Metal free Friedel-Crafts di-acetylation of resorcinol in acetic acid

Article abstract of DOI:10.14233/ajchem.2017.20224

Conventionally, the metal catalyst used for Friedel-Crafts acetylation is poisonous and the acetylating agent is not readily available. Herein, methane sulfonic acid is developed to perform Friedel-Crafts diacetylation of aromatics instead of metal catalyst. Furthermore, acetic acid is employed as acetylating agent to replace acetic anhydride. By this approach, 4,6-diacetylresorcinol, known as an indispensable intermediate in superior fiber polybenzoxazole synthesis, is prepared in high yield from resorcinol. The results obtained by ¹H NMR, FT-IR and HPLC analysis indicate that methane sulfonic acid is an efficient catalyst for Friedel-Crafts acetylation and acetic anhydride can be avoid by replacement with acetic acid, in which phosphorus pentoxide is helpful to improve conversion of desired product.

Full text of DOI:10.14233/ajchem.2017.20224

Asian Journal of Chemistry; Vol. 29, No. 4 (2017), 749-754
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20224
Metal Free Friedel-Crafts Di-acetylation of Resorcinol in Acetic Acid
2
2
FENGGUI CHEN1,2,*, YIMENG GAO and YUANJIAN XU
1College of Chemistry and Chemical Engineering, Yangtze Normal University, Fuling, Chongqing 408003, P.R. China
2Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences,
Chongqing 400714, P.R. China
*Corresponding author: Fax: +86 23 65935815; Tel: +86 23 65965813; E-mail: cfg@cigit.ac.cn
Received: 1 August 2016;
Accepted: 15 December 2016;
Published online: 31 January 2017;
AJC-18237
Conventionally, the metal catalyst used for Friedel-Crafts acetylation is poisonous and the acetylating agent is not readily available.
Herein, methane sulfonic acid is developed to perform Friedel-Crafts diacetylation of aromatics instead of metal catalyst. Furthermore,
acetic acid is employed as acetylating agent to replace acetic anhydride. By this approach, 4,6-diacetylresorcinol, known as an indispensable
intermediate in superior fiber polybenzoxazole synthesis, is prepared in high yield from resorcinol. The results obtained by 1H NMR, FT-
IR and HPLC analysis indicate that methane sulfonic acid is an efficient catalyst for Friedel-Crafts acetylation and acetic anhydride can
be avoid by replacement with acetic acid, in which phosphorus pentoxide is helpful to improve conversion of desired product.
Keywords: Acetic acid, Acetic anhydride, Acetylation, 4,6-Diacetylresorcinol, Friedel-Crafts reaction, Methane sulfonic acid.
also been synthesized in such a green way [11]. However, 2,4-
dihydroxyacetophenone cannot be further di-acetylated,
resulting in main production as mono-acetylated aromatic
compound.
INTRODUCTION
Friedel-Crafts (FC) acylation is usually ultilized to synthe-
size aromatic ketones, which are widely used in the pharma-
ceutical, fine chemical and other reactive intermediates [1,2].
Hence, a number of attempts have been devoted to improve
the catalyst and acylating agent during Friedel-Crafts acylation.
Conventionally, electrophilic acylation is mostly catalyzed by
Lewis acids (such as ZnCl2, AlCl3, FeCl3, SnCl4, TiCl4, ZrCl4
and BF3) or Bronsted acids (such as HF, PPA and H2SO4) [3-
9]. In particular, the use of metal halides causes problems
associated with the strong complex formed between ketone
product and metal halide itself, which provokes the use of
more than stoichiometric amounts of catalyst [1]. In other
words, catalyst in acylation is in great demand, which increases
cost of desired product preparation. Besides, it is poisonous to
the environment, leading to waste disposal problems and it
also complicates reaction operation by increasing viscosity of
reaction solution.
