Thermal conversion of glycerol to value-added chemicals: Pyridine derivatives by one-pot microwave-assisted synthesis

Article abstract of DOI:10.3906/kim-1312-47

One-pot syntheses of the value-added heterocyclic compounds 3-methylpyridine and pyridine using a renewablechemical, glycerol, were achieved in acidic medium by thermal conversion reactions. Condensation/cyclization reactions of the thermal degradation products of glycerol were investigated in situ using different ammonia and acidic moiety producing inorganic ammonium salts under pyrolysis or microwave heating conditions. The reaction parameters were studied in detail and satisfying product yields up to 72% were obtained under optimized conditions. Structural identification of all compounds was accomplished by spectroscopic methods. TUeBITAK.

Full text of DOI:10.3906/kim-1312-47

Turk J Chem
Turkish Journal of Chemistry
(2014) 38: 661 – 670
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1312-47
Research Article
Thermal conversion of glycerol to value-added chemicals: pyridine derivatives by
one-pot microwave-assisted synthesis
∗
¨
˘
¨
˘
¨
Duygu BAYRAMOGLU, G¨okay GUREL, Ali SINAG, Mustafa GULLU
Chemistry Department, Faculty of Sciences, Ankara University, Tandog˘an, Ankara, Turkey
Received: 20.12.2013
•
Accepted: 12.02.2014
•
Published Online: 11.06.2014
•
Printed: 10.07.2014
Abstract:One-pot syntheses of the value-added heterocyclic compounds 3-methylpyridine and pyridine using a renewable
chemical, glycerol, were achieved in acidic medium by thermal conversion reactions. Condensation/cyclization reactions
of the thermal degradation products of glycerol were investigated in situ using different ammonia and acidic moiety
producing inorganic ammonium salts under pyrolysis or microwave heating conditions. The reaction parameters were
studied in detail and satisfying product yields up to 72% were obtained under optimized conditions. Structural
identification of all compounds was accomplished by spectroscopic methods.
Key words: Microwave-assisted synthesis, glycerol utilization, pyridine synthesis, value-added chemicals
1. Introduction
In the developing world, energy demand is rapidly increasing. It is known that the most important energy
source, petroleum oil reserves, will run out in the near future. This issue has directed scientists to focus their
attention on alternative energy sources. One of the most promising renewable energy sources is biodiesel.1 The
biodiesel process involves transesterification of fats and oil with a small alcohol molecule such as methanol.
Meanwhile, large amounts of crude glycerol are formed as a by-product from the transesterification of the oils
in biodiesel production. It is known that 1 kg of glycerol is formed per 9 kg of biodiesel produced, and so the
effective utilization of glycerol is vital to reduce the cost of biodiesel production.2,3
Glycerol is an important substance, having several areas of application, and it is well known that it can
be converted into different compounds by various reactions (oxidation, reduction, dehydration, hydrogenation,
etc.). Glycerol can also be converted into valuable chemicals such as acrolein, acetaldehyde, formaldehyde, etc.
by thermal reactions.4−6 Conversion of glycerol to acrolein is well known in the literature and has been studied
in detail since the 19th century.7,8 Various solid-acid catalysts have been used for the dehydration of glycerol in
either the liquid or the gaseous phase.9−16 Liquid phase reactions are reported in which various homogeneous
or heterogeneous acid catalysts have been used, such as sulfonic acid, benzenesulfonic acid, metal phosphates,
metal sulfates, alumina, and zeolites, but only around 25% of the glycerol could be converted into acrolein with
low selectivity.17,18 Degradation, polymerization, and tar formation have been the main side reactions of the
liquid-phase dehydration of glycerol.
