Reaction of silicon with alcohols in autoclave

Article abstract of DOI:10.1007/s11172-017-1726-7

A reaction of activated silicon with alcohols in an autoclave at 240—270 °C was studied. It was found that primary alcohols form tetraalkoxysilanes Si(OR)₄ with high selectivity (up to 97%), while the secondary PrⁱOH gave a mixture of compounds HSi(OPrⁱ)₃, Si(OPrⁱ)₄, HSi(OPrⁱ)₂OSi(OPrⁱ)₂H, HSi(OPrⁱ)₂OSi(OPrⁱ)₃, and Si(OPrⁱ)₃OSi(OPrⁱ)₃ with the predominance of trialkoxysilane (up to 67%). Carrying out the reaction under the indicated conditions has the advantage of experimental simplicity, reagent availability, high conversion of silicon, good isolated yields of products.

Full text of DOI:10.1007/s11172-017-1726-7

Russian Chemical Bulletin, International Edition, Vol. 66, No. 2, pp. 260—266, February, 2017
260
Reaction of silicon with alcohols in autoclave*
I. V. Krylova, M. P. Egorov, and O. M. Nefedov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 6390. Eꢀmail: kiv@ioc.ac.ru
A reaction of activated silicon with alcohols in an autoclave at 240—270 °C was studied.
It was found that primary alcohols form tetraalkoxysilanes Si(OR)4 with high selectivity (up to
97%), while the secondary PriOH gave a mixture of compounds HSi(OPri)3, Si(OPri)4,
HSi(OPri)2OSi(OPri)2H, HSi(OPri)2OSi(OPri)3, and Si(OPri)3OSi(OPri)3 with the predomiꢀ
nance of trialkoxysilane (up to 67%). Carrying out the reaction under the indicated conditions
has the advantage of experimental simplicity, reagent availability, high conversion of silicon,
good isolated yields of products.
Key words: direct synthesis, silicon, alkoxysilanes, siloxanes, autoclave.
A reaction of silicon with alcohols discovered in
1940s1—4 still attracts attention of researchers in different
areas,5—12 since it is used in the environmentally friendly
"chlorineꢀfree" technology for the preparation of alkoxyꢀ
silanes, which are important intermediate product in the
synthesis of different siloxane products. The initial reacꢀ
tion products of silicon with alcohols is the trialkoxysilane
HSi(OR)3 (1) (Eq. (1)), which, if it remains in the reacꢀ
tion mixture, is converted to Si(OR)4 (2) (Eq. (2)). The
hydrolysis of alkoxysilanes with water, formed by the reacꢀ
tions (3) and (4), with subsequent condensation of hydrꢀ
oxysilanes leads to the formation of side diꢀ, triꢀ, and
higher siloxanes (Eqs (5)—(8)) and alkoxysilane oligoꢀ
mers. When the next members of the homologous series of
alcohols are used, the process can be complicated by the
dehydration reactions (Eqs (9) and (10)) and subsequent
reaction of formed products with alcohols (Eq. (11)).
R = Me (a), Et (b), EtOCH2CH2— (c), Prn (d), Bun (e), Bui (f), Pri (g)
In a detailed study, it was shown that granular or powꢀ
dered silicon with the main substance content ≥80% by
weight (obtained by casting, water granulation, atomizaꢀ
tion or acid treatment) actively reacted with alcohols. The
reaction is catalyzed by copper compounds.1 A convenꢀ
tional catalyst is an intermetallic and/or a solid solution of
the Cu—Si alloy obtained during the diffusion of copper
into silicon or from the reaction of silicon with a copper
compound. This requires a preliminary activation of a mixꢀ
ture of silicon with copper or its salt to remove the oxide
film from the silicon surface. This operation can be carꢀ
ried out by treatment with reagents reacting with oxides
(for example, reduction with hydrogen at 1000 °C,1 treatꢀ
ment with an aqueous solution of HF,13 etc.). Different
types of reactors used to carry out the reactions of silicon
* Dedicated to Academician of the Russian Academy of Sciences
R. Z. Sagdeev on the occasion of his 75th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 0260—0266, February, 2017.
