Phase-transfer-catalyzed synthesis of butyl 4-methoxyphenylacetate and its simultaneous hydrolysis by potassium hydroxide in oil-water biphasic system

Article abstract of DOI:10.1246/bcsj.20120055

Herein, we report for the first time simultaneous hydrolysis during the phase-transfer-catalyzed synthesis of butyl 4-methoxyphenylacetate using potassium hydroxide in biphasic medium. Effect of operating parameters on the reaction is presented. The yield of ester increases with the increase in the agitation speed up to 500 rpm and above that the conversion is not influenced by the agitation speed. At higher concentration of base, the hydrolysis product is more favored. Lower temperature and larger volume of water favors the esterfication. At high concentration of catalyst, sharp decrease in the yield of the ester is noticed. In the presence of highly polar solvents, the conversion of ester increases initially and then there was a sharp decrease. Rational explanations to account for the phenomena based on the results are made.

Full text of DOI:10.1246/bcsj.20120055

976
Bull. Chem. Soc. Jpn. Vol. 85, No. 9, 976-982 (2012)
© 2012 The Chemical Society of Japan
Phase-Transfer-Catalyzed Synthesis of Butyl 4-Methoxyphenylacetate
and Its Simultaneous Hydrolysis by Potassium Hydroxide
in Oil-Water Biphasic System
Maw-Ling Wang,*1 Perumberkandigai Adikesavan Vivekanand,1 and Ming-Chan Yu2
1Department of Environmental Engineering, Safety and Health, Hungkuang University, Shalu, Taichung 43302, Taiwan
2Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
Received February 12, 2012; E-mail: chmmlw@sunrise.hk.edu.tw
Herein, we report for the first time simultaneous hydrolysis during the phase-transfer-catalyzed synthesis of butyl
4-methoxyphenylacetate using potassium hydroxide in biphasic medium. Effect of operating parameters on the reaction
is presented. The yield of ester increases with the increase in the agitation speed up to 500 rpm and above that the
conversion is not influenced by the agitation speed. At higher concentration of base, the hydrolysis product is more
favored. Lower temperature and larger volume of water favors the esterfication. At high concentration of catalyst, sharp
decrease in the yield of the ester is noticed. In the presence of highly polar solvents, the conversion of ester increases
initially and then there was a sharp decrease. Rational explanations to account for the phenomena based on the results
are made.
Phase-transfer catalysis (PTC) finds applications in a wide
reactions can be followed by injecting the sample collected
from the organic layer into the GC system directly.
variety of reactions involving water-soluble and water-insolu-
ble reactants.1-3 It is an extremely useful method for perform-
ing nearly any type of reaction involving an ionic starting
material or intermediate. In this methodology, the transport
of a reactant from the aqueous phase into the organic phase
is carried out by phase-transfer agent. The major advantage of
employing these catalysts in organic synthesis is that a high rate
of reaction is achieved, even at a moderate operating temper-
ature. Nowadays, phase-transfer catalysis is a useful tool for
increasing efficiency, improving safety, and reducing environ-
mental impact.