Therefore, a simple, green and efficient method is in demand
to prepare acylated aromatics. As a result, some studies on
environment acceptable catalysts development are underway
to improve the acylation [1,2,10,11]. For example, 2,4-
dihydroxyacetophenone was synthesized from resorcinol in
the presence of a variety of solid acid catalysts [1,2]. Moreover,
acetylation of activated aromatics was investigated without
any added acid catalyst and 2,4-dihydroxyacetophenone has
Besides, methane sulfonic acid (MSA), recognized as a
strong, stable and biodegradable bronsted acid, is usually used
as catalyst and solvent for some condensation or rearrangement
reactions[12-17]. Generally, methanesulfonicacidbasedcatalyzed
systems containing P2O5 orAl2O3, are proven to be efficient for
acylation of activated aromatics [13,14]. Fries rearrangement
of phenolic esters was performed to synthesize acylaryl methane
sulfonates catalyzed by POCl3 with methane sulfonic acid.
Furthermore, Fries rearrangement of anilides was carried out
in the presence of mixture of P2O5 and methane sulfonic acid,
which was reported to be an efficient reagent for selective
synthesis of p-aminoaryl ketones [15]. Moreover, mixture of
alumina in methane sulfonic acid is used to prepare o-hydroxy-
arylketones by acylation of phenol and naphthol derivatives
with carboxylic acids or Fries rearrangement of phenolic esters.
However, to the best of our knowledge, the individual methane
sulfonic acid has not been employed as catalyst in Friedel-
Crafts acylation. Thus, as a biodegradable and easy-to-handle
liquid, methane sulfonic acid is expected to work in acylation
of activated aromatics.
In addition, acetyl chloride, acetic anhydride and acetic
acid are known as acetylating agent [18,19]. Acetylation
activity of acetyl chloride is high, whereas it is poisonous,

750 Chen et al.
Asian J. Chem.
giving bad smell and generating byproduct as hydrochloric
acid [2]. Owing to its high reactivity and good controllability,
acetic anhydride is in favour over acetyl chloride. However,
acetic anhydride is a restricted chemical due to its potential
use in narcotics, therefore, it is not readily available even for
laboratory purposes. It also gives acetic acid as byproduct,
resulting in poor atom economy. Besides, the anhydrides are
always commercially produced from the corresponding acids
by dehydration. In the current drive toward less wasteful and
more environmentally friendly processes, where the emphasis
is on atom efficiency and recyclability, a number of attempts
have been devoted to the acetylation of resorcinol by employing
acetic acid as acetylating agent [18]. However, acetic acid has
low acetylating activity, which is usually not effective in di-
acetylation of aromatic compounds. In other words, 4,6-
diacetylresorcinol can’t be efficiently synthe-sized with acetic
acid. Thus, it is meaningful to achieve di-acetylation of
resorcinol in acetic acid.
As known, 4,6-diacetylresorcinol, which is commercially
important intermediate for preparation of 4,6-diamino-
resorcinol dihydrochloride (DAR·2HCl) and further superior
aromatic polybenzoxazole, is usually prepared by Friedel-
Crafts di-acetylation of resorcinol [20-22]. However, the
conventional di-acetylation of resorcinol was performed with
acetic anhydride and 2 molar excess of metal containing catalyst
as zinc chloride is required, which also caused waste disposal
problems. Hence, the aim of this work is to develop greener
way for di-acetylation of resorcinol. This paper dealt with di-
acetylation of resorcinol by employing methane sulfonic acid
as catalyst and acetic acid as acetylating agent. Hopefully, 4,6-
diacetylresorcinol can be successfully synthesized in high yield
in the presence of methane sulfonic acid and acetic acid.
Furthermore, the catalyst may be recycled and reused, which
is in consonance with the principles of green chemistry.
then cooled to room temperature and poured into cool mixture
of water and methanol. Afterwards, the product was collected
by vacuum filtration and dried under vacuum at 40 °C.
Friedel-Crafts di-acetylation of resorcinol in acetic
acid: Resorcinol, P2O5 and acetic acid were added into a three-
neck round-bottomed flask with mechanical stirrer. The mixture
was stirred and methane sulfonic acid was added. Afterwards,
the reaction was allowed to proceed for several hours in a
predetermined temperature, where samples were collected for
HPLC analysis every 30 min. The mixture was then cooled
and poured into water and methanol mixture. Finally, the product
was collected by vacuum filtration and vacuum dry.
