Reaction of carboxylic acids with tetrachlorosilane

Article abstract of DOI:10.1134/S1070428010030024

Tetrachlorosilane reacted with carboxylic acids RCOOH (R = Me, Bu, t-Bu) to give the corresponding acid chlorides RCOCl in 75-95% yield. The reactions of SiCl₄ with trichloroacetic and 2-fluorobenzoic acids (R = Cl ₃C, 2-FC₆H₄) occurred more difficultly, presumably for steric reasons, and the yields of the corresponding acid chlorides were 11 and 22%, respectively. Tetrachlorosilane failed to react with stearic acid under analogous conditions. Products of the reactions of SiCl ₄ with chloroacetic and benzoic acids RCOOH (R = ClCH₂, Ph) were tetraacyloxysilanes Si(OCOR)₄, and tetrakis(chloroacetoxy) silane was formed in almost quantitative yield. The reaction of SiCl₄ with glutaric acid led to the formation of a rubber-like polymeric material with the composition C₅H₆Cl₂O₄Si. The effect of pKa values of carboxylic acids on the direction and mechanism of the examined reaction is discussed.

Full text of DOI:10.1134/S1070428010030024

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 3, pp. 318–321. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © M.G. Voronkov, A.V. Vlasov, L.I. Belousova, O.Yu. Grigor’eva, N.N. Vlasova, 2010, published in Zhurnal Organicheskoi Khimii,
2010, Vol. 46, No. 3, pp. 327–329.
Reaction of Carboxylic Acids with Tetrachlorosilane
M. G. Voronkov, A. V. Vlasov, L. I. Belousova, O. Yu. Grigor’eva, and N. N. Vlasova
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: natnicvlasova@irioch.irk.ru
Received September 19, 2008
Abstract—Tetrachlorosilane reacted with carboxylic acids RCOOH (R = Me, Bu, t-Bu) to give the corre-
sponding acid chlorides RCOCl in 75–95% yield. The reactions of SiCl4 with trichloroacetic and 2-fluoro-
benzoic acids (R = Cl3C, 2-FC6H4) occurred more difficultly, presumably for steric reasons, and the yields of
the corresponding acid chlorides were 11 and 22%, respectively. Tetrachlorosilane failed to react with stearic
acid under analogous conditions. Products of the reactions of SiCl4 with chloroacetic and benzoic acids
RCOOH (R = ClCH2, Ph) were tetraacyloxysilanes Si(OCOR)4, and tetrakis(chloroacetoxy)silane was formed
in almost quantitative yield. The reaction of SiCl4 with glutaric acid led to the formation of a rubber-like poly-
meric material with the composition C5H6Cl2O4Si. The effect of pKa values of carboxylic acids on the direction
and mechanism of the examined reaction is discussed.
DOI: 10.1134/S1070428010030024
Carboxylic acid chlorides RCOCl constitute an im-
portant class of organic compounds which attract much
interest as intermediate products in organic synthesis
[1, 2]. The simplest procedure for the preparation of
acyl chlorides is based on reaction of carboxylic acids
with thionyl chloride (SOCl2) or phosphorus chlorides
(PCl5, POCl3, PCl3) [3]. We set ourselves the task of
replacing fairly expensive reagents for the transforma-
tion of carboxylic acid moiety into acid chloride by
cheap tetrachlorosilane. In the recent time silicon tetra-
chloride has become a large-scale product in organo-
silicon industry and manufacture of semiconductor
silicon.
anhydride (yield 80%). Later on, intermediate product
in this reaction was isolated: it was tetrakis(butyryl-
oxy)silane Si(OCOPr)4 which underwent quantitative
thermal decomposition into silicon dioxide and butyric
anhydride [9] (Scheme 1). Labile tetrakis(formyloxy)-
silane was synthesized in a similar way [10].
In the present work we examined reactions of tetra-
chlorosilane with acetic, chloroacetic, trichloroacetic,
valeric, pivalic, benzoic, 2-fluorobenzoic, stearic, and
glutaric acids. The goal of this study was not only to
develop a simple and convenient procedure for the
preparation of acyl chlorides (as precursors of acyl
iodides that are the subjects of our systematic studies)
but also to reveal how the reactant ratio and dissocia-
tion constants Ka of the acids affect their reaction with
SiCl4. The reactions were carried out under solvent-
free conditions by heating the reactants, SiCl4 and
RCOOH, at a molar ratio of 2:1 at 50–70°C until
hydrogen chloride no longer evolved (4–10 h). The
reaction conditions and yields of the corresponding
acyl chlorides are given in table.
The possibility for synthesizing carboxylic acid
chlorides by reaction of carboxylic acids with SiCl4
was noted in the first half of the XXth century [4–6].
Andreanov and Dolgopolov [7] proposed a procedure
for large-scale preparation of benzoyl chloride from
SiCl4 and benzoic acid. Petrov et al. [8] reported that
butyric acid reacted with excess tetrachlorosilane to
produce up to 81% of butyryl chloride. If excess
butyric acid was used, the major product was butyric
For comparison, the reactions of SiCl4 with carbox-
ylic acids RCOOH (R = Et, Pr, i-Pr) in xylene were
Scheme 1.
Scheme 2.
4HCl
SiCl4
+
4 PrCOOH
Si(OCOPr)4
SiCl4
+
2 RCOOH
2 RCOCl + SiO2 + 2 HCl
SiO2
+
2 (PrCO)2O
R = Me, Cl3C, Bu, t-Bu, 2-FC6H4.
318