The advances of the same in recent years have made a
remarkable impact in organic synthesis and are being enor-
mously employed in a multitude of organic transformations
involving water-soluble reagents and water-insoluble organic
substrates. Ever increasing applications have expanded the use
of PTC in organic synthesis and is widely used for manufactur-
ing pharmaceuticals, agricultural chemicals, perfumes, flavors,
dyes, polymers, and other important chemicals via substitution,
displacement, condensation, polymerization, addition, alkyla-
tion, reduction, and oxidation.4-14 Phase-transfer catalysts, such
as quaternary ammonium and phosphonium salts, crown ethers,
poly(ethylene glycol)s, and cryptands, have been used to carry
out reactions between reactants, which exist in the same or
different phase(s).15-27 However, quaternary ammonium com-
pounds are the most popular catalysts due to their low toxicity
and proven effectiveness for a wide range of reactions.28-34
Conceptually, the quaternary ammonium cation, in the
aqueous phase, forms an ion pair with a water-soluble reactive
anion. The ion pair is more soluble in the organic phase than in
water, so the reactant is “ferried” through the interface into the
organic phase where the reaction takes place. The progress of
Phase-transfer-catalyzed hydrolysis of n-butyl acetate has
been studied in the past experimentally using sodium hydrox-
ide as a base. In the cyclodextrins-catalyzed hydrolysis of
carboxylic acid esters, the reaction is affected by operating
parameters viz., agitation speed, choice of cyclodextrin, and
temperature.35 Baranova et al.36,37 examined the kinetics of
alkaline hydrolysis of N-benzyloxycarbonylglycine 4-nitro-
phenyl ester in 1-butanol-borate buffer and chloroform-borate
buffer in the presence of ammonium and phosphonium salts.
Kinetics of hydrolysis of ethyl 2-bromoisobutyrate in an alka-
line solution/organic solvent two-phase medium was inves-
tigated.38 The results indicated that both the bromo-alkyl and
ester functional groups of ethyl 2-bromoisobutyrate hydrolyze
into alcohol and acid. Also, the conversion of reactants is
highly dependent on the concentration of alkaline compound
in the aqueous solution. Influence of the phase-transfer catalyst
on the overall reaction rates in the sodium hydroxide assisted
hydrolysis of n-butyl acetate catalyzed by phase-transfer cata-
lysts viz., trioctylmethylammonium chloride, (Aliquat 336) and
cetyltrimethylammonium bromide (CTMAB) was studied by
Asai et al.39,40 Recently, trioctylmethylammonium chloride-
catalyzed hydrolysis of n-butyl acetate with aqueous sodium
hydroxide was studied in the batch mode as well as in the
continuous mode in a microreactor by Cherlo et al.41
Phase-transfer catalysis has increasingly been used in
esterfication reactions over the past decade.42-49 Juang and
Liu50 explored the reaction rates for the esterification of a sub-
stituted phenylacetic acid in the presence of sodium hydrox-
ide by phase-transfer catalysis using a constant interfacial
area cell and HPLC. Yang and Li46 reported the kinetics of
esterification of sodium salicylate with benzyl bromide via
Published on the web September 1, 2012; doi:10.1246/bcsj.20120055

M.-L. Wang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
977
low alk. KOH
K2CO3
MeOPhCH2COOC4H9
C4H9Br
3
2
MeOPhCH2COOH
PTC
1000 rpm
high alk. KOH
1
+
MeOPhCH2COOH
MeOPhCH2COOC4H9
+ C4H9OH
1
3
4
Scheme 1. Competition between esterfication and unexpected hydrolysis under controlled PTC condition.
R1OR2COOH + KOH
R1OR2COOK + H2O
third-liquid phase-transfer catalysis. The investigation exam-
ined the formation of the third-liquid phase from the interaction
of aqueous reactant, inorganic salts, organic solvent, and cata-
lyst to find the characteristics of catalytic intermediate in the
triliquid system.
Q+ + X-
(QY) + X-
(QX)
With all these antecedents, we decided to investigate the
synthesis of butyl 2-(4-methoxyphenyl)acetate (3) by reacting
4-methoxyphenylacetic acid (1) with n-bromobutane (2) in
biphasic medium using aqueous potassium hydroxide as a base
and chlorobenzene as an organic solvent catalyzed by tetra-
butylammonium bromide at 1000 rpm and 60 °C (Scheme 1).
One of the interesting results of the current study is that we
were able to get exclusive ester product 3 when we utilized low
alkaline concentration (0.04 mol) and inorganic salt (K2CO3,
0.04 mol). However, when only high alkaline concentration
was employed, ester product 3 as well as hydrolyzed product 1
and n-butanol were observed. Since the study of competition
between esterfication and hydrolysis reactions under controlled
PTC reaction conditions will be interesting and challenging,
we decided to investigate the competitive chain reaction using
PTC under high alkaline concentration and in the absence of
inorganic salt. Tetrabutylammonium bromide (TBAB) served
as catalyst. Further, to the best of our knowledge, there are
meager literature reports regarding this competitive chain
reaction using potassium hydroxide, chlorobenzene (organic
solvent), and TBAB as a PTC.