Into a 1000 mL glass flask, equipped with a mechanical
stirrer and a nitrogen inlet/outlet, were placed 400 mL water.
4,6-diaminoresorcinol dihydrochloride (4.26 g, 0.02 mol) was
added. One equimolar amount of 2,6-pyridinedicarboxylic acid
(PDA, 0.02 mol) dissolved in NaOH (0.04 mol) aqueous
solution was slowly added.Afterwards, the mixture was heated
to 90 °C for 10 min and DAR-PDA salt solid precipitated, then
the product was collected in nitrogen atmosphere. The product
was dried under vacuum at 60 °C for 48 h.
RESULTS AND DISCUSSION
In general, Friedel-Crafts di-acetylation of resorcinol was
performed by using metal catalyst as ZnCl2 and the required
molar ratio of the acid catalyst to resorcinol is at least 2:1, which
causes high pollution to environment. Thus, it is meaningful
to develop a novel and green catalyst to achieve the di-acetyla-
tion. As known, methane sulfonic acid is usually used as
catalyst and solvent in some condensation or rearrangement
reactions [12-17], besides, it processes readily biodegradable
and environmentally acceptable feature, methane sulfonic acid
is therefore preferred in this system.As expected, by catalyzed
by methane sulfonic acid, 4,6-diacetylresorcinol was success-
fully synthesized in high yield shown in Scheme-I, which was
confirmed by 1H NMR, FT-IR and HPLC analysis.
EXPERIMENTAL
Resorcinol (99.5 %), ZnCl2 (≥ 98.0 %), acetic anhydride
(≥ 98.5 %), acetic acid (≥ 99.5 %), P2O5 (≥ 98.0 %), methane
sulfonic acid (≥ 99 %) and methanol (≥ 99.5 %) were purchased
from Sinopharm Chemical Reagent Co., Ltd. and used as received.
The water used was purified by filtration through Millipore
Gradient system after distillation, giving a resistivity not more
than 18.2 MΩ cm.
HO
OH
HO
OH
O
or
MSA
90–150 °C
OH
O
O
Scheme-I: Friedel-Crafts di-acetylation of resorcinol in the presence of
All proton nuclear magnetic resonance (1H NMR) spectra
were determined on a Bruker DMX-400 instrument with DMSO-
d6 as solvent and TMS as internal standard. Fourier transform
infrared (FT-IR) spectra were recorded on a Bruker VECTOR-
22 IR spectrometer. The reaction was tracked by HPLC on the
Ultimate 3000 HPLC instrument using water and acetonitrile
containing 5 mmol L-1 trifluoroacetic acid as eluent.
methane sulfonic acid
As shown in Fig. 1, the peak a at 6.4 ppm is ascribed to
the proton (1H) of aromatic ring adjacent to OH group and
the peak b at 8.2 ppm is ascribed to the proton (1H) of aromatic
ring adjacent to the acyl group and the peak c at 12.9 ppm is
ascribed to protons (2H) of OH and the peak d at 2.6 ppm is
ascribed to methane protons (6H) of acetyl group. The integra-
tion ratio of a:b:c:d is 1:1:2:6, which is consistent with the
structure of 4,6-diacetylresorcinol.
As FTIR spectra of obtained 4,6-diacetylresorcinol shown
in Fig. 2, band assignments around at about 1622 cm-1 is attri-
buted to the C=O stretching vibrations of aromatic ketone,
band at about 1578 cm-1 and 1483 is corresponding to C-C stret-
ching vibrations of aromatic rings.
Friedel-Crafts di-acetylation of resorcinol in acetic
anhydride: Resorcinol (5.5 g, 0.05 mol), acetic anhydride,
were added into a 100 mL three-neck round-bottomed flask
with a mechanical stirrer. After resorcinol dissolved in the
acetic anhydride, methane sulfonic acid was added. Then the
oil bath was raised to a predetermined temperature and the
reaction was allowed to proceed for several hours. Samples were
collected for HPLC analysis every 30 min. The mixture was

Vol. 29, No. 4 (2017)
Metal Free Friedel-Crafts Di-acetylation of Resorcinol in Acetic Acid 751
Moreover, as the reaction time increases, the content of resor-
cinol decreases and the desired product increases. It further
confirmed that 4,6-diacetylresorcinol was successfully
synthesized. The results also imply that methane sulfonic acid
is an efficient catalyst for the resorcinol di-acetylation. It
implies that methane sulfonic acid is a feasible catalyst for
Friedel-Crafts di-acetylation.
