1. In this mechanism, the dehydrogenation condensa-
tion between the Ti–OH group and the Si–H group is assumed to
be a nucleophilic substitution reaction. In the presence of water,
the nucleophilicity of the Ti–OH group is reduced due to the for-
mation of hydrogen bonds with water, lowering the reaction rate.
This implies that there may be two types of Ti–OH groups, that
is, free Ti–OH groups that react with Si–H groups at a rate con-
stant of k( free) (fast initial reaction region), and hydrogen-bonded
Ti–OH groups that react with Si–H groups with a rate constant of
References
1
2
3
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(
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2
4
5
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δ
+
k
(free)
Si
H
: O–H
Ti
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+
H2
δ
–
6
7
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k
(free) >> k(H2O)
8
9
1
H. Fukui, Hyomen, 32, 131 (1994).
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H−O
H
0
δ
+
H
O−H
k
(H2O)
Si
H
: O–H
Ti
Si–O–Ti
+
H2
δ
–
11 The preheated TiO2 powder was obtained by heating TiO2 powder at
93 K for 12 h in air.
3
Scheme 1. Proposed mechanism of the solid–liquid reaction
between TiO2 and BTMS.
12 ‘‘Handbook of Chemistry: Pure chemistry,’’ 5th ed., ed. by the
Chemical Society of Japan (2004).
Published on the web (Advance View) February 26, 2005; DOI 10.1246/cl.2005.460
Nakabayashi, Akira
