Effect of substituents on the silicon atom on etherification of organochlorosilanes with ethanol

Article abstract of DOI:10.1023/A:1013145419084

The kinetics of etherification of organochlorosilanes with various substituents on the silicon atom with ethanol were studied. The etherification rate is mostly controlled by the inductive constants of the substituents and increases as the electron-acceptor power of the substituents increases. The ρ* value in the Taft equation for etherification of organochlorosilanes with ethanol was determined.

Full text of DOI:10.1023/A:1013145419084

Russian Journal of General Chemistry, Vol. 71, No. 7, 2001, pp. 1038 1040. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 7, 2001,
pp. 1103 1105.
Original Russian Text Copyright
2001 by Chernyshev, Belyakova, Komarov, Bykovchenko.
Effect of Substituents on the Silicon Atom on Etherification
of Organochlorosilanes with Ethanol
E. A. Chernyshev, Z. V. Belyakova, E. A. Komarov, and V. G. Bykovchenko
Gosudarstvennyi nauchno-issledovatel’skii institut khimii i tekhnologii elementoorganicheskikh soedinenii
State Research Center, Moscow, Russia
Received April 10, 2000
Abstract The kinetics of etherification of organochlorosilanes with various substituents on the silicon atom
with ethanol were studied. The etherification rate is mostly controlled by the inductive constants of the sub-
*
stituents and increases as the electron-acceptor power of the substituents increases. The
equation for etherification of organochlorosilanes with ethanol was determined.
value in the Taft
There have been only a few works [1 4] on the
kinetics and mechanism of organochlorosilane etheri-
fication, which is obviously explained by the fact that
such fast reactions are difficult to study. In the present
work we studied the kinetics of etherification of five
organochlorosilanes. We focused on the effect of
substituents on the silicon atom on the rate of this re-
action. For more reliable etherification rate constants,
we used organochlorosilanes with a monochlorinated
silicon atom: chloro(chloromethyl)diethoxysilane (Ia),
chlorodiethoxyvinylsilane (Ib), chloro(chloromethyl)-
ethoxymethylsilane (Ic), chlorodiethoxymethylsilane
(Id), and chlorotrimethylsilane (Ie). The etherifying
agent was ethanol. Compounds Ia Id were prepared
by disproportionation of the corresponding chloro-
and alkoxysilanes.
For a more detailed analysis of the substituent
effect, we used in the consideration the etherification
rate constants for chlorophenylsilanes and chloro-
methylsilanes, obtained in [2, 3]. Table 2 lists the
etherification rate constants for 10 organochlorosila-
nes with various substituents on the silicon atom.
To assess the effect of inductive factors in this
reaction, we turned to the simplest Taft equation for
compounds with several substituents [7, 8].
*
*
log (k/k0) =
.
*
Here k/k0 is the ratio of rate constants, and
is
the sum of the inductive constants of the substituents.
This equation is known to reasonably fit many organo-
*
silicon reactions [8]. Table 2 also contains
log (k/k0) values. The
and
*
RSiCl3 + 2RSi(OEt)3
3RSi(OEt)2Cl,
values for Cl (2.94), OEt
(1.366), ClCH2 (1.05), Vin (0.653), Ph (0.600), and
Me (0.00) were taken from the handbook [9]. For
R = Vin, ClCH2, Me;
ClCH2(Me)SiCl2 + ClCH2(Me)Si(OEt)2
ClCH2(Me)Si(OEt)Cl.
Table 1. Rate constants of the direct (k) and reverse (k )
reactions of etherification of organochlorosilanes with
ethanol at 20 0.5 C
The etherification of organochlorosilanes occurs
with hydrogen chloride evolution.
1
Rate constant, l mol 1 s
Equilibrium
constant
K
R1R2R3SiCl + EtOH
R1R2R3SiOEt + HCl.
Comp.
no.
The concentration of HCl was determined conduc-
tometrically. The kinetic investigation was performed
by the stopped-flow technique [5, 6]. The etherifica-
tion rate constants of compounds Ia Ie are listed in
Table 1. As seen from the table, the rate constants
k
k
Ia
Ib
Ic
7.56
4.79
2.91
2.43
0.766
0.890
0.771
0.460
0.932
8.49
6.21
6.33
2.61
vary in a fairly wide range: from 7.56 l mol 1 s for
Id
1
compound Ia to 0.766 l mol 1 s for compound Ie.
Iea
1
Obviously, the reactivity of organochlorosilanes is
determined by substituents on the silicon atom.
a
k and K were not determined.
1070-3632/01/7107-1038$25.00 2001 MAIK Nauka/Interperiodica

