Comparative material study and synthesis of 4-(4-nitrophenyl)oxazol-2-amine via sonochemical and thermal method

Article abstract of DOI:10.1016/j.ultsonch.2012.09.002

The present paper deals with the synthesis of aminooxazole derivatives via thermal and ultrasonic methods using deep eutectic solvent as medium. It was observed that ultrasound-assisted method gave 90% yield in just 8 min as against 3.5 h required to get

Full text of DOI:10.1016/j.ultsonch.2012.09.002

Ultrasonics Sonochemistry 20 (2013) 633–639
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Short Communication
Comparative material study and synthesis of 4-(4-nitrophenyl)oxazol-2-amine
via sonochemical and thermal method
a
a
b
a
b
Balvant S. Singh , Hyacintha R. Lobo , Dipak V. Pinjari , Krishna J. Jarag , Aniruddha B. Pandit ,
a,
⇑
Ganapati S. Shankarling
Dyestuff Technology Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2012
Received in revised form 30 August 2012
Accepted 12 September 2012
Available online 21 September 2012
The present paper deals with the synthesis of aminooxazole derivatives via thermal and ultrasonic meth-
ods using deep eutectic solvent as medium. It was observed that ultrasound-assisted method gave 90%
yield in just 8 min as against 3.5 h required to get 69% yield by thermal method. One of the compounds
4-(4-nitrophenyl)-1,3-oxazol-2-amine synthesized by both methods were subjected to material charac-
terization study via XRD, TGA and SEM analysis. It was observed that use of ultrasound not only increased
the rate of reaction but also improved the quality of product obtained. The crystallinity of the product
from ultrasound method was 21.12% whereas thermal method fetched only 8.33% crystallinity thereby
improving crystallinity by almost 60%. In addition, sonochemical synthesis also saved more than 70%
energy as depicted by energy calculations.
Keywords:
Sonochemical
Oxazole
Deep eutectic solvent
Crystallinity
Synthesis
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Sonochemical energy delivery has been used as an excellent
oxazole derivatives [9]. Therein, we have explained the effective-
ness of this combination in improving reaction parameters and
conserving the energy. It is well known that ultrasound technique
not only improves the rates and yields of reaction but also affects
properties of material like crystallite size, percentage crystallinity
and morphology of product crystals [10]. We now apply this strat-
egy for the synthesis of amino oxazole derivatives and also perform
material characteristics study of 4-(4-nitrophenyl)-1,3-oxazol-2-
amine.
The oxazole moiety was chosen for conducting property stud-
ies due to the significant applications like versatile biological
activities [11,12], application as important precursors in organic
transformations [13,14], fluorescent whitening agents [15] and
scintillating compounds [16]. Moreover, the advantages of using
deep eutectic solvents, as reaction media, are several owing to
their obvious green characteristics like bio-degradability, non-
toxicity, non-volatility and recyclable nature [17]. Due to these
beneficial features, they have been applied in several organic
transformations [18–22] and also for electrochemical applications
[23].
alternative to thermal energy in promoting organic reactions, espe-
cially those requiring harsh conditions. The use of ultrasound has
also been known to improve the rates of reaction and yields there-
by saving tremendous amount of energy required for synthesis [1].
The origin of this energy lies in the cavitation phenomenon that
involves sequential formation, growth and collapse of several mil-
lions of microscopic vapor bubbles (voids) in the liquid [2,3]. These
fast implosions generate extremely high pressures of about
1
000 atm and temperatures of 5000 °C within the cavity thereby
sustaining heating and cooling rates above 10 billion °C per second.
Such extreme and localized hot spots act as micro reactor wherein
sound energy is transformed into useful form of chemical energy
[
4–6].
This cavitationally induced phenomenon is known to activate
reactant molecules entering into it and thereby converting them
into reactive intermediates. In addition, such a phenomenon occurs
more easily in solvents with low vapor pressure [7,8]. In this con-
text, we had applied the effective combination of deep eutectic sol-
vents and ultrasound, for the first time, in synthesis of novel
The product materials derived from sonochemical method (US)
and thermal method (NUS) were compared for various characteris-
tics such as crystallinity, thermal decomposition and morphology
of crystals via X-ray diffraction (XRD), thermogravimetric analysis
(TGA) and scanning electron microscope (SEM) respectively. In
⇑
Corresponding author.
E-mail address: gsshankarling@gmail.com (G.S. Shankarling).
1
350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2012.09.002