The gas-phase dehydration reaction of glycerol has been carried out under different conditions in the
presence of metal oxides and heteropolyacids (HPAs) supported on metal oxides in autoclaves under high
∗Correspondence: gullu@ankara.edu.tr
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pressures and at high temperatures. It was reported that in the gas-phase reactions dehydration of glycerol
was more successful and resulted in high yields of acrolein. Synthesis of valuable heterocyclic compounds has
been successfully performed from degradation and dehydration products of glycerol, such as acetaldehyde and
acrolein, in the presence of ammonia gas. Several examples of gas-phase reactions involving a heterogeneous
catalyst (silica/alumina or zeolite) can be found in the literature.19 Among these catalysts, zeolites and their
different modified versions have attracted considerable attention in the synthesis of pyridines. Not only zeolites
but also their metal supported derivatives have been used for enhancing the catalyst activity so that pyridine
and 3-methylpyridine yields reached up to 80%. Metal ions doped zeolites have been studied and a mixture of
mainly pyridine and 3-methylpyridine was obtained with a yield of 74%. The condensation reaction of different
aldehydes, such as acrolein, formaldehyde, propionaldehyde, acetaldehyde, and crotonaldehyde, or their mixture
with an ammonium salt in the liquid phase, has also been investigated in the synthesis of pyridine derivatives and
produced 3-methylpyridine more selectively than pyridine, with moderate yields in general.20−25 Polymerization
of glycerol and acrolein has been the most important problem in these reactions and caused lower yields. In
addition, the high costs of the catalysts and their harmful effect to health and the environment as well as
complicated purification processes are the main disadvantages of these methods.
In this research, one-pot syntheses of monocyclic pyridine derivatives were investigated by the thermal
reaction of glycerol in the presence of an ammonium salt under microwave irradiation. The main benefit of this
research was a one-pot reaction in a closed system under microwave heating conditions and it was expected
to prevent polymerization of both glycerol and in situ formed acrolein in the acidic medium, thus resulting in
higher yields of pyridines. An ammonium salt such as ammonium chloride or ammonium bisulfate was expected
to behave as a gaseous ammonia source and also an acid catalyst after thermal decomposition at reaction
temperatures over 200 ◦ C.
2. Results and discussion
One-pot syntheses of pyridine and derivatives were investigated by the thermal reaction of glycerol with different
ammonium salts using different heating strategies (pyrolysis, microwave) in subcritical conditions. Primarily,
the reaction conditions were determined depending on the nature and decomposition temperature of ammonium
salts. The reason for using an ammonium salt was to provide an acidic environment needed for the thermal
conversion of glycerol to acrolein. Additionally, being readily available and affordable compounds, ammonium
salts were used as a nucleophilic nitrogen source for their distinctive property of producing ammonia gas under
thermal conditions. Therefore, several ammonium salts were chosen in order to understand their utility in the
one-pot syntheses of pyridines from glycerol. The ammonium salts employed in this research work are given in
Table 1.
Table 1. Ammonium salts employed and their decomposition temperatures.
Melting or decomposition
Ammonium compound
temperature (◦C)
◦
◦
◦
(NH4)2HPO4
NH4H2PO4
(NH4)2SO4
NH4Cl
NH4OAc
H2NNH2.H2SO4
155 C
190 C (d)
235 C (d)
340 C (mp)
◦
◦
114 C (mp)
265 C (d)
◦
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Research studies were performed using pyrolysis and microwave heating and the results obtained were
evaluated under 2 headings.