EFFECT OF SUBSTITUENTS ON THE SILICON ATOM ON ETHERIFICATION
1039
the standard reaction with the rate constant k0 we took
therification of Me3SiCl.
Table 2. Effect of substituents R1, R2, and R3 on etherifi-
cation of organochlorosilanes R1R2R3SiCl with ethanol
k0
Run
no.
k,
*
Me3SiCl + EtOH
Me3SiOEt + HCl.
R1
R2
R3
log (k/k )
1
l mol 1 s
0
The figure shows the plot of log (k/k0) on
*. It
1
2
3
4
5
6
7
8
9
Me
CH2Cl Me
Me
Vin
CH2Cl OEt OEt
Me
Ph
Me
Ph
Me
Me
OEt
0
0.766a
2.91a
2.43a
4.79a
7.56a
5.84b
12.1c
20.5b
13.7c
0.343c
0
is seen that for most compounds the correlation is
fairly good. The only point that falls out of the correla-
tion is point 10 for Ph(OEt)2SiCl, a compound that
has the rather bulky phenyl substituent. Apparently,
for this compound we should use a full Taft equation
which takes account of both the inductive and steric
(Es) substituent constants. Since Ph(OEt)2SiCl definite-
ly falled out of the general regularity, we did not
include it in the corelation.
2.416
2.733
3.385
3.782
4.266
4.866
5.800
6.400
3.332
0.580
0.502
0.796
0.994
0.882
1.198
1.428
1.252
0.350
OEt OEt
OEt OEt
OEt Cl
OEt Cl
Cl
Cl
Cl
Cl
10
Ph
OEt OEt
The Taft equation for the compounds listed in
a
Table 2 [except for Ph(OEt)2SiCl)], calculated with
Notes: The k value was determined in:
the present work,
*
b
c
the log (k/k0) and
values (Table 2), has the fol-
in [3], and
in [2].
lowing form (correlation coefficient r 0.97, rms devia-
tion s 0.020).
1
3.38, 4.79, and 6.30 l mol 1 s , respectively. With
these rate constants, we obtained the following
*
Arrhenius equation: k = 104.30
e
20.3/RT, l mol 1 s ,
1
log (k/k0) = 0.014 + 0.221
.
and estimated the activation energy at 20.3 kJ/mol.
*
The positive
value (0.221) gives evidence for
the mechanism of bimolecular nucleophilic substitu-
tion at silicon SN2 Si, proposed by Sommer [10]. It
follows from the above equation that the rate of etheri-
fication of organochlorosilanes is mostly controlled
by the inductive effects of substituents at the silicon
atom. Thus, the stronger electron-acceptors are the
substituents, the faster is etherification.
EXPERIMENTAL
The kinetics of etherification were studied by a
stopped-flow technique [5] on the apparatus described
in [6]. Solutions of the reactants (organochlorosilanes
and ethanol) were simultaneously delivered with two
syringes into the reaction cell equipped with an elec-
troconductivity detector. The concentration of HCl
evolved was determined conductometrically with a
high-speed conductometer specially designed for the
The revealed regularity readily explains the known
fact that the rate of etherification of organochloro-
silanes decreases (k1 > k2 > k3) with successive sub-
stitution of chlorines by ethoxy groups [2 4].
4
apparatus used (scan time 10 s).
Kinetic measurements were performed with a large
excess of alcohol; therefore, computer treatment of
k1
RSiCl3 + EtOH
RSi(OEt)Cl2 + HCl,
RSi(OEt)2Cl + HCl,
(1)
(2)
(3)
log (k/k0)
k2
8
RSi(OEt)Cl2 + EtOH
RSi(OEt)2Cl + EtOH
1.5
7
k3
5
9
RSi(OEt)3 + HCl.
1.0
0.5
0
4
6
2
Reactions (1) (3) involve successive subsitution of
chlorines ( 2.9) by ethoxy groups ( 1.366);
thus reduced, and the reaction slows down. Therefore,
the last stage, reaction (3), is the slowest and, con-
sequently, limiting stage. Therewith, the first two re-
actions are almost irreversible, and the last stage is
reversible [2].
*
*
* is
3
1
10
4.0
for etherification of organochloro-
0.5
*
0
2.0
6.0
*
The etherification rate only slightly depends on
temperature. Thus, the rate constants of etherification
of VinSi(OEt)2Cl with ethanol at 8, 20, and 30 C are
Plot log (k/k ) on
0
silanes with ethanol. The point numbers correspond to
the entry numbers in Table 2.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 7 2001