REACTION OF CARBOXYLIC ACIDS WITH TETRACHLOROSILANE
319
Reactions of carboxylic acids RCOOH with SiCl4
Reaction time, h pKa Yield of RCOCl, %
reported to produce 49–51% of the corresponding acid
chlorides [5]. Our results (see table) indicated that the
reactions followed Scheme 2.
R
The reaction of tetrachlorosilane with chloroacetic
acid gave only stable tetra(chloroacetoxy)silane whose
yield attained 98% at a SiCl4–ClCH2COOH ratio of
1:4 (Scheme 3).
t-Bu
05
07
10
08
08
5.03
4.82
4.75
1.66
3.86
72
63
95
22
11
n-Bu
Me
Cl3C
Scheme 3.
2-FC6H4
SiCl4
+
4 ClCH2COOH
Si(OCOCH2Cl)4
+
4 HCl
polymer are terminated by carboxy groups: The IR
spectrum contains a broad band in the region 3200–
3400 cm–1 (OH) and a band at 1700 cm–1 (C=O).
Benzoyl chloride was obtained in a high yield when
the molar ratio SiCl4–PhCOOH was 2:1 [7]. The re-
action with excess benzoic acid (reactant ratio 1:2.5)
gave tetrakis(benzoyloxy)silane as the major product
(yield 65%). In addition, trichlorosilyl benzoate
PhCOOSiCl3 was formed as the first intermediate in
the reaction of SiCl4 with PhCOOH, which was not
identified previously (Scheme 4).
Scheme 6.
2n HOC(O)(CH2)3C(O)OH
+
n SiCl4
O
O
O
O
Cl
O
O
Cl Cl
Si
Cl
Si
–nHCl
( )3
( )3
( )3
HO
O
O
O
nO
OH
Scheme 4.
Presumably, the direction of the reactions of car-
boxylic acids with SiCl4 is determined by their acidity.
Such carboxylic acids RCOOH as acetic, valeric,
pivalic, propionic, butyric, and isobutyric (R = Me,
n-Bu, t-Bu, Et, Pr, i-Pr), whose pKa values range from
4.75 to 5.03 log units may be regarded as weak acids,
whereas chloroacetic acid is characterized by a pKa
value of 2.86. Therefore, chloroacetic acid more
readily undergoes dissociation with formation of car-
boxylate ion, and its reaction with tetrachlorosilane
stops at the stage of formation of tetrakis(chloro-
acetoxy)silane Si(OCOCH2Cl)4, The ionic Si–O bonds
in the latter are fairly strong, and this compound does
not decompose on heating into SiO2 and ClCH2COCl
according to Scheme 4. Increased strength of the Si–O
bond is also intrinsic to tetrakis(benzoyloxy)silane
generated from benzoic acid (pKa 4.19), and benzoyl
chloride is formed only in the presence of excess SiCl4.
SiCl4
+
2.5 PhCOOH
0.5 PhCOOSiCl3
0.5 Si(OCOPh)4
+ 2.5 HCl
+
Neither acyloxy silanes Si(OCOR)4 nor carboxylic
acid anhydrides (RCO)2O were formed in the reactions
of tetrachlorosilane with trichloroacetic and 2-fluoro-
benzoic acids. Here, the products were the correspond-
ing acid chlorides, but their yields were poor (22 and
11%, respectively; see table; Scheme 5). Presumably,
the reason is steric effect of bulky substituent in these
acids. Likewise, no reaction occurred between SiCl4
and stearic acid. On the whole, our experimental data
suggest that the reaction of SiCl4 with RCOOH can be
regarded as a two-step process.
Scheme 5.
SiCl4
+
4 RCOOH
SiCl4
Si(OCOR)4
4 RCOCl
+
4 HCl
It is interesting that even stronger trichloroacetic
acid (pKa 1.66) and weaker 2-fluorobenzoic acid
(pKa 3.86) reacted with SiCl4 fairly difficultly with
formation of the corresponding acid chlorides. A prob-
able reason is steric effect of bulky substituents in the
acids. The same factor is likely to be responsible for
the failure of tetrachlorosilane to react with stearic acid
(pKa 3.26) under analogous conditions. The formation
of polymeric product in the reaction of SiCl4 with
glutaric acid (pKa 4.34) may also be rationalized in
Si(OCOR)4
+
+
2 SiO2
In the reaction of tetrachlorosilane with dibasic
glutaric acid at a molar ratio of 2:1 we isolated only
a rubber-like polymeric material with a composition of
C5H6Cl2O4Si (Scheme 6). The product was insoluble in
water and organic solvents. In contrast, the reaction of
SiCl4 with sebacic acid was reported to afford mono-
meric dichloride ClCO(CH2)8COCl in 37% yield [5].
According to the IR data, linear chains of the obtained