1066ꢀ5285/17/6602ꢀ0260 © 2017 Springer Science+Business Media, Inc.

Reaction of silicon with alcohols
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 2, February, 2017
261
with alcohol (gas—solid compound, liquid—liquid in conꢀ
tinuous14 or periodic15 regime, or their combination) have
their advantages and disadvantages.
to the processes carried out in the "gas—solid compound"
and "liquid—liquid" type reactors with continuous distillꢀ
ing off the formed HSi(OR)3,7 tetraalkoxysilanes Si(OR)4
(2a—f) were formed with high selectivity in the autoclave
in the case of primary alcohols.
The phase Cu3Si (ηꢀphase) is a precursor of the reacꢀ
tion intermediate on the surface of the contact mass.16 As
the conversion of silicon increases during the reaction, the
accumulation of metallic copper occurs, which does not
catalyze the formation of trialkoxysilane, but acts as a
catalyst in the reaction HSi(OR)3 → Si(OR)4.13 The studꢀ
ies of the reaction of methanol with silicon in the presence
of alkenes,6,17 dienes,6 and allylic ethers18 provided an
evidence of the involvement of silylene intermediates in it.
The systematic experimental and quantum chemical
studies of the kinetics of insertion and addition reactions
of carbene analogs (silylenes, germylenes, stannylenes) in
the gas phase19,20 showed that in the case of the associaꢀ
tive process an increase in the pressure leads to a noticeꢀ
able increase in the rate of the reaction of carbene analogs
with the substrates in the gas phase. The studies of the
kinetics of the reaction of silicon with methanol21 showed
that the rate of the formation of methoxysilanes increases
upon the pressure increase, while the rate determining
step is a chemical process of the formation of a reactive
site on the silicon surface which involves alcohol.22 Runꢀ
ning the process in an autoclave17 under elevated pressure
in the presence of ethylene increases the selectivity of the
formation of the sum of products HSi(Et)(OMe)2 and
Si(Et)(OMe)3 to 26% as compared to 8% in the flowꢀtube
reactor. Nonetheless, despite the promising results this
direction has not been studied much.
Results and Discussion
A characteristic feature of the reaction of silicon with
MeOH is a complete absence of the products with the
Si—H bond. In the range of temperatures 200—250 °C,
the conversion changes from 40% (at 200 °C) to practically
quantitative 91% (at 240 °C). The main products are comꢀ
pounds 2a and 5a (Table 1). The 1H NMR spectra of these
compounds in CDCl3 exhibit one singlet for the methoxy
groups at δ 3.53. In the 13C NMR spectra, the MeO fragꢀ
ment in the products is represented by the signal at δ 51.33.
In the 29Si NMR spectra, the signals for compounds 2a
and 5a were observed at δ –78.70 and –86.08, respectively,
that agrees with the reported literature data for tetraꢀ
methoxysilane23 and hexamethoxydisiloxane.24 In the
mass spectra of compounds 2a and 5a, the primary fragꢀ
mentation is related to the elimination of the MeO group,
the peaks of more highꢀmolecularꢀweight fragments differ
from the preceding by 30 a.u., i.e., by the value correꢀ
sponding to the mass of the CH2=O aldehyde, that is
characteristic of such compounds.25 Among impurities,
the GCꢀMS analysis showed the presence of small amounts
of compounds which are similar to the products of fragꢀ
mentation of compounds with a Si—Si bond, namely,
(MeO)8Si3 (the mass spectra agree with those in Ref. 25)
formed in the amount of up to 3% and the compounds
with the structures (MeO)10Si4O and (MeO)12Si5O2 with
the content in the mixture no more than 1%.
The studies of the reaction of silicon with methanol in
a flowꢀtube and a liquidꢀphase reactor7 showed that the
highest conversion of silicon was reached at 220—260 °C.