A series of experiments were carried out to investigate
influence on Friedel-Crafts acetylation by using methane
sulfonic acid as catalyst. The results (1-1 to 1-10) are summa-
rized in Table-1. There is no doubt that temperature plays a
great role in the reaction.As list in Table-1 and shown in Fig. 4a,
different temperature from 90 to 150 °C were performed to
optimize the temperature for the reaction. Experiment 1-1, 1-
a
HO
OH
b
O
O
c
b
a
d
14
12
10
8
6
4
2
0
2, 1-3 and 1-4 were respectively carried out at 90, 110, 130,
150 °C, with molar ratio of methane sulfonic acid/acetic
anhydride/resorcinol 2:2:1. For experiment 1-1 performed at
90 °C, the conversion of di-acetylated resorcinol gradually
increases with increase of reaction time; it reaches 79 % after
4 h. In the case of reaction at 110 °C (1-2), the conversion
increases to 84.6 % in 1.5 h and then slightly decreases to
82.2 % in 4.0 h. For reaction at 130 °C (1-3), the conversion
increases to 90.5 % in 1.0 h and then slightly decreases to
84.1 % after 4.0 h. And for reaction at 150 °C (1-4), the
conversion increases to 89.6 % in 0.5 h and then drops to 39 %
after 4.0 h. Clearly, conversion of the desired product increases
to a maximum value and then decreases, which is probably
due to decomposition of desired product at high temperature.
Moreover, higher temperature results in higher reaction rate.
Hence, it can be found that 110-130 °C is a suitable temperature
range for the reaction, since conversion can reach to high value
in short time and there is only slight product decomposition.
Therefore, all further experiments were carried out at 130 °C.
It is known that Friedel-Crafts acetylation of resorcinol
is influenced not only by reaction temperature, but also the
δ
(ppm)
Fig. 1. 1H NMR spectra of the 4,6-diaceticresorinol prepared by Friedel-
Crafts di-acetylation of resorcinol
1622
1578
1483
3000
2000
1000
Wavenumber (cm–1)
Fig. 2. FTIR spectra of obtained 4,6-diacetylresorcinol prepared by Friedel-
Crafts di-acetylation of resorcinol
TABLE-1
HPLC results of the product (Fig. 3) shows the same
retention time as authentic 4,6-diacetylresorcinol sample,
which indicates that the product is the di-acetylated product.
FRIEDEL-CRAFTS DI-ACETYLATION OF RESORCINOL
WITH ACETIC ANHYDRIDE CATALYZED BY
METHANE SULFONIC ACID
HO
OH
HO
OH
MSA
Acetic anhydride
Resorcinol
(1) 0.5 h
O
O
(2) 1.0 h
(3) 1.5 h
(4) 2.0 h
(5) 2.5 h
(6) 3.0 h
Entry
1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
1-9
1-10
Temp. (°C) Molar ratioa Time (h)b Conversion (%)c
90
2:2:1
2:2:1
2:2:1
2:2:1
1:2:1
4.0
1.5
1.0
0.5
2.5
7.0
–
0.5
0.5
1.0
79.0
84.6
90.5
89.6
86.1
76.8
–
62.0
8.6
17.8
110
130
150
130
130
130
130
130
130
(0.5):2:1
(0.1):2:1
2:1:1
(7) 3.5 h
Standard sample of 4.6-diacetylresorcinol
2:2:1*
2:5:1*
aMolar ratio of methane sulfonic acid/acetic anhydride/resorcinol,
*Acetic acid is used as acetylating agent instead of acetic anhydride;
bTime to reach the highest conversion of 4,6-diacetylresorcinol from
resorcinol; cConversion of 4,6-diacetylresorcinol from resorcinol, --no
desired product was obtained.