(QX) + Y-
R1OR2COOQ + R3X
R1OR2COOR3 + QOH
R1OR2COOR3 + QX
R1OR2COOQ + R3OH
Scheme 2. Stepwise reactions involved in the esterfication
and simultaneous hydrolysis reactions.
with the quaternary cation to form the ion pair (active inter-
¹
mediate, R1OR2COO Q+) as given by Scheme 3.
Thus this ion pair (active intermediate, R1OR2COO Q+)
¹
which is readily formed in the aqueous phase is transferred
to the organic phase. Likewise, the reactions can occur near
the interface between the organic and the aqueous phase on
the aqueous phase side due to a very limited solubility of the
substrate in the aqueous phase. The ion pair (active intermedi-
ate, R1OR2COO Q+) is transferred across the interface to the
¹
organic phase where a series of reactions take place. In the first
¹
Details about the effect of experimental parameters viz.,
amount of catalysts, temperature, volume of water, type of
organic solvent, etc. on the reaction are meager in the litera-
ture. Hence, the influence of type of organic solvent, amount
of catalysts, volume of water, stirring speed, amount of base,
and temperature on the reaction was studied in detail. A
rational mechanism for the phase-transfer catalytic reaction
was proposed.
step, the ion pair (R1OR2COO Q+) reacts with R3X forming
the desired ester, R1OR2COOR3. This ester in the next step
¹
reacts with ion pair (Q+OH ), which was generated in aqueous
phase by the reaction of R1OR2COO Q+ and KOH, forming
¹
the unexpected salt of acid (R1OR2COO Q+) which is then
¹
transferred across the interface into the aqueous phase for the
reaction to proceed (Scheme 3). Thus, two reactions take place
consecutively in the organic phase and four reactions take place
consecutively in the aqueous phase. This is a typical catalytic
cycle. The mechanism of the reaction shows that the quaternary
cation of the PTC (Q+) takes up different anions for its reaction
with the substrate but most of the catalyst remains in the
reaction medium and the catalytic cycle is continued.
Results and Discussion
Overall Stoichiometry.
The overall stoichiometry of
this reaction using high alkaline concentration of potassium
hydroxide and tetrabutylammonium bromide as phase-transfer
agent is as shown in Scheme 2.
Validation of the Model.
In the presence of high
Obviously, this reaction involves a complex mechanism
due to simultaneous esterfication and hydrolysis. Taking into
consideration these complexities we propose a suitable reaction
model/mechanism for the reaction system under study.
Proposed Mechanism of Esterfication and Hydrolysis
under Biphasic Phase-Transfer Catalysis Conditions.
Since the stoichiometry of the overall reaction shows a com-
plex mechanism, the following steps were envisaged. Initially,
the acid (R1OR2COOH) reacts with the base (KOH) resulting
concentration of potassium hydroxide, we observed maximum
hydrolysis of 4-methoxyphenylacetate, when n-bromobutane
was used as a limiting agent (0.030 mol) and 4-methoxyphenyl-
acetic acid as an excess agent (0.073 mol). Thus we obtained
n-butanol as a major product (maximum yield) and ester and
acid as minor products (minor yield). Nevertheless, in presence
of 4-methoxyphenylacetic acid as a limiting agent (0.030 mol)
and n-bromobutane as a excess agent (0.073 mol), hydrolysis of
4-methoxyphenylacetate was minimized and ester was obtained
as a major product. In this reaction condition, minor by-products
¹
in potassium salt of acid (R1OR2COO K+). This salt couples

978
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
Synthesis of Butyl 2-(4-Methoxyphenyl)acetate and Simultaneous Hydrolysis
Aqueous Phase
R1OR2COO-K+ +
H2O
R1OR2COOH
KOH
+
R1OR2COO-K+
+
R1OR2COO-Q+
R1OR2COO-Q+
Q+X-
+
KX
R1OR2COO-K+
Q+OH-
+
KOH
+
Q+X-
Q+OH-
KOH
+
+
KX
Interface
Organic Phase
R1OR2COO-Q+ R3OH
+
R1OR2COOR3
R1OR2COOR3
Q+OH-
+
Q+X-
R1OR2COO-Q+ R3X
+
+
Scheme 3. Mechanism of esterfication and hydrolysis under phase-transfer catalysis conditions.