In the present work, we studied the process of the reaction
of silicon with alcohols of different structure in an autoꢀ
clave in this range of temperatures, including the cases
with the presence of alkenes. It was found that, in contrast
When carrying out the reaction of silicon with methaꢀ
nol in the presence of ethylene used as a silylene trap, we
did not find among reaction products of compounds with
Table 1. Experimental parameters of the reaction of silicon with methanol in autoclave*
Entry
Т/°С
Conversion
(%)
Pressure/atm
62
Selectivity (%)
Impurities (%)
2a
5a
1
200
40
74
18
Si3(OMe)8 (3)
(MeO)10Si4O (<0.5),
(MeO)12Si5O2 (<0.5)
Si3(OMe)8 (2)
(MeO)10Si4O (<0.5)
Si3(OMe)8 (0.4)
2
3
4
200
240
240
83
91
80
115
(50 atm С2H4)
190
81
91
88
16
7
(MeO)6Si2 (0.7)
Si3(OMe)8 (1)
340
10
(90 atm H2)
(MeO)10Si4O (<0.2),
(MeO)12Si5O2 (<0.2)
Si3(OMe)8 (1)
5
250
83
210
88
11
* A 30ꢀmL steel autoclave.

262
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 2, February, 2017
Krylova et al.
the Si—Et fragment, which should be present as a result of
the reaction of the initially formed [1+2] cycloaddition
product of silylene to ethylene with methane (see Table 1).
Among the gasꢀphase reaction products, we detected
ethane, the product of hydrogenation of ethylene with hyꢀ
drogen liberated in the reaction of silicon with methanol.
Therefore, the latter reaction proceeds faster than the adꢀ
dition at the double bond, which quite corresponds to the
data obtained in the course of experimental kinetic studies
of reactions of the carbene analogs with alkenes and alcoꢀ
hols in both the liquid26 and the gas phase.20
When the temperature of the reaction of silicon with
methanol is increased with the same amounts of reagents
(see Table 1), the conversion for the starting compounds
also increases, with the selectivity of the formation of reacꢀ
tion products changing inconsiderably.
for both directions. The compounds 1b and 2b are characꢀ
terized by the initial cleavage of the SiOCH2—CH3 bond
with subsequent elimination of the CH3CH=O and C2H4
molecules.
No reaction was observed with CF3CH2OH containing
an electronꢀwithdrawing fragment, while with EtOCH2ꢀ
CH2OH bearing an electronꢀdonating substituent, the proꢀ
cess proceeds with good conversion.
Tetraalkoxysilanes, the glycol monoether derivatives,
were earlier obtained only by the reaction with tetraꢀ
chlorosilane or by trans etherification of tetraethoxysilane
with the monoether glycol.28,34 When the reaction of siliꢀ
con with methoxyethanol (MeOCH2CH2OH) was carried
out in a liquidꢀphase reactor,3 the siliconꢀcontaining prodꢀ
ucts were detected only in small amounts and were not
isolated in the pure form. The reaction with ethyl celloꢀ
solve (EtOCH2CH2OH) in an autoclave gave tetraalkoxyꢀ
silane (2c) and hexaalkoxydisiloxane (5c) as the products
with their content in the mixture of 94 and 6%, respectively
(Table 5). When the temperature was raised from 220 to
260 °C, the conversion of silicon increased from 46 to
75%, with the selectivity of the formation of these prodꢀ
ucts changing insignificantly. The product 2c was isolated
by vacuum distillation in good yield and with high purity.
The compounds 2c and 5c were completely characterized
by 1H, 13C, and 29Si NMR spectroscopy and mass spectroꢀ
metry (see Tables 3 and 4). In the mass spectrum of comꢀ
pound 2c, the maximal peak with m/z 295 corresponds to
the molecular ion formed by the elimination of the
[O(CH2)2OEt] fragment. For compound 5c, the peak with
m/z 518 corresponds to the elimination of the aldehyde
EtOCH2CH=O with the mass of 88 and subsequent deꢀ
crease of the signals by 44 a.u. ([CH3CH=O]), 72 a.u.