2.1. Pyrolysis experiments
In our preliminary studies, 2 different methods were used, namely batch and flow system pyrolysis.26 Glycerol
was reacted as a raw material at high temperatures in these pyrolysis experiments together with an ammonium
salt chosen from the above list, to provide acidic medium for the dehydration of glycerol to acrolein as well as an
ammonium source. It was expected that the ammonium salt used would decompose and release the ammonia
gas at high temperature. In this synthetic procedure, at first, glycerol is expected to convert into acrolein by
the effect of acid catalyst formed in situ by decomposition of ammonium salt. Then a condensation reaction
between 2 moles of acrolein molecules with ammonia was anticipated to give the desired heteroaromatics after
an aromatization step. Dehydration of glycerol to acrolein was considered to be the difficult stage to handle
during the reactions, because both acrolein and other volatile aldehydes may rapidly polymerize and produce
some by-products in the reaction medium, and so it may cause tar formation and low product yields.27
In order to improve the product selectivity and the product yields, the reaction conditions were studied
in detail in both batch (method A) and flow pyrolysis (method B) experiments. Numerous experiments were
planned and carried out in order to determine the optimal reaction temperature, glycerol concentration, and
glycerol/ammonium salt ratio. Better results were obtained using 10% aqueous glycerol solution with silica
supported (NH4)2 HPO4 and H3 PO4 mixture as reagent/catalyst by heating at 450 ◦ C according to method
B, and the target products, pyridine and derivatives, were obtained in approximately 40% yield in total. Other
ammonium salts were also used under similar conditions, but lower yields of pyridines were obtained. The
chemical structures of the pyridine derivatives were identified by GC-MS analysis of crude products. A mixture
was obtained of mainly 3-methylpyridine and pyridine, together with ethylpyridine and ethyl,methylpyridines
in smaller quantities. These results demonstrated that during thermal degradation of glycerol both acrolein
and acetaldehyde were obtained as major products in various quantities depending on the acid and ammonia
concentrations. These degradation products came together in different proportions and reacted with ammonia
in the gas phase, resulting in the formation of mainly pyridine and 3-methylpyridine. On the other hand, it was
observed that the obtained yields did not exceed 40%, because of uncontrolled formation of acrolein, which also
polymerizes competitively instead of reacting with the ammonia in acidic medium at high temperatures. Below
some of the optimized results of pyrolysis experiments are given (Table 2).
Table 2. Results of some pyrolysis experiments.
Heating
rate
rate Crude
rate product
Entry
Method Catalyst
(◦C/min) rate yield∗ (%)
1.9 g Glycerol + 5 mL
0.005 M H2SO4 + 5 mL H2O
1 g Glycerol
1 g Glycerol + 3.2 mL H2O
1 g Glycerol + 7.2 mL H2O
B
(NH4)2HPO4 - 2 g
35
75
25%
A
B
B
(NH4)2HPO4 - 1.6 g
(NH4)2HPO4 -1.6 g
(NH4)2HPO4 - 1.6 g
90
90
90
15
60
60
36%
38%
40%
H3PO4 supported 0.5 g
silica gel-(NH4)2HPO4 - 1.6 g
1 g glycerol + 3.2 mL H2O
B
90
60
30%
*Calculated yields based on 3-methylpyridine
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2.2. Microwave-assisted synthesis
The pyrolysis experimental results showed that the pyrolysis conditions were not adequate because of the low
yields of target compounds and also the polymerization problem. We applied another approach for the synthesis
of pyridine and derivatives. According to this method, one-pot synthesis of pyridines was studied using glycerol
and ammonium salt in a closed vessel and the reactions were carried out by microwave heating under pressure
without using any solvent or catalyst.27 This method provided 2 important advantages: shortening of the
reaction time and achieving the reaction in a closed vessel, which serves like an autoclave.
Different ammonium salts were employed in these reactions again in order to provide ammonia gas
and acidic medium by decomposition under thermal conditions. Nontoxic, economical, and easily available
ammonium salts such as (NH4)2 HPO4 , NH4 H2 PO4 , (NH4)2 SO4 , NH4 Cl, NH4 OAc, and H2 NNH2 .H2 SO4
were preferred. Although the decomposition temperature of the ammonium salt was taken as a reference low
temperature for the experimental studies, a survey of the temperature range was carried out to determine
the optimal reaction temperature. Results of repeated temperature optimization experiments demonstrated
that conversion of glycerol to acrolein increased above 250 ◦ C, but it was observed to be a disadvantage due to
increased polymerization at high temperatures due to uncontrolled acrolein formation and caused lower product
yields. The chemical content of the products was identified by GC-MS analysis. Purification of the products was
not dealt with at this step; quantitative analyses of pyridine and 3-methylpyridine were monitored using HPLC.
3-Methylpyridine and pyridine were observed to be the main products, together with ethyl,methylpyridine and
some pyrazine derivatives obtained as by-products in lower quantities. Some of the better results obtained from
the one-pot synthesis of pyridine derivatives are given in Table 3.