Figure 2. Reaction profile for the hydrolysis study of 4-
Figure 1. Reaction profile for the conversion of n-bromo-
methoxyphenylacetate at different amounts of potassium
hydroxide and TBAB: 0.015 mol of 4-methoxyphenylace-
tate, 1 g of internal standard (biphenyl), 25 mL of H2O,
50 mL of chlorobenzene, 60 °C, 1000 rpm.
butane at different concentration of potassium hydroxide
under PTC conditions: 0.073 mol of bromobutane, 0.003
mol of TBAB, 1 g of internal standard (biphenyl), 25 mL of
H2O, 50 mL of chlorobenzene, 60 °C, 1000 rpm.
are acid and n-butanol. In relation to this, reaction profile of
conversion of n-bromobutane to n-butanol under PTC con-
ditions is shown in Figure 1. Conversion of n-bromobutane was
very low even at higher concentration of potassium hydroxide
(7.1 M). Further, we carried exclusive hydrolysis of 4-methoxy-
phenylacetate by carrying out the reaction in the absence of 4-
methoxyphenylacetic acid and n-bromobutane under different
PTC conditions (Figure 2), resulting in 4-methoxyphenylacetic
acid (major product) and n-butanol (minor product). Maximum
hydrolysis of ester was noticed only in the presence of higher
amount of TBAOH (0.003 mol) and potassium hydroxide
(0.178 mol). Nevertheless, when the reaction was carried out
in the absence of potassium hydroxide and in the presence of
TBAB, the very low conversion was observed.
The effects of different parameters on the conversion
were investigated in order to determine the suitability of the
above proposed reaction mechanism/model. In the following
subsections, the effects of the reaction conditions viz., (1)
stirring speed, (2) amount of catalyst (TBAB), (3) concen-
tration of potassium hydroxide, (4) volume of water, (5) tem-
perature, and (6) different organic solvents on the yield of ester
(R1OR2COOR3), are presented.

M.-L. Wang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
979
Figure 4. Influence of stirring speed on the esterfication of
4-methoxyphenylacetic acid under phase-transfer catalysis
condition: 0.030 mol of 4-methoxyphenylacetic acid, 0.073
mol of bromobutane, 0.1783 mol of KOH, 25 mL of H2O,
1 g of internal standard (biphenyl), 0.003 mol of TBAB,
50 mL of chlorobenzene, 60 °C.
Figure 3. Effect of different organic solvents on the
esterfication of 4-methoxyphenylacetic acid under phase-
transfer catalysis condition: 0.030 mol of 4-methoxyphen-
ylacetic acid, 0.073 mol of bromobutane, 0.1783 mol of
KOH, 25 mL of H2O, 1 g of internal standard (biphenyl),
0.003 mol of TBAB, 50 mL of organic solvent, 60 °C,
1000 rpm.
Effect of the Organic Solvents.
In liquid-liquid PTC
in order to study the effect of the reaction by ignoring the
influence of mass transfer, the agitation speed was kept at
1000 rpm in the following experimental runs. We noticed
analogous dependence52 in the epoxidation of dicyclopenta-
diene catalyzed by quaternary ammonium salt in an acidic
aqueous solution/organic solvent two-phase medium.
reaction systems, the conversions are highly affected by the
organic solvents. Various solvents have different reactivity.