([EtOCH=CH2]), 73 a.u. ([EtOCH2CH2]), and 88 a.u.
([EtOCH2CH=O]) for both ions. When the ratio of reagents
silicon : alcohol was change from 1 : 5 to 1 : 2.7, the conꢀ
tent of compound 1c in the reaction products increased to
3%. The product 1c was characterized only by GCꢀMS.
To demonstrate the prospects of the use of autoclave in
the direct synthesis of tetraalkoxysilanes, we also studied
Since a good conversion for MeOH was reached at
240—250 °C, this temperature range has been chosen for
comparative studies of the behavior of other alcohols in
this reaction.
The reaction of silicon with ethanol gave tetraethꢀ
oxysilane Si(OEt)4 (2b) as the main product, the content
of which in the mixture is as high as 90%. The product
HSi(OEt)3 (1b) and other silanes containing the
Si—H bond, such as (EtO)2HSiOSiH(OEt)2 (3b) and
(EtO)3SiOSiH(OEt)2 (4b), are formed in insignificant
amounts (Table 2). When the temperature is increased
from 240 to 270 °C, the conversion rises from 67 to 82%,
with the composition of the mixture of products remainꢀ
ing practically unchanged (see Table 2). The compound
2b was isolated in good yield and purity (Table 3), while
other alkoxysilanes failed to be separated and were charꢀ
acterized in the mixture by 1H, 13C, and 29Si NMR
spectroscopy and GCꢀMS (Table 4). The NMR spectra
are in good agreement with the literature data.24,27 The
fragmentation in the mass spectra of compounds 3b and
4b follows several directions: 1) a sequential mass loss by
28 a.u. ([C2H4]) by the [M—H] ion, 2) elimination from
the [M—H] ion of the fragment with the mass of 44 a.u.
([CH3CH=O]) with a sequential loss of the mass by 28 a.u.
Table 2. Conversion and product composition of the reaction of silicon with ethanol in
autoclave* at different temperatures (GC data)
Entry
T/°C
Conversion
(%)
Composition of mixture (%)
Other products
(%)
1b
2b
5b
1
240
67
5
90
2.5
(EtO)4Si2OH2 (0.5)
(EtO)5Si2OH (1.4)
(EtO)5Si2OH (1.9)
(EtO)4Si2OH2 (0.3)
(EtO)5Si2OH (1.0)
—
2
3
250
260
77
71
3
5
80
89
13
4
4
270
82
—
91
4
* A Roth Model II autoclave.

Reaction of silicon with alcohols
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 2, February, 2017
263
Table 3. Identification of alkoxysilanes isolated by distillation
Comꢀ
Yield/
B.p./°C
NMR spectrum (δ, J/Hz)
MS, m/z (I (%))
pound purity (%)
/(b.p. Ref.)
1H
13C
29Si
2a
2b
70/99
80/99
120—123
/(120—122)3
93—95
3.6 (s)
51.3
–78.80
152 (48) [M]+, 137 (17),
121 (100), 107 (16), 91 (75), 77 (7)
208 (12) [M]+, 193 (100), 179 (32),
163 (50), 149 (95), 135 (20), 119 (40),
107 (15), 91 (10), 79 (20), 63 (15)
295 [M – OR]
(100), 251 (8), 223 (14),
207 (18), 177 (30), 149 (22),
105 (8), 45 (2)
3.84 (q, JH—H = 7);
1.18 (t, JH—H = 7)
59.21, –82.00
18.16
/(166—168)3
2c
64/98
200—215
/(200/9)34
3.95 (t, OCH2CH2,
JH—H = 5.4); 3.52 (t,
OCH2CH2, JH—H = 5.4);
3.5 (q, OCH2CH3,
71.32, –82.55
66.48,
62.95,
15.18
JH—H = 7); 1.16
(t, CH3, JH—H = 7)
3.72 (t, OCH2,
JH—H = 6.6); 1.68—1.50
(m, OCH2CH2); 0.91
(t, CH3, JH—H = 7.4)
3.77 (t, OCH2,
JH—H = 6.6); 1.64—1.47
(m, OCH2CH2); 1.47–1.29
(m, OCH2CH2CH2); 0.91
(t, CH3, JH—H = 7.2)
2d
2e
66/96
86/98
230–250
65.21, –82.11
25.58,
10.21
264 [M] (4), 263 [M – H] (2),
235 (100), 221 (8), 205 (13),
193 (26), 177 (20), 163 (9), 151
(48), 135 (8), 121 (15), 93 (7)
