Friedel-crafts benzoylation of alkylbenzenes with brazoic anhydride catalyzed by solid superacids

Article abstract of DOI:10.1246/bcsj.77.1893

The liquid-solid phase benzoylation of mono-alkylbenzenes with methyl, ethyl, propyl, and butyl groups and xylenes was carried out with benzoic anhydride at 100-110°C over solid superacids: SO₄/ZrO ₂, WO₃/ZrO₂, and SO₄/ HfO ₂. The reactivity ratio obtained by the competitive method of reaction over WO₃/ZrO₂ was 1 to 4.6 for toluene to p-xylene and 1.1:10:1 among o-, m-, and p-xylenes, respectively. Although the SO₄/ZrO₂ catalyst gave high yields of 92 and 97% for toluene and ethylbenzene in a 3:7 distribution of o- and p-isomers, respectively, low yields were observed with propyl and butylbenzenes over the catalyst: that is, 5 and 2% for propylbenzene and isopropylbenzene, 14% for isobutylbenzene, and trace yields for butylbenzene, s-butylbenzene, and t-butylbenzene, respectively. The usual Friedel-Crafts benzoylation using AlCl₃ was examined in the present alkylbenzenes in order to confirm the low reactivity of both propyl and butylbenzenes. The results were similar to those with the SO₄/ZrO₂ catalyst; that is, the yields at 0°C for 1 h were 37, 21, 6, 1, 0, 3, and 2% for toluene, ethylbenzene, propylbenzene, isopropylbenzene, butylbenzene, s-butylbenzene, and t-butylbenzene, respectively, showing an unexpected result where there was no distinction between homogeneous and heterogeneous conditions.

Full text of DOI:10.1246/bcsj.77.1893

ꢀ 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 1893–1896 (2004) 1893
Friedel–Crafts Benzoylation of Alkylbenzenes with Benzoic
Anhydride Catalyzed by Solid Superacids
ꢀ
ꢀ
Hideo Nakamura and Kazushi Arata
Department of Science, Hokkaido University of Education, Hachiman-cho, Hakodate 040-8567
Received January 8, 2004; E-mail: karata@cc.hokkyodai.ac.jp
The liquid–solid phase benzoylation of mono-alkylbenzenes with methyl, ethyl, propyl, and butyl groups and xy-
ꢁ
lenes was carried out with benzoic anhydride at 100–110 C over solid superacids: SO4/ZrO2, WO3/ZrO2, and SO4/
HfO2. The reactivity ratio obtained by the competitive method of reaction over WO3/ZrO2 was 1 to 4.6 for toluene
to p-xylene and 1.1:10:1 among o-, m-, and p-xylenes, respectively. Although the SO4/ZrO2 catalyst gave high yields
of 92 and 97% for toluene and ethylbenzene in a 3:7 distribution of o- and p-isomers, respectively, low yields were ob-
served with propyl and butylbenzenes over the catalyst: that is, 5 and 2% for propylbenzene and isopropylbenzene, 14%
for isobutylbenzene, and trace yields for butylbenzene, s-butylbenzene, and t-butylbenzene, respectively. The usual
Friedel–Crafts benzoylation using AlCl3 was examined in the present alkylbenzenes in order to confirm the low reac-
tivity of both propyl and butylbenzenes. The results were similar to those with the SO4/ZrO2 catalyst; that is, the yields
at 0 ꢁC for 1 h were 37, 21, 6, 1, 0, 3, and 2% for toluene, ethylbenzene, propylbenzene, isopropylbenzene, butylbenzene,
s-butylbenzene, and t-butylbenzene, respectively, showing an unexpected result where there was no distinction between
homogeneous and heterogeneous conditions.
The catalytic acylation of aromatics via the Friedel–Crafts
reaction for introducing functional substituents to the aromatic
rings is an attractive challenge for clean technology as well as
for organic synthesis. Thus, there have been several efforts to
conduct the reaction using catalytic amounts of acidic promot-
ers.1–9 Yadav and Pujari synthesized 4-chlorobenzophenone, a
