Selective hydrolysis of pentafluorobenzotrichloride

Article abstract of DOI:10.1023/B:RUJO.0000003188.49417.22

Selective hydrolysis of pentafluorobenzotrichloride has been studied in the presence of a catalytic amount of iron(III) chloride. The main products are pentafluorobenzoyl chloride, pentafluorobenzoyl fluoride, and 4-chlorotetrafluorobenzoyl chloride. The two latter products are formed by hydrolysis of dichlorofluoromethylpentafluorobenzene and 4- chlorotetrafluorobenzotrichloride which result from halogen exchange between the methyl group and aromatic ring in the initial pentafluorobenzotrichloride. A probable mechanism for halogen exchange is proposed.

Full text of DOI:10.1023/B:RUJO.0000003188.49417.22

Russian Journal of Organic Chemistry, Vol. 39, No. 7, 2003, pp. 975 978. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 7, 2003,
pp. 1036 1039.
Original Russian Text Copyright
2003 by Shipilov, Kolpashchikova, Igumnov.
Selective Hydrolysis of Pentafluorobenzotrichloride
A. I. Shipilov, L. A. Kolpashchikova, and S. M. Igumnov
Perm Division, Russian Scientific Center Prikladnaya khimiya Federal State Unitary Enterprise,
ul. Voronezhskaya 41, Perm, 614034 Russia
e-mail: shipilov@perm.ru
Received November 15, 2002
Abstract Selective hydrolysis of pentafluorobenzotrichloride has been studied in the presence of a catalytic
amount of iron(III) chloride. The main products are pentafluorobenzoyl chloride, pentafluorobenzoyl fluoride,
and 4-chlorotetrafluorobenzoyl chloride. The two latter products are formed by hydrolysis of dichlorofluoro-
methylpentafluorobenzene and 4-chlorotetrafluorobenzotrichloride which result from halogen exchange
between the methyl group and aromatic ring in the initial pentafluorobenzotrichloride. A probable mechanism
for halogen exchange is proposed.
A large-scale procedure for the synthesis of ben-
zoyl chlorides is based on selective hydrolysis of the
corresponding trichloromethyl derivatives (benzotri-
chlorides) with water in the presence of iron(III)
chloride [1, 2].
presence of a catalytic amount of iron(III) chloride
(1 2 mol %), pentafluorobenzoyl chloride (II) was
obtained in 70 90% yield. Unexpectedly, among the
reaction products we detected pentafluorobenzoyl
fluoride (III) and 4-chlorotetrafluorobenzoyl chloride
(IV) (Scheme 1). The results are summarized in
Table 1. In some experiments (run nos. 1 3), the
overall yield of benzoyl halides III and IV reached
30%. The goal of the present study was to elucidate
the ways and mechanism of formation of compounds
III and IV.
Heating of pentafluorobenzoyl chloride (II) in the
presence of iron(III) chloride for 3 h at 120 C (with
no water added) did not result in formation of appreci-
able amounts of other products, specifically of com-
pounds III and IV (Table 1, run no. 6). When the
hydrolysis of pentafluorobenzotrichloride (I) was per-
Depending on the substituents present in the aro-
matic ring, the reaction can follow both SN1 and
SN2 mechanism [1]. As a rule, the yield of benzoyl
chlorides ranges from 90 to 98%. We have studied
an analogous process with a view to obtain penta-
fluorobenzoyl chloride. When the hydrolysis of penta-
fluorobenzotrichloride (I) was performed with equim-
olar amounts of the reactants at 110 120 C in the
Scheme 1.
Scheme 2.
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SHIPILOV et al.
Table 1. Hydrolysis of pentafluorobenzotrichloride in the presence of iron(III) chloride
Amounts of reactants, g (mol)
Reaction conditions
Product composition,a
%
Run
no.
tempera- addition reaction
I
FeCl3 6H2O
H2O
II
III
IV
V
ture,
C
time, h time, h
1
2
3
4
5
6
7
8
285.5 (1)
285.5 (1)
4.28 (0.017) 18.0 (1)
4.28 (0.017) 18.0 (1)
110 115
120 125
110 115
110 115
115 120
120
3.0
3.0
3.5
3.0
3.0
1
1
1
1
2
3
3
1
90
73
69
73
32
98
4.8
15.0
17.7
9.5
23.0
Traces
4.0 Traces
8.5 Traces
9.8 Traces
190.0 (0.665) 2.85 (0.011) 12.0 (0.665)
285.5 (1)
285.5 (1)
4.28 (0.017) 16.2 (0.9)
4.28 (0.017) 12.9 (0.72)
5
11
20.5
24.4
153.0 (0.665)b 1.43 (0.01)c
285.5 (1)
4.28 (0.017)
120
120
15d
Reaction mixture from run 18.0 (1)
no. 7 was used
3
60
23
17
Traces
a
b
c
According to the GLC data.
Compound II contained 99% of the main substance.
Anhydrous FeCl3 was used.
d
The mixture also contained 11% of VI, 2% of VII, 1.5% of VIII, and 70% of I.
formed with insufficient water (the I-to-H2O molar
ratio was 1 : 0.9), the reaction mixture contained
a series of new products, among which we identified
dichlorofluoromethylpentafluorobenzene (V) and
small amounts of chlorodifluoromethylpentafluoroben-
zene (VI) and 1-chloro-4-dichlorofluoromethyltetra-
fluorobenzene (VII) (Scheme 2; Table 1, run no. 4).
unambiguously indicate that benzoyl halides III and
IV are formed as a result of hydrolysis of intermediate
products V and VIII (Scheme 4).
Scheme 4.
The fraction of compound V in the product mixture
increased to 25% at a I-to-H2O ratio of 1:0.72 under
more severe conditions (Table 1, run no. 5). When
pentafluorobenzotrichloride was kept under conditions
similar to the hydrolysis conditions (120 C, 3 h) but
in the absence of water, the yield of products of halo-
gen exchange between the methyl group and aromatic
ring also increased: 4-chlorotetrafluorobenzotri-
chloride (VIII) and compound V were obtained in
an overall yield of greater than 25% (Scheme 3,
Table 1, run no. 7).
Scheme 3.
Compounds V and VIII are formed in turn via
halogen exhange between the methyl group and
aromatic ring. Presumably, this process begins with
formation of a complex between iron(III) chloride
and benzotrichloride (I), which is converted into the
corresponding ion pair Ia (Scheme 5). The formation
of analogous cationoid intermediates in the reaction
of PhCCl3 with Lewis acids was reported previously
[3]. It should be noted that, according to the high-
resolution NMR data, complex Ia is characterized by
a considerably stronger charge separation, as com-
pared to the corresponding benzoyl chloride Lewis
The subsequent hydrolysis at 120 C of the mixture
obtained in run no. 7 gave 60% of II, 23% of III,
and 17% of IV (Table 1, run no. 8). These data
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 7 2003