320 (4) [M], 277 (100), 263 (14),
247 (7), 221 (18), 205 (11), 179 (8),
165 (17), 149 (12), 123 (29),
93 (15), 79 (21)
/(120/20)3
139
63.27, –81.92
34.51,
18.90,
/(134—136/5)32
13.88
2f
59/95
42/95
138—141
3.53 (d, OCH2, JH—H = 6.6); 70.08, –82.70
320 (1) [M], 277 (46), 263 (6),
221 (24), 191 (8), 165 (100),
151 (8), 135 (15), 93 (10), 79 (21)
/(255—258)35
1.81 (hept, OCH2CH2,
JH—H = 6.6); 0.91 (t,
CH3, JH—H = 6.6)
30.65,
18.98
1g
50
4.33 (s, Si—H); 4.25(pent,
JH—H = 6, OCH); 1.22
(d, JH—H = 6, OCH);
1.22 (d, CH3, JH—H = 6)
65.41, –62.60
25.50
205 [M – H] (6), 191 (100)
[M – CH3], 149 (59), 107 (25),
63 (43)
/(164—166)29
the process of the reaction of silicon with PrnOH and
BunOH. It is known that the running this process in the
gas—solid compound and liquid—liquidꢀtype reactors
gives the corresponding trialkoxysilanes 1d3,4 and 1e29 as
the main products, though in quite low yields. The correꢀ
sponding tetraalkoxysilanes are obtained, as a rule, by the
reaction of tetrahalosilanes with alcohols or alkoxides or
by trans etherification of ethylsilicates.30 The reaction of
silicon with PrnOH and BunOH in an autoclave gives tetraꢀ
alkoxysilanes 2d,e as the main products, respectively,
which were isolated in good yields (see Tables 3 and 5).
These compounds were completely characterized by
1H, 13C, and 29Si NMR spectroscopy and mass spectroꢀ
metry (see Table 5). In the mass spectra of these comꢀ
pounds, the peak with the maximal intensity correꢀ
sponds to the molecular ions [Si(OPr)3(OCH2)]+ and
[Si(OBun)3(OCH2)]+, respectively. This means that the
cleavage occurs at the SiOCH2—CH2 bond, the further
fragmentation in both cases includes a sequential loss of
the masses by 42 a.u. [C3H6] and 58 a.u.[CH3CH2CH=O]
for 2d, as well as by 42 and 56 a.u. [CH3CH2CH=CH2]
for 2e. For hexaalkoxysiloxanes 5d and 5e, the peaks with
the maximal intensity correspond to the loss of the alkoxy
group [(PrO)5Si2O]+, that agrees with the known data,31
and [(BunO)5Si2O]+, respectively, with subsequent mass
loss similar to the same process for tetraalkoxysilanes (see
Table 4).
Among the products of the reaction of silicon with
BuiOH, together with the expected tetra(isobutoxy)silane
2f, there were detected the products with the Si—H bond
(see Tables 4 and 5) formed in insignificant amounts. The
1
signals in the H NMR spectrum at δ 4.27 and 4.33 were
assigned to the Si—H protons for compounds 1f and 3f
(4f), respectively. In the 29Si NMR spectrum, these prodꢀ
ucts are represented by the signals at δ –58.4, –67.8, and
–89.1 assigned to the compound 1f, the SiOSiH(OBui)2
fragment with JSi—H = 197 Hz, and the (BuiO)3SiOSi group,
respectively. In the mass spectrum of silane 1f, the fragꢀ
mentation of the [M – H]+ ion follows the pathway of the
mass loss by 42 and 56 a.u. with the signal for the
[(HO)2(BuiO)Si]+ ion being the most abundant. It is inꢀ
teresting that for the siloxane compounds 3f and 4f, the
primary fragmentation consists in the elimination of the
alkoxy group BuiO with subsequent sequential loss of the
fragments with the mass of 56 a.u., which resembles the
process in the siloxane compounds described above. The

264
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 2, February, 2017
Krylova et al.