2
4
6
8
10
12
14
Retention time (min)
Fig. 3. HPLC spectra of the generated 4,6-diacetylresorcinol as a function
of time during Friedel-Crafts di-acetylation of resorcinol, which was
performed in acetic anhydride catalyzed by methane sulfonic acid

752 Chen et al.
Asian J. Chem.
catalyst dosage. Therefore, the amount of methane sulfonic
acid was screened and the results are listed in Table-1 and
Fig. 4b, molar ratio of methane sulfonic acid to resorcinol
varies from 2:1 to 0.1:1. Experiment 1-3, 1-5, 1-6 and 1-7 are
respectively corresponding to the reaction performed with
molar ratio of methane sulfonic acid/resorcinol 2:1, 1.0:1, 0.5:1
and 0.1:1, with molar ratio of acetic anhydride/resorcinol 2:1
at 130 °C. For experiment 1-5 operated with molar ratio of
methane sulfonic acid/resorcinol only 1.0:1, the conversion
can reach 86.1 % after 2.5 h. And for 0.5:1, the conversion
reaches 76.8 % after 7 h (data not shown). However, when the
ratio is 0.1:1, no di-acetylated resorcinol can be observed. It
indicates that methane sulfonic acid is indispensable in the
reaction but small amount of catalyst used can also result in
high yield of 4,6-diacetylresorcinol preparation. Note that more
than 2:1 of molar ratio of catalyst/resorcinol was required in
conventional method.
Furthermore, as shown in Fig. 4c, influence of the molar
ratio of acetylating agent/reactant on Friedel-Crafts di-
acetylation of resorcinol was also examined. Compared with
reaction performed with acetic anhydride/resorcinol molar
ratio 2:1, the conversion obtained with ratio of acetic anhy-
dride/resorcinol 1:1 is much lower, which can only reach 62.0
%. Hence, acetylating agent dosage plays a critical role in the
di-acetylation of resorcinol. It also indicates that more than
2:1 of acetylating agent/reactant is required to achieve the di-
acetylation and the generated acetic acid is unable to further
acetylate the mono-acetylated resorcinol into di-acetylated
product. In other words, atom economy is poor by using acetic
anhydride as acetylating agent.
Besides, acetic anhydride is usually made from acetic acid
and it also gives acetic acid as byproduct after acetylation. In
addition, it is strictly controlled because it is a precursor chemical
of narcotics, thus, acetylating agent is not readily available
even for laboratory purposes. Based on the disadvantages of
these most frequently used acetylating agents, it is expected
to be replaced by low-cost and benign agents. In this connection,
acetic acid is a better choice. Therefore, it would be meaningful
if the acetylation could be achieved using acetic acid as acety-
lating agent. As the experiment 1-9 and 1-10 shown in Table-
1 and Fig. 4d, Friedel-Crafts di-acetylation is carried out in
acetic acid. Clearly, the conversion of di-acetylated resorcinol
is low, only 8.6 % while performed with 2:1 ratio of acetic
acid/resorcinol at 130 °C. It is understandable that the
generated water induces hydrolysis of reaction intermediate
and lowers the yield [12]. Although the conversion of 4,6-
diacetylresorcinol using acetic acid as acetylating agent
increases when more acetic acid is used (8.6 % in experiment
(a)
(b)
90
60
30
0
90
60
30
0
(1-3)
(1-2)
(1-1)
(1-3)
(1-5)
(1-6)
(1-4)
(1-1): 90 °C, 2:2:1
(1-2): 110 °C, 2:2:1
(1-3): 130 °C, 2:2:1
(1-4): 150 °C, 2:2:1
(1-3): 130 °C, 2:2:1
(1-5): 130 °C, 1:2:1
(1-6): 130 °C, (0.5):2:1
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
(c)
(d)
90
60
30
0
90
60
30
0
(1-3)
(1-3)
(1-3): Acetic anhydride/Resorcinol = 2/1
(1-9): Acetic acid/Resorcinol = 2/1
(1-10): Acetic acid/Resorcinol = 5/1
(1-10)
(1-9)
(1-8)
(1-3): 130 °C, 2:2:1
(1-8): 130 °C, 2:1:1
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Fig. 4. Conversion of 4,6-diacetylresorcinol as a function of reaction temperature (a), methane sulfonic acid dosage (b), acetic anhydride
dosage (c) and acetylating agent (d) by Friedel-Crafts di-acetylation of resorcinol performed in acetic anhydride

Vol. 29, No. 4 (2017)
Metal Free Friedel-Crafts Di-acetylation of Resorcinol in Acetic Acid 753
1-9 vs. 17.8 % in experiment 1-10), the acetylation of resorcinol
in acetic acid is still too low for practical use. Therefore, di-
acetylation of resorcinol with acetic acid is expected to be
achieved in high conversion.