In this work, three organic solvents were applied to investigate
the effect of their polarities and dielectric constants (¾) on the
reaction system. The solvents that are employed in this inves-
tigation are: toluene, methyl isobutyl ketone (MIBK) and
chlorobenzene. The dielectric constants of these solvents are:
toluene (¾ = 2.38), methyl isobutyl ketone (¾ = 13.11), and
chlorobenzene (¾ = 5.62). As shown in Figure 3, the reactivity
of the three solvents is of the following order: MIBK >
chlorobenzene > toluene. It was clearly found that the yield of
ester 3 increased with the dielectric constant of the solvents.
However, in the case of MIBK a peculiar trend was observed.
The yield of 3 initially increases and then it falls drastically,
which we attribute to its high dielectric constant value aiding
the hydrolysis of ester. Although toluene is very common in
industry and academic PTC studies, using a solvent with a low
dielectric constant is unfavorable to the reaction. As a result
of the low dielectric constant of toluene, its conversion is low.
Recently we observed a similar dependence51 in the synthesis
of 1-(3-phenylpropyl)pyrrolidine-2,5-dione under solid-liquid
phase-transfer catalytic conditions. Hence, it is decided to
employ chlorobenzene as solvent for the reaction under study
in other experimental variations.
Effect of Catalyst Loading. Generally it was expected
that as the amount of catalyst increases, the conversion also
increases. But in the current study we did not notice this posi-
tive trend. The catalyst (TBAB) concentration was varied in
the range of 0.2 to 3.0 g under otherwise similar conditions
(Figure 5). It was observed that with increase in catalyst
concentration, the yield of 3 increases only up to 1 g. The
yield corresponding to 1.2, 1.4, and 1.6 g decreases slightly.
However, the yield at 3.0 g of catalyst amount decreases
drastically. The aforementioned decrease in conversion is
attributed to hydrolysis of ester in the given reaction conditions.
Effect of Varying Potassium Hydroxide Concentration.
In the PTC/OH catalyzed reaction, the reaction rate is known
to be greatly affected by the concentration of the alkaline
compound. Hence, in the next set of variations, we studied the
influence of potassium hydroxide concentration under other-
wise similar conditions and the initial rates were measured.
The variation was carried out using 1.43, 2.85, 5.70, 7.13,
8.56, and 10.70 M. As expected the yield of ester increases
from 1.43 to 7.13 M (Figure 6). However, to our surprise we
noticed that the yield of esterfication decreases at 8.56 and
10.70 M. This is because at higher concentration the aqueous
solution becomes saturated resulting salting out effect. Further,
at these higher concentration, we expect the hydrolysis of
Effect of Agitation Speed. In principle, the homogeneous
reaction is independent of the agitation speed. However, in
heterogeneous reaction systems such as the present one agita-
tion speed does have an influence on the reaction. Hence we
investigated the influence of agitation speed on the reaction at
five different speeds viz., 200, 500, 800, 1000, and 1200 rpm.
As shown in Figure 4, the yield of ester 3 is highly dependent
on the agitation speed for agitation speed up to 500 rpm. This
can be attributed to constancy in mass transfer values. Thus,
¹
the ion pair (active intermediate, R1OR2COO Q+) to be more
extensive.
Effect of Volume of Water. The effect of water on the rate
of reaction was studied by varying the quantity of water in the

980
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
Synthesis of Butyl 2-(4-Methoxyphenyl)acetate and Simultaneous Hydrolysis
Figure 5. Influence of catalyst amount on the esterfication
of 4-methoxyphenylacetic acid under phase-transfer catal-
ysis condition: 0.030 mol of 4-methoxyphenylacetic acid,
0.073 mol of bromobutane, 0.1783 mol of KOH, 25 mL of
H2O, 1 g of internal standard (biphenyl), 50 mL of chloro-
benzene, 60 °C, 1000 rpm.