Table 4. Identification of alkoxysilanes in the mixture
Comꢀ
pound
1H NMR
29Si NMR
MS, m/z (I (%))
(δ, J/Hz)
5а
3.6
—
—
–86.1
258 [M]+ (0.5), 227 (100), 196 (55), 167 (22), 121 (10)
332 [M]+ (100), 287 (42), 257 (46), 227 (20), 197 (20), 167 (10),
Si3(OMe)8
—
—
—
151 (4), 136 (8), 121 (10), 113 (4), 91 (0.5)
(MeO)10Si4O
—
438 [M]+ (100), 393 (80), 363 (20), 347 (50), 333 (10), 317 (20), 303 (2),
287 (20), 273 (4), 257 (18), 241 (1), 227 (8), 181 (20), 173 (1), 121 (1)
163 [M – H] (0.2), 149 (90), 135 (100), 121 (34), 105 (72),
91 (70), 77 (55)
1b
3b
4b
4.28 (s, Si—H)
4.33 (s, Si—H)
4.33 (s, Si—H)
–58.6
–68.0 (JSi—H= 285)
253 [M – H] (8), 209 (100), 181 (25), 153 (20),
125 (45), 107 (20), 91 (5)
297 [M – H] (3), 269 (5), 253 (100), 239 (10),
225 (22), 209 (20), 197 (15),
–68.0 (OSiH(OEt)2,
JSi—H = 285);
–89.1 (OSi(OEt)3)
–89.2
181 (15), 169 (15), 153 (10), 141 (30), 125 (20), 123 (25), 107 (10), 79 (2)
342 [M]+ (1), 297 (100), 269 (24), 253 (30), 241 (10),
225 (20), 213 (8), 197 (12), 185 (8), 157 (30), 141 (24), 123 (20)
295 [M – H] (0.5), 237 (100), 177 (30), 149 (20), 137 (8), 119 (8)
518 [M – EtOCH2CH=O]+ (100), 445 (30), 429 (70), 399 (30), 355 (15),
327 (12), 311 (30), 281 (44), 267 (45), 239 (36), 209 (58), 183 (38),
165 (25), 157 (24), 149 (14), 139 (14), 73 (44), 59 (18), 45 (54)
426 [M]+ (0.5), 385 (6), 367 (100), 325 (36), 283 (16),
267 (4), 241 (14), 225 (6), 199 (22), 171 (19), 157 (74), 141 (13)
510 [M]+ (0.5), 455 (8), 437 (50), 381 (36), 325 (14), 269 (8),
157 (100), 141 (7)
247 [M – H]+ (10), 205 (50), 191 (38), 149 (100), 135 (14),
119 (20), 93 (14), 63 (38), 57 (34)
364 (0.1), 293 (50), 267 (2), 237 (3), 211 (8), 197 (21),
181 (29), 155 (45), 141 (30), 125 (42)
438 [M]+ (0.5), 395 (1), 383 (1),365 (80), 339 (1), 325 (1), 309 (20),
283 (3), 269 (2), 253 (8), 227 (11), 213 (11), 197 (50), 171 (80),
157 (38), 141 (64), 57 (100)
5b
—
1c
5c
—
—
—
–89.3
5da
5e
1fb
3f
—
–89.1
—
–89.0
4.27 (s, Si—H)
4.33 (s, Si—H)
—
–58.4 (JSi—H = 288)
–67.8 (JSi—H= 297)
4f
–67.8 (JSi—H= 297),
–89.1
2g
3g
—
–86.0
–70.9 (JSi—H = 291)
263 [M – H] (4), 249 (100), 207 (32), 165 (90), 121 (36), 79 (70)
295 [M – CH3]+ (5), 253 (20), 251 (37), 209 (30), 211 (30),
167 (75), 141 (30), 125 (100), 123 (26), 107 (12)
367 [M – H] (0.5), 353 (20), 309 (90), 267 (28), 225 (62),
183 (82), 141 (100), 123 (20)
4.36 (s, Si—H)
4g
4.36 (s, Si—H)
–70.9 (JSi—H = 291),
–92.0
a
δ (13C): 65.15, 25.50, 10.18.
