938
Chemistry Letters 2001
Ultrasonic Enhancement on the Hydrolysis of Diethyl 1,2-Benzenedicarboxylate
Bongbeen Yim,* Youngeok Yoo,† Yoshio Nagata, and Yasuaki Maeda
Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531
†College of Education, Taegu University, Kyungsan-city, Kyungpook 712-714, Korea
(Received June 18, 2001; CL-010572)
The ultrasonic enhancement on the hydrolysis of diethyl
1,2-benzenedicarboxylate results from the dissipated heat when
the cavitation micro-bubbles were vigorously collapsed. The
effective temperature in a solution depends on the nature of the
dissolved gas. The relatively broad region that spreads from the
interface of the cavitation micro-bubble plays an important role
in the sonolytic hydrolysis of esters.
Sonication of an aqueous solution leads to a process called
cavitation, which is the formation, growth, and adiabatic col-
lapse of the micro-bubbles. The temperature and pressure
derived from the violent collapse of the cavitation micro-bub-
bles are estimated to be up to 5000 K and 1000 bar, respective-
ly. Reactive free radicals in aqueous solution are generated
under these extreme conditions.1 The sonochemical effects are
the result of the cavitation phenomenon.
Table 1 shows the ultrasonic enhancement on the hydrolysis
of aqueous diethyl 1,2-benzenedicarboxylate solutions under
alkaline conditions. The formation rates of the monoethyl 1,2-
benzenedicarboxylate during non-sonication (stirring) as well as
during sonication were dependent on the increased pH. The
average ultrasonic enhancement on the hydrolysis was observed
to be approximately 3.8 times. This is a fairly significant
hydrolysis enhancement compared to the reported previous stud-
ies.2−4 The evolution of gases by the thermal reaction during the
hydrolysis by sonication was confirmed. On the other hand, in
the sonolytic hydrolysis of dimethyl 1,2-benzenedicarboxylate,
its enhancement was not great. In the presence of ultrasounds, it
was observed that the dimethyl 1,2-benzenedicarboxylate was
not more rapidly hydrolyzed, due to its physicochemical proper-
ties, than diethyl 1,2-benzenedicarboxylate under the same con-
ditions.7 The hydrophobic solute would be expected to concen-
trate around the cavitation micro-bubbles (i.e., the gas–liquid
interfacial region between the cavitation micro-bubble and the
bulk solution) during the sonication, due to the partition into the
interfacial region although it would not enter the cavitation
micro-bubbles. Therefore, the solute with higher hydrophobici-
ty should be likely to be affected by the chemical effect arising
from the collapsing cavitation micro-bubbles in the vicinity of
the cavitation micro-bubbles.
Figure 1 shows Arrhenius plots of the hydrolysis rate con-
stants of diethyl 1,2-benzenedicarboxylate by non-sonication at
pH 12. From the kinetic data, the activation energy (Ea) was
experimentally determined to be 49.9 kJ mol−1 and the activa-
tion parameters, such as ∆H‡, ∆G‡, and ∆S‡ were also calculat-
ed to be 47.2 kJ mol−1, 98.4 kJ mol−1, and −156.5 J·K−1mol−1,
respectively. If it is postulated that the sonolytic hydrolysis is a
reaction with the activation energy obtained from the non-soni-
cation experiments and the linear Arrhenius-type behavior, the
reaction temperature for the sonolytic hydrolysis can be calcu-
lated from the Arrhenius plot of the rate constants of the non-
sonication hydrolysis (see Figure 1).
The hydrolysis of esters such as methyl acetate,2 carboxylic
acid esters,3 and nitrophenyl esters4 by sonication has been
reported, but the description with respect to the sonochemical
effect is limited. It was recently reported that the formation of
transient supercritical water was an important factor in the
acceleration of the sonolytic hydrolysis.5 However, the forma-
tion and role of supercritical water for the hydrolysis in the
presence of ultrasounds are still uncertain.6
In this paper, we report the ultrasonic enhancement on the
hydrolysis of aqueous diethyl 1,2-benzenedicarboxylate solu-
tions under homogeneous conditions. The formation rate of
monoethyl 1,2-benzenedicarboxylate regarded as the product of
hydrolysis was determined. In order to investigate the chemical
effect induced by sonication, sonications under an atmosphere
of five dissolved gases were also carried out.
All the reagent grade chemicals were purchased from
Wako. The water used for the experiments was purified by a
Millipore Milli-Q system. Before sonication, the sample solu-
tion was prepared by bubbling a high purity gas (> 99.99%),
and the pH was adjusted with phosphate buffer and sodium
hydroxide solution. For the purpose of sonication, the experi-
mental apparatus consisted of an ultrasonic generator and a bar-
ium titanate oscillator operating at 200 kHz (200 W). The tem-
perature of the solution was kept at approximately 20 °C by a
cooling water bath during the sonication. The cylindrical glass
vessel (150 mL) had a side arm with a septum for gas bubbling
or for withdrawing liquids sample and its bottom was made as
thin as possible (1 mm) for good transmission of the ultrasonic
waves. The vessel was fixed at 3.8 mm from the oscillator and
closed during the sonication. Monoethyl and diethyl 1,2-ben-
zenedicarboxylates were analyzed using a high-performance
liquid chromatograph equipped with an ODS-18 column, and
the mixture of methanol and phosphate buffer solution was used
as the eluent at the flow rate of 1.0 mL min−1.
Copyright © 2001 The Chemical Society of Japan
Yim, Bongbeen