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34
B.S. Singh et al. / Ultrasonics Sonochemistry 20 (2013) 633–639
R
O
NUS METHOD
Br
O
2
Deep eutectic solvent
5 oC
N
6
NH2
H
2
N
NH
2
O
US METHOD
R
Deep eutectic solvent
3
a-3d
Ultrasound, 35 o
C
1
a-1d
where R = -NO , -H, -Br, -OCH
2
3
Scheme 1. Thermal and ultrasound assisted synthesis of oxazole derivatives 3a–d using deep eutectic solvent as reaction medium.
addition, the energy efficiency of sonochemical method is also
described.
tracted using dichloromethane (DCM). The DCM layer was sub-
jected to evaporation under reduced vacuum to obtain the final
product 3a–d.
2
. Materials and methods
2.5. Synthesis of oxazole derivative 3a by ultrasound method in DES
2.1. Materials
medium (US method)
All the solvents and chemicals were procured from S D fine
A mixture of phenacyl bromide derivative 1a–d (1.0 eq) and
urea 2 (1.0 eq) was added to the deep eutectic solvent (7.0 g) and
the mixture was stirred initially for the mixing of substrates. The
reaction mixture was then placed under sonication using an ultra-
sonic horn (ACE horn, 22 kHz frequency) at 40% amplitude for re-
quired time with a 5 s ON and 5 s OFF cycle from time t = 0 h.
The temperature of the process was maintained at 35 ± 2 °C by
means of supply of water to the jacketed reactor, used for the syn-
thesis. The reaction was monitored by thin layer chromatography
(TLC). The complete consumption of reactant was observed after
certain time (as stated in Table 1). The reaction mass was extracted
using dichloromethane (DCM). The DCM layer was subjected to
evaporation under reduced vacuum to obtain the final product 3.
The reaction time was estimated by repeating the same reaction
three times.
chemicals (India) and were used without further purification. The
reactions were monitored by TLC using 0.25 mm E-Merck silica
gel 60 F254 precoated plates, which were visualized with UV light.
Key intermediate 2-bromo-1-(4-methoxyphenyl) ethanone,
2
-bromo-1-(4-nitrophenyl)ethanone,
2-bromo-1-(4-bromophe-
nyl)ethanone was prepared by bromination of their respective
acetophenone derivative with N-bromosuccinimide (NBS) in
accordance with the reported reference [24]. The deep eutectic
solvent used in the present work was easily prepared from choline
chloride (1 eq) and urea (2 eq) at 80 °C by a previously reported
method [25] with 100% atom economy. The resulting viscous liquid
with freezing point of 12 °C [17], was used directly without any
purification.
The deep eutectic solvent could be easily isolated after extrac-
tion of product as mentioned in procedure owing to its immiscibil-
ity in DCM. The recovered DES was further used for the next run
wherein the reaction between 2-bromo-1-(4-nitrophenyl)etha-
none and urea was considered as a standard reaction.
2
.2. Reaction scheme
The reaction scheme of phenacyl bromide derivatives and urea
in deep eutectic medium is shown in Scheme 1.
2.3. Ultrasound set-up
2.6. Characterization and spectral data
Ultrasound for sonochemical synthesis is generated with the
4
-(4-Nitrophenyl)-1,3-oxazol-2-amine (3): m.p. = 186 °C (lit.
help of ultrasonic instrument set-up (horn type). The specification
and details of the set-up, processing parameters used during the
experiments are:
ꢁ1
m.p. = 190 °C [26]); (
667 (C@N), 1567 (C@C), 1493 (C@C), 1447 (C@C), 1068 (C–O);
H NMR(300 MHz; CDCl
mmax/cm
3400 (N–H), 3128 (Ar. C–H),
1
1
3
4
; Me Si) d = 8.25–8.20 (2H, m, CH),
8
NH
.19–8.17 (1H, s, CH), 7.90–7.84 (2H, m, CH), 7.00–6.90 (2H, bs,
Make: ACE, USA.
Operating frequency: 22 kHz.
Rated output power: 750 W.
1
3
2
); C NMR(300 MHz; CDCl
3
; Me
4
Si) d = 161.9, 145.9, 137.5,
, calculated
1
30.8, 130.4, 125.4, 124.0; EIMS m/z = 206.2, C
9 7 3 3
H N O
ꢁ2
m/z: 205.1.
Diameter of stainless steel tip of horn: 1.3 ꢀ 10 m.
ꢁ4
2
Surface area of ultrasound irradiating face: 1.32 ꢀ 10 m .
5
2
Intensity: 3.4 ꢀ 10 W/m .
3
. Characterization and material study
The oxazole derivative was characterized by FT-IR, H NMR, 13C
1
2
.4. Synthesis of oxazole derivative 3 by thermal heating in DES
medium (NUS method)
1
13
NMR spectroscopy and mass spectrometry. H NMR and C NMR
spectrums were recorded on Varian 300 MHz mercury plus spec-
trometer, and chemical shifts are expressed in d ppm using TMS
as an internal standard. Mass spectral data were obtained with
micromass-Q-Tof (YA105) spectrometer. In order to study the
properties of materials, the oxazole samples synthesized by both
methods were initially subjected to X-ray diffraction (XRD) on a
Rigaku Mini-Flex X-ray Diffractometer. The XRD patterns were
A mixture of phenacyl bromide derivative 1a–d (1.0 eq) and
urea 2 (1.0 eq) was added to the deep eutectic solvent (7.0 g) with
stirring. The reaction mixture was stirred at (65 ± 2 °C). The reac-
tion was monitored by thin layer chromatography (TLC). The
phenacyl bromide derivative was almost completely consumed
after certain time (as given in Table 1). The reaction mass was ex-