δ (13C): 69.26, 30.65, 18.98.
b
fragmentation of compound 2f occurs similarly to that of
other tetraalkoxysilanes with the initial cleavage of the
SiOCH2—CH2 bond and the formation of the
[(BuiO)3SiOCH2]+ fragment ion, which, in turn, underꢀ
goes fragmentation with sequential loss of the mass 56 a.u.
Compounds 3f and 4f were not isolated in the individual
state, their distilled fractions contain a mixture of the inꢀ
dicated products.
The tetraalkoxysilanes obtained by the reaction of siliꢀ
con with primary alcohols were isolated by vacuum distilꢀ
lation in good yields and with the purity up to 99% (see
Table 3).
pound1 reactor, nor in the liquid—liquid3 reactor. However,
we found that when the reaction was carried out in an
autoclave, PriOH reacted with silicon with good converꢀ
sion (50% at 250 °C and 52% at 270 °C). The main reacꢀ
tion product at both temperatures was trialkoxysilane
HSi(OPri)3 (1g) (see Table 5). The content of Si(OPri)4
(2g) in the mixture of products did not exceed 9% even
after a prolonged (7 h) stirring of the reaction mixture in
an autoclave at 270 °C. The product 1g was isolated from
the reaction mixture by vacuum distillation with 95% purity
and 42% yield (calculated on the amount of converted
silicon). The signal in the 1H NMR spectrum at δ 4.33 was
assigned to the Si—H proton of the product 1g, while the
broad signal at δ 4.36 was assigned to the products 3g
and 4g; in the 29Si NMR spectrum, the signals at δ –62.6
It was mentioned not once that secondary alcohols do
not react with silicon: no products of the reaction of silicon
with PriOH were obtained neither in the gas—solid comꢀ

Reaction of silicon with alcohols
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 2, February, 2017
265
Table 5. The composition (%) of the mixture of reaction products of silicon with alcohols in
autoclave at 250 °C and 3 h of time (GLC data)
Alcohol
1
2
3
4
5
Other products (%)
MeOH (a)
—
15
—
—
—
14
18
58
67
88
89
94
84
96
88
76
18
18
—
0.3
—
—
—
3
—
9
9
—
1
—
—
—
3
2
6
4
11
14
16
16
14
—
12
—
—
(MeO)8Si3 (1.3)
EtOHa (b)
—
—
—
—
12
12
13
18
EtOCH2CH2OHb (c)
PrnOH (d)
BunOH (e)
BuiOH (f)
BuiOHb (f)
PriOH (g)
PriOHc (g)
ButOH
Only the starting alcohol and isobutylene
Only the starting alcohol and cyclohexene
Only the starting alcohol and 4% of cyclohexene
cycloꢀC6H11OH
cycloꢀC6H11OH3
a 260 °C, 3 h.
b 270 °C, 3 h.
c 270 °C, 7 h.
(JSi—H = 285 Hz), –70.9 (JSi—H = 291 Hz), –86.0, and
–92.0 were assigned to compounds 1g (the OSiH(OPri)2
group) and 2g (the OSi(OPri)3 group), respectively. In the
13C NMR spectrum, the reliable assignment was made
only for the signals at δ 65.40 and 25.50, attributing them
to the CH and CH3 groups, respectively, in compound 1g.