SELECTIVE HYDROLYSIS OF PENTAFLUOROBENZOTRICHLORIDE
977
acid complex [3]. This explains the formation of
benzoyl halides as hydrolysis products and their
failure to undergo further transformations.
acceptor. However, Dvornikova et al. [7] and Cherst-
kov et al. [8] showed that the reaction of octafluoro-
toluene with insufficient AlBr3 at 50 C initially gives
pentafluorobenzotribromide which is converted into
p-bromoheptafluorotoluene and a small amount of
p-bromotetrafluorobenzotribromide on further heating
of the reaction mixture at the boiling point. Taking
into account the high strength of the aluminum
fluorine bond in aluminum fluoride formed in the first
reaction stage, it is possible that in this case halogen
exchange between the methyl group and aromatic ring
also occurs (Scheme 7).
Scheme 5.
Scheme 7.
Nucleophilic attack by FeCl4 on the second
molecule of initial pentafluorobenzotrichloride gives
a
complex. Decomposition of the latter is accom-
panied by elimination of fluoride ion and its replace-
ment by chlorine; fluoride ion adds to the cationic
part of ion pair Ia at the methyl group. The process
is likely to involve five-center transition state A
(Scheme 6). We believe that Scheme 6 explains
halogen exchange between the methyl group and
aromatic ring.
Thus, our results show that the hydrolysis of penta-
fluorobenzotrichloride to pentafluorobenzoyl chloride
in the presence of iron(III) chloride is accompanied by
exchange of the aromatic fluorine atom and chlorine
atom in the methyl group between pentafluorobenzo-
trichloride molecules with intermediate formation of
dichlorofluoromethylpentafluorobenzene and 4-chloro-
tetrafluorobenzotrichloride. Hydrolysis of these inter-
mediate products yields pentafluorobenzoyl fluoride
and 4-chorotetrafluorobenzoyl chloride.
Scheme 6.
EXPERIMENTAL
The 19F NMR spectra were recorded on a Bruker
WP 80 instrument (75.398 MHz) in acetone-d6 using
hexafluorobenzene as internal reference. The mass
spectra (70 eV) were run on a Finnigan Mat ITD 800
spectrometer (batch inlet temperature 200 C). GLC
analysis was performed on a Chrom-5 chromatograph
equipped with a thermal conductivity detector; 5-m
3.5-mm column packed with 20% of FS-169 on
Chromaton-W; oven temperature 150 C (isothermal
mode); carrier gas helium (flow rate 50 ml/min).
Iron(III) chloride hexahydrate FeCl3 6H2O of
pure grade was used without additional purification.
Pentafluorobenzotrichloride was synthesized by the
procedure described in [10]. Its purity was 99%,
according to the GLC data.
This assumption does not contradict published data.
It is known that both aromatic and benzylic fluorine
atoms in poly- or perfluorinated aromatic compounds
are smoothly replaced by chlorine or bromine by the
action of the corresponding aluminum halide (AlCl3
and AlBr3) [4 9]. As a rule, benzylic fluorine atom
is replaced under milder conditions than those neces-
sary for replacement of aromatic fluorine. Here,
aluminum chloride or bromide acts as fluorine
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 7 2003