in short time. Fig. 5b shows the results of di-acetylation of
resorcinol performed with different molar ratio of acetic acid/
resorcinol. The molar ratio of acetic acid to resorcinol was varied
from 5:1 to 2:1. For comparison with experiment 2-3 with molar
ratio of P2O5/methane sulfonic acid/acetic acid/resorcinol
0.70:2:5:1, the experiment 2-4 operated with molar ratio of P2O5/
MSA/acetic acid/resorcinol 0.70:2:3:1 has the conversion of
87.8 % in 1.0 h (Table-2). Similarly, for experiment 2-5 with 2:1
of acetylating agent/resorcinol used, the conversion can reaches
83.5 % (Table-2). It is clearly that less acetic acid can also achieve
high di-acetylation conversion, which could improve atom
economy of the di-acetylation. In order to keep the conversion
in a constant high value for further studies, the subsequent
experiments were performed with molar ratio of acetic acid/
resorcinol 5:1.
Furthermore, effect of the catalyst dosage on 4,6-diacetyl-
resorcinol preparation was presented in Fig. 5c. Compared
with experiment 2-3 performed with the molar ratio of methane
sulfonic acid/resorcinol 2:1, conversion of experiment 2-8
carried out with molar ratio of methane sulfonic acid/resorcinol
only 1:1 can reach to 87.1 % after 4.0 h reaction, which indicates
that the di-acetylation of resorcinol can be efficiently achieved
in high conversion with small amount of methane sulfonic
acid used, but longer reaction time is needed. By contrast, for
By assuming that generated water prevents the further
acetylation of resorcinol, P2O5 was introduced into methane
sulfonic acid based catalytic system to improve the di-
acetylation of resorcinol with acetic acid in high yield. More
specifically, the generated water can be absorbed by P2O5
during the di-acetylation process, which facilitates the reaction
[11].As expected, P2O5 significantly help to improve the yield
of desired product. Similarly, effect of temperature, acetylating
agent and catalyst dosage on Friedel-Crafts di-acetylation of
resorcinol is examined. As shown in Fig. 5a, different tempe-
ratures varied from 90-130 °C were studied to consider the proper
temperature for the reaction with molar ratio of P2O5/methane
sulfonic acid/acetic acid/resorcinol 0.70:2:5:1. Consequently, as
time increases, the conversion of the desired product increases
to a maximum value and then decreases. For experiment 2-1, 2-
2 and 2-3 were respectively performed at 90, 110, 130 °C, they
reaches 74.2 % after 5.0 h, 89.0 % in 1.5 h and 90.6 % in 1.0 h
(Table-2). That is, the reaction at 130 °C exhibits a highest
conversion. Therefore, 110-130 °C is suitable for the reaction,
whose conversion of the desired product can reach a high value
(a)
(b)
90
60
30
0
(2-2)
(2-3)
(2-1)
90
60
30
0
(2-3)
(2-4)
(2-5)
(2-1): 90 °C, (0.70):2:5:1
(2-3): 130 °C, (0.70):2:5:1
(2-4): 130 °C, (0.70):2:3:1
(2-5): 130 °C, (0.70):2:2:1
(2-2): 110 °C, (0.70):2:5:1
(2-3): 130 °C, (0.70):2:5:1
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
(c)
(d)
90
60
30
0
90
60
30
0
(2-8)
(2-3)
(2-3)
(2-3): 130 °C, (0.70):2:5:1
(2-6): 130 °C, (0.35):2:5:1
(2-7): 130 °C, (0.10):2:5:1
(1-10): 130 °C, 0:2:5:1
(2-6)
(2-9)
(2-3): 130 °C, (0.70):2:5:1
(2-8): 130 °C, (0.70):1:5:1
(2-9): 130 °C, (0.35):1:5:1
(2-7)
(1-10)
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 5. Conversion of 4,6-diacetylresorcinol as a function of reaction temperature (a), methane sulfonic acid dosage (b), acetic acid dosage (c)
and P2O5 dosage (d) by Friedel-Crafts di-acetylation of resorcinol performed in acetic acid

