Esterification of dicarboxylic acids with benzyl alcohol under the action of the microwave radiation

Article abstract of DOI:10.1134/S1070363208100162

Reaction of dicarboxylic acid with benzyl alcohol under the microwave irradiation proceeds faster as compared to the thermal conditions. The main reaction products are alkyl dicarboxylates, and the monoester and dibenzyl ether are formed as the side products. A proposal about the nature of the nonthermal effect in the reactions stimulated by the microwave irradiation is considered.

Full text of DOI:10.1134/S1070363208100162

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 10, pp. 1920–1923. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © D.N. Aver’yanov, A.V. Batrakova, Ya.D. Samuilov, R.R. Spiridonova, A.M. Kochnev, S.S. Galibeev, O.I. Gnezdilov, 2008, published
in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 10, pp. 1684–1688.
Esterification of Dicarboxylic Acids with Benzyl Alcohol
under the Action of the Microwave Radiation
D. N. Aver’yanov, A. V. Batrakova, Ya. D. Samuilov, R. R. Spiridonova,
A. M. Kochnev, S. S. Galibeev, and O. I. Gnezdilov
Kazan State Technological University,
ul. K. Marksa 68, Kazan, 420015 Tatarstan, Russia
e-mail: denisaveryanov@yandex.ru
Received March 31, 2008
Abstract—Reaction of dicarboxylic acid with benzyl alcohol under the microwave irradiation proceeds faster
as compared to the thermal conditions. The main reaction products are alkyl dicarboxylates, and the monoester
and dibenzyl ether are formed as the side products. A proposal about the nature of the nonthermal effect in the
reactions stimulated by the microwave irradiation is considered.
DOI: 10.1134/S1070363208100162
Intensification of chemical processes by the
application of microwave irradiation is an important
method of the energy saving technologies. This
method of activation is used successfully for
performing a wide range of chemical processes [1–4].
The thermal and specific (nonthermal) effects of the
microwave activation of chemical processes are
distinguished [5, 6]. In the first case the rates of
chemical reactions under the conditions of the
microwave heating are equal to the rates of thermal
reactions when the temperatures of the reaction
mixtures are the same. In the second case the
micriwave-stimulated reactions proceed significantly
faster than the thermal ones. Specific effect of the
microwave irradiation presents a great interest because
it permits to increase the reaction rate without the
introduction of catalysts.
A successful industrial examination of a pilot flow-
type plant for esterification of monocarboxylic acids
with alcohols in the presence of alumosilicates under
the microwave irradiation heating has also been
reported [8].
Effect of the microwave radiation on the
esterification of carboxylic acids is not unambiguous.
No specific effect was found during the esterification
of propionic acid with propanol-1 [5] and of benzoic
acid with ethanol [9]. In both cases the microwave
irradiation was only a means for the heating the
reaction mixture. At the same time esterification of
benzoic acid with metanol [10] and of metacrylic acid
with phenol [11] demonstrated clear specific effect.
Thermal polycondensation of lactic acid does not take
place at 200°C, but it proceeds successfully at the same
temperature under the microwave irradiation [12].
Alkyl carboxylates are important products of the
industrial organic chemistry. They are widely used as
the plasticizers of the polymeric materials, unsaturated
polyesters are used for preparation of polyester resins
and serve also as the starting products in the synthesis
of polyurethanes. Microwave irradiation is an effective
tool for the stimulation of esterification of carboxylic
acids with alcohols. For example the almost
quantitative esterification of acetic and benzoic acids
with higher alcohols in the presence of heteropolyacids
under the conditions of the microwave irradiation has
been reported [7].
Reported data on the esterification of carboxylic
acids under the action of microwave irradiation deal
mainly with monobasic acids, while the reports on
esterification of dibasic acids are scarce [13]. At the
same time dicarboxylic acid esters have important
practical use.
With the purpose of revealing the rules of
formation of the dicarboxylic acid esters under the
microwave irradiation of reaction mixtures we studied
esterification of a series of dicarboxylic acids with
benzyl alcohol.
1920

ESTERIFICATION OF DICARBOXYLIC ACIDS WITH BENZYL ALCOHOL
1921
HOOC–R–COOH + HOCH2C6H5
HOOC–R–COOCH2C6H5
–H2O
Iа–Ig
II
IIIа–IIIg
HOOC–R–COOCH2C6H5 + HOCH2C6H5
IIIа–IIIg II
C6H5CH2OCO–R–COOCH2C6H5
–H2O
IVа–IVg
R = –(СН2)2– (а), –(СН2)3– (b), –(СН2)4– (c), –(СН2)6– (d), –(СН2)7– (e), –(СН2)8– (f), trans–СН=СН– (g).
The reaction under investigation is a two-stage
process with the intermediate formation of acid
monoesters IIIa–IIIg. Benzyl alcohol II was chosen
by us as a high-boiling compound, convenient for the
investigation of the transformations with its participa-
tion under the conditions of microwave irradiation.
should result in establishing of equilibrium between
the reagents and the products.
In the course of the microwave-activated process a
decrease in mass of the reaction mixture takes place
(Fig. 3). This is caused by the evaporation of the
benzyl alcohol–water azeotrope [14].
In the first step of our studies we tried to reveal a
non-thermal microwave effect on the esterification of
dicarboxylic acids. With this purpose we carried out
comparative investigation of carboxylic group con-
sumption in the reaction of the acid Ia with the alcohol
II under thermal conditions and microwave irradiation.
The results obtained are presented in the Fig.1.
The data on the time dependence of the acid
number in the microwave-activated reactions of the
acids Ia–Ig with alcohol II are listed in Table 1. From
these data follows that esterification of the acids Ia–Ig
with the alcohol II proceeds in high yield. Besides the
formation of the incomplete and complete esters IIIa–
IIIg, IVa–IVg formation of dibenzyl ether V and of
polycondensation product VI can take place.
From the data presented follows that at the com-
parable temperature the thermal reaction proceeds
significantly slower than the microwave-activated one.
Hence, in the reaction under investigation the specific
microwave effect is observed. From the fig.1 it follows
that the acid number dependence on the reaction time
2 II → C6H5CH2OCH2C6H5 + H2O,
V
n II → C6H5CH2(C6H4CH2)n–1OH + (n – 1) H2O.
VI
1
under the microwave activation is linear. H NMR
studies of reaction mixtures showed that partial ac-
cumulation of water formed in the course of esterifica-
tion takes place in the reaction mixture (Fig. 2). This
These reactions are catalyzed by Brønsted acids
[15]. With the purpose of establishing the selectivity of
the microwave-activated esterification, a chromato-
350
300
250
200
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
150
100
50
1
0
10
20
30
40
50
2
0
t, min
0
10
20
30
40
50 t, min
Fig. 2. Plot of water concentration ([H2O]) against the
reaction time (t) in the reaction of the acid Ia with the
alcohol II under the conditions of microwave irradiation.
Fig. 1. Time (t) dependence of the acid number at the
reaction of acid Ia with alcohol II at (1) 180°C and (2)under
the microwave irradiation.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 10 2008