978
SHIPILOV et al.
Table 2. 19F NMR spectral parameters of polyfluorobenzenes
Chemical shifts F, ppm (relative to CCl3F)
Compound
no.
1-F
2-F
3-F
4-F
5-F
6-F
I
II
133.8
138.4
134.4
139.3
133.5
137.8
134.8
134
161.1
159.9
160.4
134.6
161.0
159.8
137.8
140.3
150.0
146.0
143.3
161.1
159.9
160.4
134.6
161.0
159.8
137.8
140.3
133.8
138.4
134.4
139.3
133.5
137.8
134.8
134
III
IV
V
VI
VII
VIII
46.3 (COF)
45.2 (CCl2F)
55.1 (CClF2)
45.9 (CCl2F)
149.0
147.1
Hydrolysis of pentafluorobenzotrichloride.
Pentafluorobenzotrichloride, 285.5 g, was heated to
110 120 C, and 18 g of water containing 4.28 g of
FeCl3 6H2O was added with stirring over a period
of 3.5 h. The mixture was then stirred for 1 h at that
temperature and analyzed. The other experiments
were carried out following the same procedure. The
amounts of the reactants, reaction conditions, and
yields of the products are collected in Table 1. The
Reference Book), Oshin, A.L., Ed., Moscow: Khi-
miya, 1972, p. 450.
3. Ayyangar, N.R., Lahoti, R.J., Srimiasan, K.V., and
Daniel, T., Synthesis, 1991, p. 322.
4. Yakobson, G.G. and Shteingarts, V.D., Zh. Obshch.
Khim., 1964, vol. 34, p. 932.
5. Kobrina, L.S. and Furin, G.G., Izv. Sib. Otd. Akad.
Nauk SSSR, Ser. Khim., 1968, no. 4, p. 98.
6. Dvornikova, K.V. and Platonov, V.E., Izv. Akad.
Nauk SSSR, Ser. Khim., 1990, no. 2, p. 468.
19F NMR spectra of the products are given in Table 2.
The mass spectra of all the products contained the
molecular ion peaks.
7. Dvornikova, K.V., Platonov, V.E., and Yakob-
son, G.G., Zh. Org. Khim., 1991, vol. 27, p. 609.
8. Cherstkov, V.F., Sterlin, S.R., and German, L.S., Izv.
Akad. Nauk SSSR, Ser. Khim., 1985, no. 11, p. 2647.
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10. Sintezy ftororganicheskikh soedinenii (Syntheses of
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