Figure 7. Influence of volume of water on the esterfication
of 4-methoxyphenylacetic acid under phase-transfer catal-
ysis condition: 0.030 mol of 4-methoxyphenylacetic acid,
0.073 mol of bromobutane, 0.1783 mol of KOH, 1 g of
internal standard (biphenyl), 0.003 mol of TBAB, 50 mL of
chlorobenzene, 60 °C, 1000 rpm.
Figure 6. Effect of amount of KOH on esterfication of 4-
methoxyphenylacetic acid under phase-transfer catalysis
condition: 0.030 mol of 4-methoxyphenylacetic acid, 0.073
mol of bromobutane, 1 g of internal standard (biphenyl),
25 mL of H2O, 0.003 mol of TBAB, 50 mL of chloro-
benzene, 60 °C, 1000 rpm.
Figure 8. Influence of temperature on the esterfication:
0.030 mol of 4-methoxyphenylacetic acid, 0.073 mol of
bromobutane, 0.1783 mol of KOH, 25 mL of H2O, 1 g of
internal standard (biphenyl), 0.003 mol of TBAB, 50 mL of
chlorobenzene, 1000 rpm.
aqueous phase by varying its volume from 15 to 75 mL. The
reactions were carried out under otherwise similar conditions.
At lower volume of water (15 mL), the yield of ester is less
than at higher volume of water (Figure 7). When the volume of
water is low, the aqueous phase is saturated by base concen-
tration and hence the hydrolysis reaction is favored, thereby
decreasing the yield.
Influence of temperature on the yield of ester (Figure 8) indi-
cates that the yield increases with temperature only up to 60 °C.
At, 70 °C, there is a sharp decrease in the yield in ester. We
attribute it to the competition from hydrolysis of ester.
Conclusion
In this work it has been demonstrated that carboxylic acid
ester (butyl 4-methoxyphenylacetate) synthesis and hydrolysis
could take place simultaneously by controlling reaction condi-
tions i.e., at high alkaline concentration and in the absence
of inorganic salt (potassium carbonate) under phase-transfer-
Effect of Temperature.
The effect of temperature on
conversion was studied at 30, 40, 50, 60, and 70 °C under
otherwise similar conditions. Generally, it is expected the yield
of the product will increase with the increase in temperature.

M.-L. Wang et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
981
catalyzed two-phase organic/aqueous conditions. Variation of
experimental parameters substantially influences the ester yield.
At high agitation speed mass transfer influence was found to
be negligible. The higher the temperature (70 °C), amount of
catalyst (3 g), and amount of base (10.70 M) of the reaction,
lower will be the yield of ester. When lower volume of water
(15 mL) was employed, the hydrolysis product was more
visible. Among the solvents, MIBK favors the hydrolysis than
other solvents viz., toluene and chlorobenzene. Rational
explanation is presented for the observed trends.
taining 100% methyl polysiloxane; injection temperature,
250 °C; detector, flame ionization detector (250 °C); column
temperature: 50-220 °C. The products formed were analyzed
by comparison of their retention times with authentic samples.
Product yields were determined from standard curve and using
biphenyl as internal standard.
The authors thank the National Science Council, Taiwan, for
financial support under Grant NSC-98-2221-E-241-013-MY3.
Supporting Information
Experimental
Characterization of butyl 4-methoxyphenylacetate is avail-
able free of charge on the Web at: http://www.csj.jp/journals/
bcsj/.
Chemicals.
Potassium hydroxide, chlorobenzene, 4-
methylphenylacetic acid, n-bromobutane (RBr), biphenyl, tera-
butylammonium bromide and other reagents are all reagent-
grade chemicals from Fluka, Echo, Merck, SHOWA, BDH,
ACROS, TEDIA, Lancaster, and TCI. These were used as
received without any further purification. All other chemicals
were of analytical grade.
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