B.S. Singh et al. / Ultrasonics Sonochemistry 20 (2013) 633–639
635
Table 1
Synthesis of oxazole derivatives from phenacyl bromide derivatives with urea by thermal and ultrasound methods.
Entrya Acyl bromide
Product
Reaction time
Yield (%)
Melting point (°C)
Observed
NUSb (h) USc (min) NUSb (h) USc (min) Literature
O N
2
O
Br
N
NH2
3.5
8
69
90
190 [26]
186
3
a
O
O N
2
O
Br
N
O
4.5
12
63
88
140 [26]
138
3
b
NH2
O
Br
Br
N
4
5
13
20
53
51
82
79
185–186 [27] 182
3
3
c
NH2
Br
O
O
H CO
3
Br
N
180 [26]
188
d
NH2
H CO
O
3
a
b
c
All reactions were carried out with derivative of phenacyl bromide (1 eq), urea 2 (1 eq), DES (7.0 g).
Isolated yields of oxazoles by thermal method (NUS), reaction temperature = 65 ± 2 °C.
Isolated yields of oxazoles by ultrasonic method (US) at room temperature (35 ± 2 °C).
Fig. 1. XRD pattern of the product 3a obtained by thermal (NUS) and ultrasound method (US).
recorded at angles between 2° and 80°, with a scan rate of 2°/min.
The crystallite sizes were obtained using the Debye–Scherrer equa-
tion. The thermal studies of these samples were conducted by ther-
mo-gravimetric analysis (TGA) done with the help of TA
Instruments (SDT R600v8.2 Build 100), USA at heating rate of
4. Results and discussions
4.1. Synthesis of oxazole derivatives by thermal (NUS) and ultrasound
(US) method
1
0°/min. The Scanning Electron Microscopy was performed on a
Keeping in mind the parameters derived in our earlier report
[9], we considered extending it towards the synthesis of substi-
tuted aminooxazole derivatives. The method showed good toler-
ance towards both electron donating and withdrawing
JOEL JSM 680LA 15 kV SEM by deposition of platinum on sample
powder thereby giving a prediction about the surface characteris-
tics of samples.

