One billionhertz microwave athermal action on the synthesis of aromatic esters at normal pressure

Article abstract of DOI:10.1081/SCC-100105121

Benzoic acid or p-hydroxybenzoic acid undergoes rapid reaction with alcohol using H₂SO₄ as catalyst at normal pressure, under irradiating continuously of the 1 GHz and 50 mW microwave for 22-60 minutes to afford aromatic esters in 76-90% yields.

Full text of DOI:10.1081/SCC-100105121

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ONE BILLIONHERTZ MICROWAVE ATHERMAL ACTION
ON THE SYNTHESIS OF AROMATIC ESTERS AT NORMAL
PRESSURE
Zhibin Zhang a , Lanxiang Zhou a , Ming Zhang a , Hao Wu a & Zhongjun Chen b
a Science College of Yangzhou University , Yangzhou, 225002, People's Republic of China
b Chemistry Department , Nanjing University , Nanjing, 210093, People's Republic of
China
Published online: 09 Nov 2006.
To cite this article: Zhibin Zhang , Lanxiang Zhou , Ming Zhang , Hao Wu & Zhongjun Chen (2001) ONE BILLIONHERTZ
MICROWAVE ATHERMAL ACTION ON THE SYNTHESIS OF AROMATIC ESTERS AT NORMAL PRESSURE, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:16, 2435-2439,
DOI: 10.1081/SCC-100105121
To link to this article: http://dx.doi.org/10.1081/SCC-100105121
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SYNTHETIC COMMUNICATIONS, 31(16), 2435–2439 (2001)
ONE BILLIONHERTZ MICROWAVE
ATHERMAL ACTION ON THE SYNTHESIS
OF AROMATIC ESTERS AT NORMAL
PRESSURE
Zhibin Zhang1, Lanxiang Zhou,1 Ming Zhang,1
*
2
Hao Wu,1 andZhongjun Chen
1Science College of Yangzhou University, Yangzhou 225002,
People’s Republic of China
2Chemistry Department, Nanjing University, Nanjing 210093,
People’s Republic of China
ABSTRACT
Benzoic acid or p-hydroxybenzoic acid undergoes rapid reac-
tion with alcohol using H2SO4 as catalyst at normal pressure,
under irradiating continuously of the 1 GHz and 50 mW
microwave for 22–60 minutes to afford aromatic esters in
76–90% yields.
The application of microwave (MW) energy in organic chemistry is
increasing rapidly1–2 since Gedye3 and Giguere4 reported it. There exist heat-
ing action5 and athermal action in chemical reaction under microwave irra-
diation.6–7 In our laboratory, a new method for synthesizing aromatic esters
has been developed using a special microwave condition. This microwave
*Corresponding author.
2435
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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ZHANG ET AL.
condition has three features. First, the microwave frequency is 1.0 Â 10E9 Hz
other than 2.45 Â 10E9 Hz. Second, the microwave power (50 mW) is far
lower than microwave oven’s, and there isn’t microwave heating action but
microwave athermal action. Finally, this microwave condition can act on
classical synthetic systems in the state of reflux at normal pressure.
Richard Gedye had reported esterification systems8 of benzoic acid
with alcohol in a sealed Teflon pressure-vessel in a microwave oven
(2450 MHz, about 700 W) previously. However, these systems were less
attractive because of abnormal pressure.3 The systems we reported here
for synthesizing aromatic ester were accomplished in reflux state at
normal pressure under athermal action of microwave9 (1 GHz and 50 mW).
1:MW;ÀH2O
p-R1C6H4CO2H þ R2OH ÀÀÀÀÀÀÀ! p-R1C6H4CO2R2
2:H2SO4
1
2
3a–h
Scheme 1. Esterification of aromatic acid with alcohol.
3aÀh R1R2: a: H-; Me-; b: H-; Et-; c: H-; Pr-; d: H-; Bu-
e: HO-; Me-; f: HO-; Et-; g: HO-; Pr-; h: HO- Bu-
Quantities and concentrations of reactants were kept same in all sets of
reactions. Under traditional reflux conditions, when benzoic acid or
p-hydroxybenzoic acid was mixed with alcohol and some sulfuric and sub-
jected to microwave (1 GHz and 50 mW) Irradiation for 22 ꢀ 60 minutes, the
aromatic esters were obtained in 76 ꢀ 90% yield as shown in Scheme 1 and
Table 1. Aromatic esters were identified by boiling point or melting point,
IR or Elemental analysis data.
In conclusion, the present method of esterification had advantages
over known methods. High yield, short time, safe condition and easy control
of reaction made the method more attractive. The microwave athermal
action is especially important to promoting those synthetic reactions in
which temperature is strictly demanded at normal pressure.
EXPERIMENTAL
All reagents were commercially available and used without further
purification. Continuous microwave irradiation was carried out with a
microwave & laser therapeutic instrument (1 GHz, 50 mW)9 at atmospheric
pressure. IR spectra were obtained on a Nicolet 740 FT-IR spectrometer.
Elemental analyses were done on Perkin-Elmer 240C elemental analyzer.