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VORONKOV et al.
320
terms of sterically hindered formation of primary
mono- and bicyclic products.
12.2 g (100 mmol) of benzoic acid, and the mixture
was heated for 7 h at 55–60°C. The mixture was
cooled and distilled under reduced pressure to isolate
7.4 g (29%) of trichlorosilyl benzoate, bp 84°C (5 mm)
[IR spectrum, ν, cm–1: 1680 (C=O), 1280 (C–O), 920
(Si–O), 600 (Si–Cl). Found, %: C 32.42; H 2.06;
Si 10.92. C7H5Cl3O2Si. Calculated, %: C 32.87;
H 1.96; Si 10.95], and 8.3 g (65%) of amorphous tetra-
kis(benzoyloxy)silane, mp 125–126°C. IR spectrum, ν,
cm–1: 1680 (C=O), 1280 (C–O), 920 (Si–O). Found,
%: C 65.47; H 5.20; Si 7.08. C28H20O8Si. Calculated,
%: C 65.62; H 5.20; Si 7.29.
Thus synthesis of carboxylic acid chlorides via
reaction of the corresponding carboxylic acid with
tetrachlorosilane may be recommended as preparative
only for weak acids (pKa < 4.7).
EXPERIMENTAL
The IR spectra were recorded from thin films on
a UR-20 spectrometer.
Reaction of acetic acid with tetrachlorosilane.
Tetrachlorosilane, 8.5 g (50 mmol), was slowly added
to 6.0 g (100 mmol) of acetic acid, and the mixture
was heated for 10 h at 50°C. The product was acetyl
chloride, yield 7.4 g (95%), bp 50–51°C, nD20 = 1.3890;
published data [11]: bp 51.8°C, nD20 = 1.3897. IR spec-
trum, ν, cm–1: 1760 (C=O), 600 (C–Cl).
Reaction of 2-fluorobenzoic acid with tetra-
chlorosilane. A mixture of 14 g (100 mmol) of
2-fluorobenzoic acid and 8.5 g (50 mmol) of tetra-
chlorosilane was heated for 8 h at 50–55°C. We
isolated 1.8 g (11%) of 2-fluorobenzoyl chloride,
bp 103°C (30 mm). IR spectrum, ν, cm–1: 1800 (C=O),
610 (C–Cl). Found, %: C 52.59; H 3.12; Cl 22.24;
F 11.97. C7H5ClFO. Calculated, %: C 52.66; H 3.13;
Cl 22.25; F 11.91.
Reaction of chloroacetic acid with tetrachloro-
silane. Tetrachlorosilane, 2.2 g (12.5 mmol), was
added to 4.7 g (50 mmol) of chloroacetic acid, and the
mixture was heated for 4 h at 50–55°C. We isolated
5.1 g (98%) of crystalline tetrakis(chloroacetoxy)-
silane, mp 153–154°C; published data [12]: mp 150°C.
IR spectrum, ν, cm–1: 1730 (C=O), 1190, 780
(Si–O–C), 630 (C–Cl).
Reaction of glutaric acid with tetrachlorosilane.
Tetrachlorosilane, 20.4 g (120 mmol), was slowly
added to 8.0 g (60 mmol) of glutaric acid. Vigorous
evolution of gaseous hydrogen chloride was observed
(it was identified by qualitative test with AgNO3).
Unreacted tetrachlorosilane was removed from the
reaction mixture by simple distillation. We isolated
a polymeric product which decomposed at 300°C and
was insoluble in water, diethyl ether, acetone, and
chloroform. IR spectrum, ν, cm–1: 1700 (C=O), 1050
(Si–O), 680 (Si–Cl). Found, %: C 28.10; H 3.01;
Cl 28.93; Si 12.57. C5H7Cl2O4Si. Calculated, %:
C 26.20; H 2.62; Cl 31.00; Si 12.22.
Reaction of trichloroacetic acid with tetrachloro-
silane. Tetrachlorosilane, 8.5 g (50 mmol), was added
to a solution of 16.4 g (100 mmol) of trichloroacetic
acid in 10 ml of hexane, and the mixture was stirred
for 8 h at 50–55°C. We isolated 4.0 g (22%) of tri-
chloroacetyl chloride, bp 118°C; published data [13]:
bp 118–120°C. IR spectrum, ν, cm–1: 1790 (C=O),
600 (C–Cl).
Reaction of valeric acid with tetrachlorosilane.
Tetrachlorosilane, 17.0 g (100 mmol), was added to
20 g (200 mmol) of valeric acid, and the mixture was
heated for 7 h at 50–55°C. We isolated 15.9 g (63%) of
valeroyl chloride, bp 126–128°C, nD20 = 1.4200; pub-
lished data [11]: bp 128°C, nD20 = 1.4207. IR spectrum,
ν, cm–1: 1790 (C=O), 600 (C–Cl).
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 05-03-32096) and by the Council for Grants at the
President of the Russian Federation (project no. NSh-
4575.2006.3).
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REACTION OF CARBOXYLIC ACIDS WITH TETRACHLOROSILANE
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