1922
AVER’YANOV et al.
Table 1. Dependence of the acid number on time in the
microwave-activated esterification of acids Ia–Ig with
alcohol II
Table 2. Composition of the reaction mixtures of acids I
with alcohol II established by the chromato-mass spectro-
metric measurements
Acid number, mg KОН g–1
Composition of the reaction mixture, wt %
Com-
Reaction
pound
initial
341
325
310
283
278
255
388
5 min
275
261
259
277
246
247
340
40 min
12.7
11.9
11.3
9.35
9.7
III
IV
V
Iа
Ib
Ic
Id
Ie
If
Iа + II
Ib + II
4.7
95.2
0.1
4.7
91.7
3.6
Id + II
Ig + II
4.4
1.6
93.2
95.9
2.4
2.5
9.4
4.6
Ig
mass spectrometric investigation of a series of crude
reaction mixtures was carried out. The data obtained
are listed in Table 2.
proceeds faster than under the thermal activation. From
that it follows that the specific microwave effect is
observed. This effect has been discussed about, and
some workers even reject its existence [16]. In this
connection we want to note the following two
curcumbstances.
We found that in all the cases complete esters IV
are the main reaction products. Content of the acid
esters is insignificant. Formation of the by-product V
was observed in all the cases, while no oligomer VI
was found. Amount of the admixtures is too small that
At first, the microwave-activated reactions are often
carried out in the thermostable glass vessels. This ma-
terial absorbs strongly in this spectral range. Due to
that in the course of chemical transformations only the
thermal effect of microwave activation is mainly
observed because the reaction mixture is to a great
extent is heated by the wall of the vessel. Teflon and
quartz glass are more suitable materials for such
apparatus because they do not practically absorb the
microwave radiation.
1
to be rgistered by the method of H NMR spec-
troscopy.
Hence, the above-described studies show that the
microwave-activated esterification of dicarboxylic
acids is a convenient procedure for preparing their
complete esters. Under these conditions the reaction
m, g
The second statement is that the microwave
radiation energy is consumed for excitation of the
rotation levels of molecules. Such levels in turn are
formed by the splitting of the vibrational levels. From
this point of view excitation of the rotation levels must
be always accompanied by partial excitation of the
vibrational ones. In the course of chemical reaction the
reacting system is transfered from the vibrational
levels of the reagents onto the vibrational levels of the
transition state. If the microwave radiation leads to
diminishing in the energy difference of these two
vibrational levels then the specific microwave effect
becomes inevitable.
5.00
4.95
4.90
4.85
4.80
4.75
4.70
4.65
4.60
4.55
4.50
0
10
20
30
40
EXPERIMENTAL
t, min
1H NMR spectra were taken on a Bruker DPX-400
spectrometer (400 MHz). DMSO-d6 was used as a
solvent and as the reference substance.
Fig. 3. Dependence the mass (m) of the reaction mixture on
the reaction duration (t) at the microwave-activated
esterification of acid Ia with alcohol II.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 10 2008

ESTERIFICATION OF DICARBOXYLIC ACIDS WITH BENZYL ALCOHOL
1923
Chromatomass spectrometric studies were carried
out on a Perkin-Elmer (Precisely) Clarus 500 mass
spectrometer equipped with a MSD detector. The
evaporator temperature 260°C, the initial column
temperature 100°C. After 50 seconds the column
temperature was elevated to 220°C at a rate of 15°C/min.
Then the column temperature was elevated to 306°C at
a rate of 8°C/min. The device was equipped with a
25000×0.2 mm capillary column, Elite-17 MS
stationary phase, helium carrier gas.
Compound IVd, δ, ppm: 1.26 m (4H, CH2CH2),
1.53 m (4H, CH2CH2), 2.34 m (4H, CH2CH2), 5.09 s
(4H, OCH2–h), 7.36 m (10H, aromatic protons).
1
Compound Vg. H NMR spectrum, δ, ppm: 5.24 s
(4H, OCH2–Ph), 6.86 s (2H, CH=CH), 7.39 m (4H,
aromatic protons), 7.41 m (6H, aromatic protons).
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used, the main substance content being above 99.0%.
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Benzyl alcohol was purified by vacuum distillation
according to [14]. Its physicochemical constants were
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