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B.S. Singh et al. / Ultrasonics Sonochemistry 20 (2013) 633–639
Table 2
Crystallinity measurements, percentage yield and energy data for synthesis of 4-(4-nitrophenyl)-1,3-oxazol-2-amine synthesizd by thermal (NUS) and ultrasound method (US).
Method
Reaction time
Crystallinity (%)
Average
Crystallite size, d (nm)
Average
Yield (%)
Average
Energy utilized (kJ/g)
Thermal (NUS)
Ultrasonic (US)
3.5 h
8 min
8.33
21.12
15.865
51.414
69
90
6.46
0.52
Fig. 2. Thermogravimetric analysis of product 3a synthesized by thermal (NUS) and ultrasound (US) method.
substituents. The thermal method took a longer time and gave
product with moderate yields. However, it was interesting to know
that ultrasound method led to complete consumption of reactant
within just 8–20 min in good yields (Table 1). The impact of acous-
tic energy was evident in reduction of the processing time and
increasing yield of the reaction mass. This is due to the rapid
micromixing that results in faster reaction.
NUS synthesized oxazole (8.33%). The reasons for obtaining such
results are due to the useful environments created by ultrasound
during sonication that facilitates faster reaction, allowing crystal
to form properly. The possible reasoning behind increase in the
crystallinity of US sample than NUS sample have been explained
in our earlier work [10].
4.3. Thermogravimetric analysis (TGA)
4.2. Prediction of X-ray diffraction (XRD) data
The oxazole product 3a derived from both methods were sub-
jected for study of its thermal stability under nitrogen atmosphere
via thermogravimetric analysis (TGA). It can be observed from
Fig. 2 that product 3a obtained from NUS method decomposed
slowly over a range of temperature. However, the phase transfor-
mation was very sharp and rapid in case of product derived from
US method, which is consistent with its observed higher crystallin-
ity. The reason behind slow decomposition may be because of
higher amorphous phase content. Higher the percentage of amor-
phous phase content in any organic derivative, lower is the decom-
position rate. It is due to the better micro or molecular level mixing
which enhances the formation of clear crystal structure.
X-ray diffraction analysis represents a versatile technique for
characterization of various crystalline forms or phases in powder
and solid samples. It also assists in deriving the percentage crystal-
linity and crystallite size of samples.
From the XRD patterns (Fig. 1), calculating percentage crystal-
linity and crystallite size are possible. The amorphous phase frac-
tion of the sample may be determined by taking the ratio of the
amorphous area of the X-ray diffractogram to the total exposed
area (amorphous and crystalline). The area covered by the amor-
phous region means that the area of the diffractogram not con-
tained by any sharp diffraction peaks. A method for estimation of
amorphous phase fraction from XRD patterns have been developed
by Pandit and workers [28,29].The results summarizing the% crys-
tallinity and crystallite size of material for compound 3a are de-
picted in Table 2.
It can be seen from Fig. 1 that the peaks at different crystal
planes of NUS synthesized oxazole derivatives matches exactly
with that of US synthesized oxazole derivatives which shows that
there is no difference in the two synthesized products. Broadening
peaks, for both samples, in the XRD pattern also clearly shows that
there may be formation of very small nanocrystals. The crystallin-
ity of all the samples is still considerable because of the inherent
characteristics of the sonically assisted process. It was found that
US synthesized oxazole shows more crystallinity (21.12%) than
4.4. Scanning electron microscope (SEM) analysis
The scanning electron microscope images were taken for prod-
uct 3 synthesized by both thermal and ultrasound method. The
magnification at 1000ꢀ clearly shows sharper rod-like natured
crystals of the product derived by US method (Fig. 3B
1
) as com-
) that gave clustered aggregate
which were amorphous in nature. This aspect became clearer on
further magnification (Fig. 3A and 3B ). It is clearly seen that les-
pared to thermal method (Fig. 3A
1
2
2
ser or no agglomeration and sharply defined structures are ob-
served in the case of the US synthesized sample. The use of
ultrasound as an intensification tool has had its influence in the

B.S. Singh et al. / Ultrasonics Sonochemistry 20 (2013) 633–639
637
Fig. 3. SEM micrograph of product 3a at: 1000ꢀ: (A
1
) thermal method (NUS) and (B
1 2
) ultrasound method (US). 3000ꢀ: (A ) thermal method (NUS) and (B
2
) ultrasound
method (US).
Fig. 4. Recyclability studies in synthesis of amino oxazole derivative (3a) from 2-bromo-1-(4-nitrophenyl)ethanone and urea.
reaction step by considerably changing crystals at the synthesis
4.5. Recyclability of deep eutectic solvent
stage itself. This not only helps in improving its effect on material
properties by reducing the overall reaction time, improving prod-
uct quality but also aids in decreasing the difference in the types
of crystal shapes formed and bringing more uniformity.
The deep eutectic solvent medium derived from choline chlo-
ride and urea was recycled and re-used up to five consecutive runs.
Reaction of 2-bromo-1-(4-nitrophenyl)ethanone with urea was