Table 3. Summarized results obtained from optimization experiments.
Temperature Reaction period Crude product
Glycerol (g) Ammonium reactive (g)
(◦C)
280
250
270
280
200
(min)
20
20
40
20
yield* (g)
68%
4%
66%
34%
0.54
0.59
0.54
0.58
0.50
(NH4)2HPO4 - 0.81
(NH4H2PO4) - 0.88
NH4Cl - 0.81
(NH4)2SO4 - 0.87
H2NNH2.H2SO4 -0.86
30
23%
*Calculated yields based on 3-methylpyridine
The results summarized in Table 3 demonstrated that crude product yields are lower than expected
due to polymerization, which was observed as tar formation in the reaction vessel. It is well understood that
acrolein formation, or in other words glycerol degradation, is the key step of the reaction. Glycerol degradation to
acrolein and acetaldehyde was followed by 2 competitive reactions, coupling/condensation/cyclization of acrolein
to heteroaromatics and polymerization of acrolein. Therefore, excessive formation of acrolein is supposed to
increase the polymerization rate. Therefore, it is assumed that controlling the rate of this step could be crucial
to reduce the polymerization. In order to reduce the amount of free glycerol in the reaction medium, as in the
Skraup synthesis, which is well known in the literature, acetic acid has been used for converting glycerol into
its mono- and/or diacetate esters in situ.28−32 Thus, formation of acrolein could have been controlled, so that
formation of pyridines would be increased, even in mild conditions. Three different acids, acetic acid, benzoic
acids, and p−toluensulphonic acids, were used in these experiments to suppress polymerization. The effect
of various ammonium salts at different temperatures and reaction times in the presence of an acid catalyst
was investigated in detail and the better results are summarized in Table 4. These reactions showed that the
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presence of an acid reduced the polymerization reaction and provided satisfactory product yields and acetic
acid gave the best results.
Table 4. Effect of an organic acid on the synthesis of pyridine derivatives.
No. Ammonium reactive (g) AcOH (mmol) Temperature (◦C) Reaction period (min) Yield* (%)
1
2
3
4
5
(NH4H2PO4) - 0.88
NH4Cl - 0.78
(NH4)2SO4 - 0.81
NH4OAc - 0.67
H2NNH2.H2SO4 - 0.42
7.3
5.3
5.5
8.3
3.1
180
220
220
250
200
30
20
30
30
15
52%
64%
35%
26%
72%
Ammonium reactive (g) p−TosOH (g)
Temperature (◦C) Reaction period (min) Yield* (%)
6
7
8
9
10
NH4H2PO4 - 0.62
NH4Cl - 0.29
4.5
1.2
4.7
3.5
2.6
180
200
220
180
200
30
20
30
20
20
37%
14%
12%
6%
(NH4)2SO4 - 0.81
NH4OAc - 0.60
H2NNH2.H2SO4 - 0.42
42%
Ammonium reactive (g) PhCOOH (g)
Temperature (◦C) Reaction period (min) Yield* (%)
11
12
13
14
(NH4H2PO4) - 0.88
NH4Cl - 0.78
(NH4)2SO4 - 0.81
H2NNH2.H2SO4 - 0.42
4.3
5.6
5.9
2.1
220
250
220
200
20
30
30
15
41%
56%
36%
24%
*Calculated yields based on 3-methylpyridine, **5 mmol glycerol was used in each experiment.
It was proved that the formation of acrolein and other volatile aldehydes was slowed down in the presence
of an acid, which indirectly controlled the degradation reaction of glycerol, thus reducing the polymerization.
Under optimized reaction conditions, the best result was 72% yield of mainly 3-methylpyridine containing a
product mixture and it was observed with acetic acid and hydrazine sulfate at 200 ◦ C in 15 min. Analysis of
crude products was carried out by characterization with GC-MS and quantitation with HPLC against pyridine
and 3-methylpyridine standards and showed that mainly 3-methylpyridine formed together with pyridine.