754 Chen et al.
Asian J. Chem.
TABLE-2
which also significantly reduces the operation difficulty. More
specifically, it can be found that the optimized temperature is
110-130 °C, small amount of methane sulfonic acid usage can
also result in high yield of desired product; P2O5 dosage plays
a great role in the acetylation with acetic acid. Based on the
improved atom efficiency and the use of environmental benign
catalyst, it is in consonance with the practice of green chemistry.
DI-ACETYLATION OF RESORCINOL IN ACETIC
ACID WITH P2O5/METHANE SULFONIC ACID
HO
OH
HO
OH
CH3COOH
P2O5/MSA
O
O
ACKNOWLEDGEMENTS
Entry
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
Temp. (°C) Molar ratioa Time (h)b
Conversion (%)
74.2
90
(0.70):2:5:1
(0.70):2:5:1
(0.70):2:5:1
(0.70):2:3:1
(0.70):2:2:1
(0.35):2:5:1
(0.10):2:5:1
(0.70):1:5:1
(0.35):1:5:1
5.0
1.5
1.0
1.0
0.5
1.0
2.0
4.0
2.0
Financial support from National Natural Science
Foundation of China (51303206) was gratefully acknowledged.
The youth innovation fund projects of Chongqing Institute of
Green and Intelligent Technology (Y23H040M10) is also
acknowledged.
110
130
130
130
130
130
130
130
89.0
90.6
87.8
83.5
50.8
17.7
87.1
36.4
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sulfonic acid/P2O5 molar ratio 1:5:1:0.35, the conversion is only
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As known, the generated water will prevent the reaction
and reduce the conversion of Friedel-Crafts di-acetylation [18].
Therefore, P2O5 were introduced to get rid of water in this
system, which avoids the degradation of reaction intermediate
and promote the desired product formation. Since P2O5 dosage
in the reaction has remarkable effects on the di-acetylation,
the experiments were performed with different amount of P2O5
to further understand its corresponding influence. As shown
in Fig. 5d, experiment 1-10, 2-3, 2-6 and 2-7 are respectively
corresponding to the reaction operated with molar ratio of P2O5/
resorcinol 0:1, 0.70:1, 0.35:1, 0.10:1. Obviously, when the
ratio of P2O5/resorcinol is 0.70:1, the corresponding conversion
can reaches 90.6 %; but when the ratio of P2O5/resorcinol is
0.10:1, the conversion is only 17.7 %. It is understandable
that 0.70 equivalent of P2O5 can completely get rid of the
generated water and 0.35 or 0.10 equivalent of P2O5 used leads
to some water left and residual water prevent the reaction. In
other words, insufficient P2O5 used results in lower conversion
of desired product. Thus, it is optimized to carry out the experi-
ment with P2O5/resorcinol molar ratio more than 0.67:1. On
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Conclusion
In conclusion, methane sulfonic acid was employed as
catalyst for Friedel-Crafts acetylation, which provides an
environmentally acceptable way for acetylation of aromatic
compounds. Moreover, resorcinol is successfully di-acetylated
by acetic acid instead of acetic anhydride and P2O5 is helpful
to improve the conversion. And the di-acetylation with acetic
anhydride or acetic acid processed high region-selectivity. By
this method, 4,6-diacetylresorcinol, a monomer precursor of
high performance fiber PBO, was synthesized in high yield,
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