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MICROWAVE SYNTHESIS OF AROMATIC ESTERS
2437
Table 1. Esters 3a–h Prepared Under Microwave Athermal Action
Classical method
Reaction
temp.
(ꢁC)
Average
Yield
(%)a
Time
(min)
Time
(min)
Temp.
(ꢁC)
Yield
(%)
Product
3a
3b
3c
3d
3e
3f
60
60
45
22
60
30
30
30
65
79
97
118
65
79
97
84
90
85
87
81
86
78
76
480
720
240
60
480
600
240
60
65
79
97
117
65
79
97
7410
9711
7812
8212
8313
8711,13
8113
7913
3g
3h
118
118
aYield of pure isolated product; at least two experiments.
3a: Benzoic acid, methyl ester; yield 84%, b.p. 197 ꢀ 200ꢁC (lit12. b.p.
198ꢀ200ꢁC) IR n (cmÀ1): 2960, 2880, 1720, 1600, 1452, 1315, 1276, 1176,
1111, 1070, 1027, 711. Anal. Calcd. for C8H8O2: C, 70.58; H, 5.87. Found:
C, 70.10; H, 5.71.
3b: Benzoic acid, ethyl ester; yield 90%, b.p. 211ꢀ213ꢁC (lit12. b.p.
211 ꢀ 213ꢁC) IR n (cmÀ1): 2960, 2934, 2880, 1720, 1600, 1452, 1315, 1280,
1182, 1111, 1070, 1027, 950, 711. Anal. Calcd. for C9H10O2: C, 71.98; H,
6.66. Found: C, 71.59; H, 6.58.
3c: Benzoic acid, propyl ester; yield 85%, b.p. 226 ꢀ 228ꢁC (lit12. b.p.
227 ꢀ 228ꢁC) IR n (cmÀ1): 2960, 2880, 1720, 1600, 1452, 1390, 1317, 1276,
1176, 1111, 1070, 1030, 940, 711. Anal. Calcd. for C10H12O2: C, 73.76; H,
7.33. Found: C, 73.20; H, 7.19.
3d: Benzoic acid, butyl ester; yield 87%, b.p. 242 ꢀ 249ꢁC (lit12. b.p.
243.5 ꢀ 249.5ꢁC) IR n (cmÀ1): 2960, 2920, 2880, 1740, 1600, 1450, 1315,
1260, 1176, 1105, 1070, 1027, 711. Anal. Calcd. for C11H14O2: C, 74.15;
H, 7.81. Found: C, 73.69; H, 7.63.
3e: p-Hydroxybenzoic acid, methyl ester; yield 81%, m.p. 125 ꢀ 128ꢁC
(lit12. m.p. 125 ꢀ 128ꢁC) IR n (cmÀ1): 3300, 2960, 1680, 1600, 1580, 1512,
1440, 1315, 1276, 1230, 1152, 1120, 952, 711. Anal. Calcd. for C8H8O3: C,
63.15; H, 5.26. Found: C, 62.87; H, 5.21.
3f: p-Hydroxybenzoic acid, ethyl ester; yield 86%, m.p. 115 ꢀ 119ꢁC
(lit12. m.p. 116 ꢀ 119ꢁC) IR n (cmÀ1): 3210, 2960, 1680, 1600, 1580, 1440,
1360, 1310, 1276, 1230, 1172, 1170, 852, 761. Anal. Calcd. for C9H10O3: C,
65.04; H, 6.17. Found: C, 65.27; H, 6.03.

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ZHANG ET AL.
3g: p-Hydroxybenzoic acid, propyl ester; yield 78%, m.p. 96ꢁC (lit.12
m.p. 96.2ꢁC) IR n (cmÀ1): 3280, 2970, 1680, 1600, 1580, 1512, 1450, 1371,
1276, 1230, 1162, 1111, 952, 781. Anal. Calcd. for C10H12O3: C, 66.64; H,
6.66. Found: C, 66.19; H, 6.41.
3h: p-Hydroxybenzoic acid, butyl ester; yield 76%, m.p. 67 ꢀ 69ꢁC
(lit.12 m.p. 67 ꢀ 69ꢁC) IR n (cmÀ1): 3370, 2950, 1680, 1600, 1580, 1508,
1450, 1315, 1276, 1210, 1162, 1120, 952, 850, 771, 699. Anal. Calcd. for
C11H14O3: C, 68.02; H, 7.20. Found: C, 67.81; H, 7.03.
General Procedure for 3a–h
A mixture of aromatic acid 1(0.50 mol), alcohol 2(1.00 mol) and oil of
vitriol (0.02 mol) was placed in a 250 mL four-necked and round-bottomed
flask connected with refluxing equipment. Under reflux condition, the reac-
tion system was subjected to microwave (1 GHz and 50 mW) irradiation for
22ꢀ60 minutes. The evolved water was collected in Dean-Start apparatus.
After irradiation, the excessive alcohol was evaporated, and the reaction
mixture was cooled to room temperature. Then, the mixture was transferred
to a 500 mL beaker containing 200 mL cool water. Water phase (or liquid
phase) was removed while organic phase (or solid phase) was washed with
saturated aqueous sodium bisulfite solution and water, dried with magne-
sium sulfate, and evaporated to give crude product. Products 3a–d were
purified by distillation and products 3e–h were purified by recrystallization
from CH3CH2OH.
ACKNOWLEDGMENTS
This work is supported by the Provincial Natural Science Foundation
of Jiangsu. The analyses work was partially supported by the Center of
Materials Analysis, Nanjing University.
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27, 4945.
5. Michael, D.; Mingos D.; David R. Chem. Soc. Rev. 1991, 20, 2.

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2439
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Received in the USA November 6, 2000

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