pharmaceutical intermediate, via the acylation of benzene with
4-chlorobenzoyl chloride over sulfated zirconia, a solid super-
acid.10
Solid superacids of sulfated and supported metal oxides,
such as sulfated zirconia (SO4/ZrO2) and tungstated zirconia
(WO3/ZrO2), were synthesized by the addition of sulfate spe-
cies to oxide surfaces of Fe, Ti, Zr, Hf, Sn, Al, and Si, or by
supporting SnO2, TiO2, and Fe2O3 with WO3 and ZrO2 with
MoO3 and B2O3.11,12 A previous paper reported a systematic
study of those superacids for the benzoylation of toluene with
benzoic anhydride and benzoyl chloride in the liquid–solid
phase.13 Most of the superacids showed satisfactory results,
and several catalysts were also examined in a less reactive ac-
ylation: the acetylation of toluene; however, satisfactory results
were not obtained. Thus, the acetylation of toluene with acetic
anhydride was investigated in detail using catalysts selected
from the above results, but it was demonstrated that the cata-
lysts could not act efficiently to complete the reaction because
of the most difficult acylation owing to a difficulty in the for-
mation of an intermediate acetyl-cation (MeCOþ) from acetic
anhydrides and acetyl halides. In this sense, the catalytic action
was more effective for propionylation and butyrylation, more
reactive acylations with longer hydrocarbon chains.14 The ace-
tylation, however, was quite effective on anisole, a more reac-
tive substrate, over the SO4/ZrO2 catalyst.14,15
and the development of wide substrates is in demand. This pa-
per is concerned with a study of the catalytic benzoylation of
alkylbenzenes, such as xylenes, ethylbenzene, propylbenzenes,
and butylbenzenes, over superacid materials. The results were
also compared with those by the usual reaction procedures us-
ing AlCl3 as a catalyst.
Experimental
Catalyst Preparation. SO4/ZrO2 was prepared by calcining
sulfated zirconia of Wako Pure Chemical Industries, Ltd., at 550
ꢁC for 3 h. WO3/ZrO2 and SO4/HfO2 were prepared following
the literature.13,14
Reaction Procedure. Benzoylation was carried out with a
mixture of 140 mmol (15–22 mL) of alkylbenzene (purified by
distillation before use), 2 mmol of benzoic anhydride (0.45 g),
0.5 g of catalyst, and 0.185 g (1 mmol) of tridecane as an internal
standard with stirring under an argon atmosphere. A small amount
of the sample was withdrawn periodically with a 1 mL syringe,
diluted with ethyl acetate, separated from the catalyst by filtration,
and analyzed by gas-liquid chromatography with an FIDꢁdetector
using a 25 m column of OV-1701 BONDED (120–220 C). The
yields were calculated based on the acylating reagents.
Reactions catalyzed by AlCl3 (Wako Pure Chemical Industries,
Ltd.) were performed at 0 ꢁC in the same manner as the above pro-
cedures, the catalyst amount being three-times as much as the ac-
ylating agent, 6 mmol. After the reaction mixture was diluted with
ethyl acetate, the organic phase was washed with water, saturated
NaHCO3, and brine followed by drying over MgSO4. After the
solvent was removed under reduced pressure, the products were
analyzed.
Isolation and Identification of the Product. Silica-gel col-
umn chromatography was performed using Merck Kiesel 60
(70–230 mesh). The product isomers were identified by a compar-
ison of their spectral data (1H and 13C NMR spectra on a JEOL
In a series of the Friedel–Crafts acylations catalyzed by su-
peracids, the substrates have been limited mainly to toluene,
Published on the web October 9, 2004; DOI 10.1246/bcsj.77.1893