Structural analysis of the 3-methylpyridine and pyridine mixture was also carried out with FTIR and 1 H NMR
spectra. Integrations values of the protons from the 1 H NMR spectrum of the product mixture demonstrated
that 3-methylpyridine formed more than pyridine in a ratio of 60:40. These results also proved that under
optimal conditions glycerol produces much acrolein by dehydration and degrades to small carbonyl compounds
in lower quantities. This result also explains the formation of the main product, 3-methylpyridine, which is
formed by coupling and condensation of 2 moles of acrolein. Microwave heating affected the course of the
reaction dramatically, shortened the reaction period, and even decreased the reaction temperature. Another
advantage of one-pot synthesis was provided by the closed reaction system during microwave heating, where
gaseous products were kept inside the reaction medium and forced to react; hence no loss of dehydration
and degradation products happened. Five different ammonium salts were employed as a nucleophilic nitrogen
source in these experiments within the scope of our work. Although hydrazine sulfate gave the best yield in the
presence of acetic acid (Table 4, example 5), other ammonium salts were also found to be useful with different
acid reagents (Table 4; experiments 2 and 12).
This one-pot synthesis in a closed vessel under pressure of gaseous products by microwave heating was
proved to be a better method for the synthesis of heteroaromatics from glycerol instead of open system pyrolysis.
A mechanism could be proposed for the one-pot synthesis of 3-methylpyridine and pyridine via formation
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of acrolein and acetaldehyde by thermal dehydration and degradation of glycerol in near and supercritical
conditions by ionic and radical mechanism, respectively (Schemes 1 and 2).31 The composition of the product
mixture from glycerol dehydration depends on the applied temperature. It means that the proportions of
acrolein, formaldehyde, and acetaldehyde in the mixture could be changed with the temperature by competing
ionic and free radical reaction pathways.33 Dehydration of glycerol to acrolein is increased at low temperatures
as a result of the ionic mechanism and acrolein is obtained as the main product (Scheme 1).
OH
OH2
H
C
H3O
H3O
+H2O
H
HO
O
- H3O+
- H2O
- H2O
OH OH
O
-H2O
OH OH
OH OH
acrolein
- H2O
H3O
OH
H
H
H
OH
+
CH2
O
O
- H2O
OH OH2
OH
acrolein >>acetaldehyde>formaldehyde
Scheme 1. Synthetic route for the dehydration of glycerol via ionic mechanism.
acetaldehyde formaldehyde
On the other hand, acetaldehyde occurs mainly at higher temperatures dominantly by a free radical
mechanism (Scheme 2).
OH
OH
OH
H
CH
OH OH
- H2O
OH OH
- H2O
- OH
O
OH OH
acrolein
- H2O
OH
OH
C
OH
H
O
OH
CH
+
+
OH
CH2
- H2O
OH
OH OH
acetaldehyde
H
H
T>600oC
rapidly
+
H
O
formaldehyde
acetaldehyde>> acrolein> formaldehyde
Scheme 2. Synthetic route for the dehydration of glycerol via free radical mechanism.
After the dehydration/degradation step, coupling and condensation of acrolein and acetaldehyde with
ammonia result in dihydropyridines, which then aromatize to produce pyridine and 3-methylpyridine. The
suggested condensation/cyclization mechanism is given in Schemes 3 and 4.
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H
NH3
H
H
H
NH3
NH
O
N
H
O
O
- H2O
HO
H
CH
N
+ H3O
- H2O
CH
N
HC NH
H
- H2O
O
H
HO
H
H
H
+ H2O
CH2
H2C
N
N
- H3O
N
Scheme 3. Suggested mechanism for the synthesis of 3-methylpyridine.
H
H
NH3
N
NH3
NH
O
- H2O
O
HO
-O
H
CH
N
+ H3O
- H2O
H2
C
HC NH
O
CH
HN
H
H
- H2O
H
OH
H
H
+ H2O
- H3O
(H)
(Arom.)
H2C
NH
N
N
H
N
Scheme 4. Suggested mechanism for the synthesis of pyridine.