The other fractions isolated by distillation are unseparable
mixtures of compounds 1g, 2g, 3g, and 4g in different
ratios. The compounds in these mixtures were identified
by NMR spectroscopy and GCꢀMS. The mass spectra of
all the compounds are characterized by the initial elimiꢀ
nation of a methyl ion with subsequent sequential deꢀ
crease of the fragment masses by 42 a.u. [CH3CH=CH2].
It was found that other secondary alcohols, such as
cyclohexanol and secꢀbutanol, did not react with silicon.
After the reaction completion, analysis by NMR spectroꢀ
scopy showed the presence in the reaction mixtures of
insignificant amounts of dehydration products, namely,
cyclohexene and a mixture of butenes, respectively. The
absence of products of the reaction of silicon with ButOH
is not a surprise, since even in the reaction of the latter
with tetrahalosilanes the corresponding alkoxysilanes were
not obtained.32 A slightly increased pressure in the autoꢀ
clave after the reaction completion is given by isobutylene
formed at dehydration of the alcohol.
process are undoubted advantages of the direct autoclave
process of the reaction of silicon with alcohols.
Experimental
Powdered silicon with particle size of 140 nm used in the
work was obtained by grinding KRꢀ1 metallurgical silicon, the
reactant CuCl (97% purity) was commercially available from
Acros Organics. Alcohols were dried according to the described
procedures33 and distilled.
Reactions were carried out in a Roth Model II laboratory
autoclave (250 mL, Germany) and in a steel thermostated autoꢀ
clave (30 mL) with stirring on a shaker.
The mixture compositions were analyzed on a SRI Instruꢀ
ment gasꢀliquid chromatograph on a 8610C basis with a 30ꢀm
MTXꢀ5 capillary column; carrier gas helium; a flameꢀionizing
detector. GCꢀMS analysis was carried out on a Thermo Scienꢀ
tific Trace GC Ultra equipment with a DSQII detector with
a 30ꢀm capillary column. 1H, 13C, and 29Si NMR spectra were
recorded on Bruker AC (200 MHz) and Bruker Avance II
(300 MHz) spectrometers in CDCl3. Chemical shifts are given
relative to tetramethylsilane.
The contact mass (CM) was prepared according to the proꢀ
cedure described earlier.13 A weighed amount of CM was placed
into an autoclave liner, followed by the addition of freshly disꢀ
tilled alcohol (a fiveꢀfold molar excess) and purging with Ar.
After sealing the autoclave, the mixture was stirred at a given
temperature. Then, the mass was cooled to ~20 °C and the gas
phase composition analyzed by passing the gas flow through
CDCl3. The mixture was removed from the autoclave and centriꢀ
fuged, the liquid phase was decanted. The conversion of CM was
evaluated from the difference in masses obtained after washing the
CM residue with acetone and drying in vacuo. The liquid phase was
distilled. The alkoxysilanes isolated by distillation were identiꢀ
fied by their physical data and spectral characteristics (see Table 3).
The compounds, which were not isolated in the individual state,
were identified analyzing the mixtures by 1H, 13C, and 29Si NMR
spectroscopy, chromatography, and GCꢀMS (see Table 4).
In conclusion, the reaction of silicon with primary
alcohols carried out in an autoclave at 240—270 °C gave
good yields of tetraalkoxysilanes 2 with high selectivity.
The main reaction product of silicon with isopropyl alcoꢀ
hol is triꢀisopropoxysilane, which was isolated in the indiꢀ
vidual state in moderate yield. The absence of the reaction
of silicon with CF3CH2OH and sterically hindered alcoꢀ
hols can indicate an important role of chemosorption proꢀ
cesses. The experimental simplicity, high conversion of
silicon, good product yields, environmental safety of the

266
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 2, February, 2017
Krylova et al.
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The authors are grateful to E. V. Shulishov for the help
in the recording the NMR spectra, D. N. Platonov and
R. F. Salikov for carrying out GCꢀMS analysis.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 13ꢀ03ꢀ12237ꢀ
ofiꢀm).
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Received April 7, 2016;
in revised form May 23, 2016