1894 Bull. Chem. Soc. Jpn., 77, No. 10 (2004)
Benzoylation of Alkylbenzenes
Table 1. Benzoylations of Toluene and Xylenes with
ꢁ
70
60
50
40
30
20
10
Benzoic Anhydride at 100 C for 2 h
Reactant
Catalyst Yield % Products and isomer ratio
SO4/ZrO2 82
WO3/ZrO2 41
PhCO
COPh
p-Xylene
,
SO4 /HfO2
20
Toluene
3:7
SO4/ZrO2
99
COPh
WO3/ZrO2 32
SO4/HfO2
14
0
0.5
1
1.5
2
2.5
3
Reaction time/h
PhCO
SO4/ZrO2
98
Fig. 1. Competitive reaction between p-xylene ( ) and tol-
ꢁ
WO3/ZrO2 92
COPh
COPh
,
uene ( ) over WO3/ZrO2 at 90 C.
SO4/HfO2
88
1:10
Table 2. Competitive Reaction among Toluene and Xylenes
ꢁ
SO4/ZrO2
99
COPh
over WO3/ZrO2 at 90 C for 30 min
WO3/ZrO2 49
SO4/HfO2 19
YieldaÞ/%
Yield/%
Reactivity ratio
,
Reactant
1:3
o-Xylene
m-Xylene
p-Xylene
Toluene
4
30
3
1.1
10
1
JNM-EX270) and retention times in GLC with those of authentic
samples.
23
5
4.6
1
a) Reaction in Fig. 1.
Results and Discussion
Benzoylations of Toluene and Xylenes. The benzoylation
of toluene along with o-, m-, and p-xylenes with benzoic anhy-
dride was carried out in the liquid phase at 100 C over SO4/
The yields at 30 min of time are summarized in Table 2 to-
gether with their reactivity ratio, estimated by the yields. The
ratio is 1 to 4.6 for toluene to p-xylene and 1.1 to 10 to 1 for o-
to m- to p-xylenes, respectively. A large difference in the reac-
tivities is reasonably based on the electron density on the ben-
zene ring. The high reactivity of p-xylene against toluene is
explained by the double electron-releasing effect of two meth-
yl groups, and the extremely high value of m-xylene against o-
and p-xylenes is interpreted by both o- and p-orientation ef-
fects of xylene brought by two methyl groups. The relative re-
activity among o-, m-, and p-xylenes was determined to be
6:16:1, respectively, in the homogeneous reaction of benzoyl
chloride with AlCl3.16
Benzoylation of Isobutylbenzene. Since the benzoylation
of methyl-activated benzenes was examined, the reaction of
isobutylbenzene was accomplished in order to investigate
how longer hydrocarbon chains work and how side chains
act. Benzoylation following the method of toluene was per-
formed over the SO4/ZrO2 catalyst at 110 ꢁC for 4 h, but
the yield was quite low, 14%. Supplementary examinations
were attempted several times; the results were identical with
the above. The time-variation of the reaction showed a failure
of the catalyst to react further after 2–3 h, being different from
the cases of toluene and xylenes, showing a continual increase
of yield up to 100%. A question arose concerning the deacti-
vation of the catalyst on account of an excess of the substrate.
Thus, the reaction was carried out in solvents of cyclohexane,
chlorobenzene, and nitrobenzene, frequently used in homoge-
neous Friedel–Crafts reactions. However, yields close to the
above were just obtained by stirring a mixture of 2 mL of iso-
butylbenzene, 2 mmol of the anhydride, and 20 mL of solvent
over the catalyst for 5 h.
ꢁ
ZrO2, WO3/ZrO2, and SO4/HfO2, the former two of which
showed the highest activity for the benzoylation of toluene
among the solid superacids of sulfated and supported metal ox-
ides, respectively, in the previous paper;13 the results for 2 h
are given in Table 1. As for the reaction of toluene, the previ-
ous results were reproduced, and the activity ratio was estimat-
ed to be 4:2:1 for SO4/ZrO2, WO3/ZrO2, and SO4/HfO2, re-
spectively.
In the benzoylation of xylenes, 2,5-dimethylbenzophenone
was formed from the reaction of p-xylene, 2,4- and 2,6-di-
methylbenzophenones in the ratio 10:1 from m-xylene, and
2,3- and 3,4-dimethylbenzophenones in the ratio 1:3 from o-
xylene. Although similar activities to those for toluene among
the catalysts were observed with o- and p-xylenes, the yields
obtained by the reaction of m-xylene were quite high in all cas-
es, showing its high reactivity.
Competitive Reaction among Toluene, o-, m-, and p-
Xylenes. In order to confirm the intrinsic reactivities of the
substrates, a competitive reaction method among the three xy-
lenes along with toluene was tried. The competitive benzoyla-
tion between toluene and p-xylene was carried out over WO3/
ZrO2 with the anhydride in equimolar amounts of toluene
(7 mL) and p-xylene (8 mL); Fig. 1 shows the time variations
of the total yields of isomers. Variations among the three iso-
mers of xylene (5 mL each) were also conducted in the same
manner as mentioned above. The reactions showed a continual
increase of yield with time, close to first-order reaction kinetics
with respect to the anhydride concentration. The reactions are
thus judged to be heterogeneously catalytic.