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38
B.S. Singh et al. / Ultrasonics Sonochemistry 20 (2013) 633–639
R
H
H
N
N
H
H
CH3
N
H C
CH3
O
O
3
Br
R
R
R
Cl-
OH
O
O
O
H
OH
H
N
N
H
H
N
N
H
N
H
CH2
O
H2N
(
-HBr)
H N
2
O
H N
2
O
(
-H O)
2
O
C
H
R
N
NH2
H
N
H2N
O
Scheme 2. Plausible mechanism for synthesis of oxazole derivatives.
selected as the model reaction. A slight darkening of the eutectic
mixture was observed after recycling, however, no significant de-
crease in yields of products was obtained as shown in Fig. 4. This
indicates the fact that ultrasound does not hold any negative im-
pact on the eutectic mixture thereby highlighting the significance
of using DES as reaction medium in sonochemical approach.
derivatives. The results were compared with the thermal method
wherein it was observed that use of ultrasound improved the yield
and rates of reaction. The ultrasonic irradiation also gave better
crystallinity of product (improvement by 60%) as observed by
XRD studies. This fact was confirmed by SEM images which
showed better rod-shaped crystals in comparison to clustered
aggregates obtained by thermal method. Moreover, the energy cal-
culation data indicated more than 70% saving in energy during syn-
thesis. In conclusion, the method was green, rapid, energy-efficient
and gave better properties of product.
4.6. Efficacy of energy utilization
Appendix A show the comparison of the energy based perfor-
mance of the two types of synthesis methods, conventional
NUS) and sonochemical (US) used for synthesizing aminooxazole
(
Acknowledgements
derivative. The energy utilized for the synthesis of oxazole material
is the total energy supplied (kJ) per unit weight of the material pro-
cessed/obtained (g). It is already explained in Section 4.1 that reac-
tion time to synthesize oxazole was 8 min for US method and 3.5 h
for NUS method. Total energy required per unit weight of the
material processed to synthesize oxazole is 0.523 (kJ/gm) for US
method and 6.46 (kJ/gm) for NUS method. Thus, US method proved
to be energy efficient which saved more than 70% of energy uti-
lized by NUS method and also a reduction in the reaction duration.
Authors (B.S.S. and H.R.L.) are thankful to University Grants
Commission-CAS, New Delhi for providing fellowship; World
Bank-TEQIP and SAIF IIT-Bombay, Mumbai for recording Mass
1
13
spectra, H NMR and C NMR spectra.
Appendix A
A.1. Energy calculations
4.7. Mechanism of reaction
Following sample calculation is done on the basis of data taken
from Table 1 for the synthesis of compound 3a.
A suitable mechanism is proposed that illustrates the role of
DES in oxazole synthesis (Scheme 2). This mechanism is in sync
with our earlier predicted mechanism [9]. The ability of urea com-
ponent in DES [21] to catalyze reactions is well known and hence
has been applied in our case too wherein the urea component
can stabilize the oxygen atom of carbonyl group thereby promot-
ing attack of amide on acyl bromide and consequent cyclization.
The sonochemical energy might help in creating highly reactive
species that assists in faster cyclization and dehydration steps con-
sidering previous reports using ultrasound and ionic liquids [1].
A.1.1. Energy delivered during sonication
ꢂ Energy delivered during sonication = Energy required to synthe-
size Oxazole material.
ꢂ Electrical energy delivered during sonication using horn for
8
min (indicated by the power meter) = 14.45 kJ.
ꢂ Efficiency of Horn taken for the calculation = 30% (estimated
independently using calorimetric studies).
ꢂ Actual energy delivered by horn during sonication = Energy
delivered during sonication using horn in 8 min ꢀ Efficiency of
Horn = 14.45 ꢀ 30/100 = 4.33 kJ.
5
. Conclusions
ꢂ Quantity of material processed = Quantity of 2-bromo-1-
phenylethanone + Quantity of urea + Quantity of DES =1 (g) +
0.24 (g) + 7 (g) = 8.24 (g).
We successfully extended the ultrasound assisted methodology
in deep eutectic solvent towards the synthesis of aminooxazole

B.S. Singh et al. / Ultrasonics Sonochemistry 20 (2013) 633–639
639
ꢂ Net energy supplied for processing of material using sonochem-
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[
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(
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1034 Joules
ð1 cal ¼ 4:184 JoulesÞ
[
ꢂ Total energy supplied for heating reaction mixture to 65 °C from
room temperature (35 °C)
[
ꢄ
¼
Energy supplied for heating reaction mixture to 65 C
ꢄ
from room temperature ð35 CÞ ꢀ 3:5 h ꢀ ð1=30Þ h
707.
ꢀ
ð30 min are required for coolingÞ
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¼
1034 Joules ꢀ 10
(
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[
[
[
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10; 340 Joules ¼ 10:34 kJ
ꢂ Net energy delivered during conventional method = Energy
delivered during conventional method for stirring + Total
energy supplied for heating reaction mixture to 65 °C from
room temperature (35 °C) = 42.89 + 10.34 = 53.23 kJ
ꢂ Net energy supplied for processing of material using conven-
tional method = Net energy delivered during conventional
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(
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[
[
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¼
6:46 ðkJ=gÞ
ðBÞ
A.1.3. Percentage of energy saved (%)
[
ꢂ Net energy saved = (Net energy supplied for processing of mate-
rial using conventional method (B)) ꢁ (Net energy supplied for
processing of material using sonochemical method (A))/(Net
energy supplied for processing of material using conventional
method (B)) ꢀ 100 = (6.46 ꢁ 0.52)/8.24 ꢀ 100 = 72.08%
(
2001) 2010–2011.
[
[
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