1040
CHERNYSHEV et al.
the conductometric data gave apparent first-order rate
constants (kapp). The computer treatment was per-
formed using a special program. The computed kapp
values nicely fitted those estimated manually by the
Guggenheim method [11].
The reaction temperature was maintained within
0.5 C. The solutions were prepared using ultrapure
grade acetone distilled over P2O5 and handled over
3
molecular seives.
REFERENCES
The k values listed in Table 1 were determined by
the equation kapp = kcROH (k is the etherification rate
constant and cROH is the concentration of ethanol)
using the kapp values obtained at the reaction onset,
when the reverse etherification reaction could be
neglected.
1. Allen, A.D., Charlton, J.C., Eaborn, C., and Mode-
na, G., J. Chem. Soc., 1957, no. 8, pp. 3668 3670.
2. Utkin, O.V. and Kolobkov, V.S., Zh. Prikl. Khim.,
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sil’ev, O.A., Zh. Prikl. Khim., 1986, vol. 53, no. 3,
pp. 653 655.
For the reactions of compounds Ia Id from the
equilibrium chemical compositions of the starting and
final products we determined equilibrium constants
K. From the equation K = k/k (k and k the rates of
the direct and reverse reactions) we obtained the k
values listed in Table 1. The errors in k, k , K for a
95% confidence level did not exceed 12%.
4. Speier, J.L. and Tzou, M.S., Organometallics, 1993,
vol. 12, no. 5, pp. 1981 1982.
5. Caldin, E.F., Fast Reaction in Solution, Oxford:
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Compounds Ia Id were prepared by disproporti-
onation from the corresponding chlorosilanes and
ethoxysilanes in a 1:2 molar ratio at room tempera-
ture (1 2 days) and isolated by rectification on a
column of a 10000 plates efficiency. Compound Ie
was isolated from the products of direct synthesis
of chloromethylsilanes. Further purification, when
required, was performed by preparative GLC. The
purity of compounds used in kinetic studies was no
less than 99% (by GLC).
7. Taft, R.U., Steric Effects in Organic Chemistry,
Newman, M.S., Ed., New York: Wiley, 1956. Trans-
lated under the title Prostranstvennye effekty v
organicheskoi khimii, Moscow: Inostrannaya Litera-
tura, 1960, pp. 562 686.
8. Mileshkevich, V.P. and Novikova, N.F., Usp. Khim.,
1981, vol. 50, no. 1, pp. 85 110.
9. Spravochnik khimika (Chemist’s Handbook), Nikol’-
skii, B.P., Ed., Moscow: Khimiya, 1965, vol. 3,
pp. 954 955.
Gas chromatography was performed on an LKhM-
72 chromatograph, detector katharometer, carrier gas
helium (60 ml/min). Stainless-steel columns (0.3
200 cm) were packed with Chromaton N-AW-DMCS
(0.25 0.31 mm); liquid phase E-301 or PMS-20000
(15 wt %). Injector temperature 390 C, detector
temperature 300 C. The oven temperature was prog-
rammed from 30 to 200 C at a rate of 15 deg/min.
Kinetic measurements were performed with acetone
solutions of organochlorosilanes and ethanol (0.01
0.02 and 0.9 1.1 M, respectively).
10. Sommer, L.H., Stereochemistry, Mechanism, and
Silicon, New York: McGraw-Hill, 1965. Translated
under the title Stereokhimiya i mekhanizmy reaktsii
kremniiorganicheskikh soedinenii, Moscow: Mir,
1966, pp. 78 98.
11. Polekhin, A.M., Baranov, M.K., Loshadkin, N.A., and
Markov, S.M., Zh. Vses. Khim. O va, 1965, vol. 10,
no. 4, pp. 467 469.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 7 2001