H. Nakamura et al.
Bull. Chem. Soc. Jpn., 77, No. 10 (2004) 1895
Table 3. Benzoylations of Alkylbenzenes with Benzoic
ꢁ
80
70
60
50
40
30
20
10
Anhydride at 110 C for 4 h
Isomers/%
Reactant
Catalyst
Yield/%
o-
p-
Isobutylbenzene
SO4/ZrO2
SO4/SnO2
SO4/HfO2
WO3/ZrO2
SO4/ZrO2
SO4/ZrO2
SO4/ZrO2
SO4/ZrO2
SO4/ZrO2
SO4/ZrO2
SO4/ZrO2
SO4/ZrO2
14
7
4
9
8
8
7
91
92
92
93
4
Butylbenzene
s-Butylbenzene
t-Butylbenzene
Tetralin
Propylbenzene
Isopropylbenzene
Ethylbenzene
Toluene
Trace
Trace
Trace
Trace
5
2
97
92
0
4
2
6
8
Reaction time /h
28
15
30
27
72
85
70
71
Fig. 2. Benzoylations of toluene ( ), ethylbenzene ( ),
propylbenzene ( ), isopropylbenzene ( ), butylbenzene
( ), isobutylbenzene ( ), and s-butylbenzꢁene ( ) with
benzoic anhydride catalyzed by AlCl3 at 0 C.
The benzoylation of isobutylbenzene was performed over
other catalysts, WO3/ZrO2 and SO4/HfO2, together with
SO4/SnO2, where the SnO2 catalyst was recently reported to
be a solid superacid with its acid strength higher than that of
SO4/ZrO2, and was prepared by following the literature.17,18
The results are given in Table 3. Although yields higher than
14% were expected, SO4/SnO2 gave just 7% yield.
50
40
30
isoPropylbenzene
20
The product distribution was 7–9% 2- and 91–93% 4-isobu-
tylbenzophenones, and the meta isomer was not detected. The
low ortho-substitution compared with the case of toluene is
based on the steric hindrance of a large isobutyl group.
Benzoylations of Benzenes with Methyl, Ethyl, Propyl,
and Butyl Groups. Since the results in the benzoylation of
isobutylbenzene were unexpected, the reactivities of other al-
kylated benzenes were compared. Table 3 summarizes the re-
sults in the benzoylation of toluene, ethylbenzene, propylben-
zene, isopropylbenzene, butylbenzene, s-butylbenzene, and t-
butylbenzene over SO4/ZrO2 under the same conditions as
those of isobutylbenzene. Toluene and ethylbenzene showed
reasonable conversions of 92 and 97% in the distribution 3:7
of o- and p-isomers, respectively. On the other hand, the con-
versions of propyl- and butyl-activated benzenes were surpris-
ingly low, far from the results that we expected when consid-
ering the reactivity of the organic compounds, the highest yield
being 5% for propylbenzene. Their inactivity is independent of
the interaction of the hydrocarbon chains with the catalyst sur-
face, because tetralin, equivalent to cyclic butylbenzene, is
also inactive.
Butylbenzene
10
0
1
4
6
2
3
5
Reaction time /h
Fig. 3. Benzoylations of isopropylbenzene ( ) and butyl-
benzene ( ) with benzoic anhydride and AlCl3 by the ad-
dition of H2O (10 mL) at 0 C.
ꢁ
the coordination of AlCl3 with the acylating reagent, followed
by forming complexes with both the carbonyl product and pro-
duced benzoic acid.
The results given in Fig. 2 lead to a question whether the ac-
tive species are created during the reaction, for instance HCl
derived from the reaction of AlCl3 with contaminated H2O.
Thus, the benzoylations of isopropyl and butylbenzenes, show-
ing wide S-curves in Fig. 2, were performed with the addition
of H2O (10 mL); the results are shown in Fig. 3. Both of the
reactions show a continual increase of yield with time in strik-
ing contrast with those in Fig. 2, indicating the catalytic effect
of the produced HCl. However, benzoylation did not occur
with HCl only without AlCl3; that is, the reaction of toluene
with benzoyl chloride and H2O, where HCl was supposed to
be produced, did not take place, but subsequently proceeded
by the addition of AlCl3.
In consideration of the above results, it is proposed that the
reaction is performed with benzoyl chloride as the acylating
agent, derived from the reaction of benzoic anhydride with
HCl, produced by the hydrolysis of AlCl3. Thus, the possibility
of benzoyl chloride being formed was examined, and the chlo-
ride was detected by the analysis of TLC after 30 min of reac-
tion in theꢁreaction of toluene with benzoic anhydride and
AlCl3 at 0 C, and not detected after 2 h because of probable
completion of the reaction. Therefore, it would be advisable
Homogeneous Benzoylations of Benzenes with Methyl,
Ethyl, Propyl, and Butyl Groups. The present low conver-
sions of various propyl and butylbenzenes lead to a question
regarding whether active species on the solid surface are pois-
oned, that is, whether there is a distinction between homogene-
ous and heterogeneous conditions. Therefore, the present ben-
zoylation was carried out under the usual, homogeneous,
Friedel–Crafts conditions using AlCl3 as a catalyst. The results
are shown in Fig. 2 as the time-variations of conversion. It can
be seen that all of the reactions progress continuously with
ꢁ
time at 0 C, but the process of the reaction shows S-shaped
curves without following the first-order kinetics with respect
to the anhydride, particularly in the cases of propyl and butyl-
benzenes. The reaction homogeneously proceeded based on

1896 Bull. Chem. Soc. Jpn., 77, No. 10 (2004)
Benzoylation of Alkylbenzenes
Table 4. Benzoylations of Alkylbenzenes with Benzoic
ꢁ
interpreting the activity of the benzene ring by the electron-
releasing effect of the alkyl groups.
Anhydride Catalyzed by AlCl3 at 0 C for 1 h
Isomers/%
Reactant
Yield/%
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