The mechanisms described show above that 3-methylpyridine and pyridine might form in separate
pathways; while 3-methylpyridine was formed only from acrolein, pyridine formation requires both acrolein
and acetaldehyde in equal amounts. Ionic dehydration/degradation produces acrolein as the main product, but
together with small amounts of acetaldehyde. In our results, 3-methylpyridine/pyridine ratios were found to be
60:40, which supports the ionic mechanism in the presence of an acid catalyst instead of the radical mechanism.
In conclusion, 3-methylpyridine and pyridine were successfully produced by one-pot synthesis from a
renewable chemical, glycerol, by the thermal conversion reaction in the presence of an ammonia producing
inorganic reactive under microwave irradiation and pyrolysis conditions. Both products were obtained under
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pyrolysis conditions in lower yields (40%) due to undesired polymerization, tar formation, and loss of gaseous
products. On the other hand, microwave-assisted heating in a closed vessel resulted in much better yields in
much shorter reaction periods. Addition of an acid, such as acetic acid, prevented the polymerization and
increased the product yields. Ammonium salts were proved to be highly successful in providing ammonia gas
as a nucleophilic nitrogen source and an acid catalyst for ionic dehydration/degradation of glycerol. Direct
conversion of glycerol to 3-methylpyridine and pyridine mixture was successfully realized by one-pot synthesis
under optimized conditions with yields up to 72%. In conclusion, glycerol, a by-product of biodiesel production,
was found to be highly successful as a renewable raw material in the one-pot synthesis of valuable heteroaromatics
by microwave-assisted heating in a closed system.
3. Experimental
3.1. Chemicals and instrumentation
All chemicals, including (NH4)2 HPO4 , NH4 H2 PO4 , (NH4)2 SO4 , NH4 Cl, NH4 OAc, and H2 NNH2 .H2 SO4 ,
were purchased from Sigma Aldrich or Fluka and used without further purification.
Pyrolytic reactions were carried out using a Protherm pyrolysis furnace and temperature control system.
Thermal experiments were carried out in a Quartz tube reactor. A double-walled flask was used for collecting
gaseous products in liquefied form, by condensing with a double-walled condenser fitted to the flask. Both
collection flask and condenser were cooled by a cryostat at –20 ◦ C set temperature.
A CEM Discovery S-Class (Single Mode) Microwave Synthesis System was used with 35-mL glass vials
for the microwave-assisted synthesis reactions. GC-MS analyses were performed using an Agilent Technologies
6890 N Network GC System/Agilent Technologies 5975B VL MSD mass spectrometry. 1 H NMR and 13 C NMR
spectra were recorded at room temperature on a Varian-Mercury 400-MHz high performance digital Fourier-
transform FT-NMR spectrometer with tetramethyl silane (TMS) as the internal standard. All chemical shifts
were given in ppm. IR spectra were recorded on a PerkinElmer Spectrum 100 spectrometer. Quantitative
analyses of the products were carried out by Essence HPLC Workstation using an ACE 5 C18 column with 30%
aqueous MeOH mobile phase.
3.2. Reactions
3.2.1. Pyrolysis reactions
Two different methods were used for the synthesis of pyridine derivatives, namely batch (method A) and flow
pyrolysis (method B).
Batch pyrolysis (method A): 3 g of glycerol and 2.5 g of (NH4)H2 PO4 were packed into a glass tubular
reactor. Nitrogen gas was passed through the system with a flow rate of 40 mL min−1 for 20 min before the
reaction. The system was heated to a pre-adjusted temperature (450 ◦ C) and the reaction was carried out at
that temperature for about 1.5 h. During the reaction, the products were collected in a cooled (–20 ◦ C) reflux
condenser fitted flask containing a mixture of H2 SO4 /H2 O (pH 1–2). The products were dissolved in the form
of sulfate salt in this solution; therefore after the reaction was completed, this aqueous solution was added to
saturated Na2 CO3 solution to free pyridine derivatives. Pyridine derivatives were then extracted with CH2 Cl2
(50 mL × 3) and the combined extracts was dried over Na2 SO4 and evaporated to obtain the crude product.
Flow pyrolysis (method B): Different from the batch pyrolysis, an aqueous solution of glycerol was
continuously added dropwise to the sample tub, placed inside the tubular reactor by a thermally stable glass
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tube. Thus, (NH4)H2 PO4 (2.5 g) and H3 PO4 impregnated silica gel (0.5 g) were prepared in the sample tub
and placed into the quartz tubular reactor. Aqueous glycerol solution (20% m/m) was prepared and taken in
a dropping funnel. Nitrogen gas was passed through the system with a flow rate of 40 mL min−1 for 20 min
before the reaction. The Quartz tube reactor was heated to a pre-adjusted temperature (450 ◦ C) and then
glycerol solution was added drop by drop and the reaction was carried out for 1.5 h at this temperature, while
all the gaseous products were collected in a cooled (–20 ◦ C) reflux condenser fitted flask containing a mixture of
H2 SO4 /H2 O (pH 1–2). The products were dissolved in the form of sulfate salt in this solution; therefore after
the reaction was completed, this aqueous solution was added to saturated Na2 CO3 solution to free pyridine
derivatives. The pyridine derivatives were then extracted with CH2 Cl2 (50 mL × 3) and the combined extracts
were dried over Na2 SO4 and evaporated to obtain the crude product.
3.2.2. Microwave-assisted reactions
A mixture of glycerol (1 mmol), an ammonium salt (1.1 mmol), and an acid catalyst was prepared in a 35-mL
glass vial and closed with a cap tightly. After stirring, the reaction mixture was irradiated by microwave energy
for a few minutes to complete the reaction as required. The reaction mixture was cooled, then treated with water,
and then neutralized with aqueous NaOH. The products were extracted with CHCl3 and dried over Na2 SO4 .
The extract was evaporated to obtain the crude product, which was purified by column chromatography over
silica gel (ethyl acetate:hexane 4:1).
3.3. Spectral data for pyridine and derivatives
The contents of crude products from various reactions were characterized using GC-MS. It was determined
that a mixture of 3-methylpyridine and pyridine was obtained together with small amounts of ethylpyridine
and ethylmethylpyridine. Quantitative analyses of the 3-methylpyridine and pyridine were carried out with
HPLC using a mobile phase consisting of 30% MeOH/70% H2 O. Standard stock solutions were purchased
from Sigma-Aldrich and correlation curves for each one were prepared. Calibration curves of the standards
ranging from 200 to 2000 ppm were constructed for both eluents. Utilizing the peak area of each component,
it was demonstrated that 3-methylpyridine occurred as the main product with a yield of 35%–51%. Chemical
structures of the compounds were supported by MS, 1 H NMR, and FTIR analysis.
MS of pyridine (EI) m/z (%) calcd. for C5 H5 N m/z: 79 (M+ , C5 H6 N, found: 79 (16)), 52 (100), 26
(10).
MS of 3-methylpyridine (EI) m/z (%) calcd. for C6 H7 N m/z: 93 (M+ , C6 H7 N, found: 93 (15)), 78
(10), 66 (100), 51 (16), 39 (73), 28 (21).
1 H NMR spectrum of pyridine (CDCl3), δH /ppm: 8.62–8.60 (dd, H, J = 4.8, J = 1.2); 7.68–7.64 (tt,
H, J = 7.2, J = 4.8, J = 1.4); 7.28–7.25 (m, H).
1 H NMR spectrum of 3-methylpyridine (CDCl3), δH /ppm: 8.44 (d, H, J = 1.2); 8.41–8.40 (dd, H, J
= 4.8, J = 1.2); 7.48–7.46 (dd, H, J = 7.2, J = 1.4); 7.18–7.15 (dd, H, J = 4.8, J = 2.4); 2.3 (s, 3H).
IR spectrum of 3-methylpyridine ν /cm−1 : 3075 (aromatic C–H); 2855 (aliphatic C–H); 1660 (C=N);
1420–1375 (aliphatic C–H); 1594 (aromatic C=C).
IR spectrum of pyridine ν /cm−1 : 3046 (aromatic C–H); 1663 (C=N conj.); 1594 (aromatic C=C).
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