Synthesis and evaluation of the thermal behavior of flavonoids: Thermal decomposition of flavanone and 6-hydroxyflavanone

Article abstract of DOI:10.1007/s10973-016-5896-6

Flavonoids in the broad sense of the term are virtually universal plant pigments. Synthesis of flavanone and 6-hydroxyflavanone was carried out and identified by H and C NMR. Details concerning the thermal behavior were evaluated by thermogravimetry under oxygen and nitrogen purge gases. Additionally, the kinetic studies were evaluated from several heating rates (5, 10 and 20?°C?min^?1) and sample mass of 2?mg in open crucibles. The obtained data were evaluated with the isoconversional method, where the values of activation energy (E_a/kJ?mol^?1) were plotted in function of the conversion degree (α). The results of thermal behavior of the flavanone under a nitrogen purge gas showed that this compound has a homogeneous degradation process while the hydroxyl group in the aromatic ring of 6-hydroxyflavanone there are two mass losses.

Full text of DOI:10.1007/s10973-016-5896-6

J Therm Anal Calorim
DOI 10.1007/s10973-016-5896-6
Synthesis and evaluation of the thermal behavior of flavonoids
Thermal decomposition of flavanone and 6-hydroxyflavanone
Leonardo Miziara Barboza Ferreira1 Marcelo Kobelnik2 Luis Octavio Regasini3
•
•
•
Luiz Antonio Dutra3 Vanderlan da Silva Bolzani3 Clovis Augusto Ribeiro
1
•
•
´
Received: 18 September 2015 / Accepted: 5 October 2016
´
´
Ó Akademiai Kiado, Budapest, Hungary 2016
Abstract Flavonoids in the broad sense of the term are
virtually universal plant pigments. Synthesis of flavanone
and 6-hydroxyflavanone was carried out and identified by H
and C NMR. Details concerning the thermal behavior were
evaluated by thermogravimetry under oxygen and nitrogen
purge gases. Additionally, the kinetic studies were evalu-
ated from several heating rates (5, 10 and 20 °C min-1) and
sample mass of 2 mg in open crucibles. The obtained data
were evaluated with the isoconversional method, where the
values of activation energy (Ea/kJ mol-1) were plotted in
function of the conversion degree (a). The results of thermal
behavior of the flavanone under a nitrogen purge gas
showed that this compound has a homogeneous degradation
process while the hydroxyl group in the aromatic ring of
6-hydroxyflavanone there are two mass losses.
absorb near-UV radiation, due to extensive chro-
mophores, and are visible to many arthropod and bird
pollinators [2]. Flavanones (or 2-phenylchroman-4-ones)
are flavonoids containing one sigma bond between
carbons C-2 and C-3 (ring C) carbons. They are
responsible for several organoleptic properties of food,
including the taste. Interestingly, naringin-7-rutinoside,
a glycosylated compound obtained from the peel of the
grapefruit (Citrus paradisi), is very bitter and astringent
[3]. Nowadays, the study of natural and synthetic fla-
vanone derivatives has significantly increased because
these compounds exhibited potent chemopreventive and
pharmacological activities such as: antioxidant, anti-
inflammatory, anticancer, anti-diabetic and antimicro-
bial [4–8].
Although the antioxidant activity of the 6-hydroxyfla-
vanone is not as good as flavonoids with multiple hydroxyl
substituents [9], this compound shows other good medici-
nal properties such as vaso-relaxing effects [10] and an
anticancer effect through extrinsic and intrinsic apoptotic
pathways [11].
Keywords Flavanones Á NMR spectra Á Thermogravimetry
Introduction
Flavonoids in the broad sense of the term are virtually
universal plant pigments [1]. They are responsible for
the color of flowers, fruits and leaves. Their structures
In this work, we report on the synthesis and thermal
characterization of flavanone and 6-hydroxyflavanone. The
results obtained with the present study improve the
knowledge on these flavonoids. The experiments were
carried out with one only mass sample in nitrogen and
oxygen purge gases, in order to determine the effect of this
change on obtained results as well as on the kinetic
behavior. Moreover, the isoconversional method is used as
a way of obtaining reliable and consistent kinetic infor-
mation and also because it avoids the use of explicit kinetic
& Marcelo Kobelnik
mkobelnik@gmail.com
1
´
Departamento de Quımica Analıtica, Instituto de Quımica,
UNESP—Universidade Estadual Paulista,
´
´
C.P. 355, Araraquara, SP 14800-900, Brazil
2
3
´
Centro Universitario do Norte Paulista, UNORP,
Sa˜o Jose do Rio Preto, SP, Brazil
models [12–16]. Thus, the activation energy (Ea/kJ mol-1
)
´
data were obtained applying the method proposed by
Capela et al. [16].
´
Departamento de Quımica Organica, Instituto de Quımica,
UNESP—Universidade Estadual Paulista,
ˆ
´
C.P. 355, Araraquara, SP 14800-900, Brazil
123

L. M. B. Ferreira et al.
3.0 Hz, H-2), 3.16 (dd; J = 17.0, 13.0 Hz, H-3b), 2.78 (dd;
J = 17.0, 3.0 Hz, H-3a). 13C NMR (125 MHz, DMSO-d6)
d = 191.6 (C-4), 154.3 (C-6), 151.5 (C-9), 139.1 (C-10),
128.4 (C-30 and C-50), 128.3 (C-40), 126.4 (C-20 and C-60),
124.5 (C-10), 120.8 (C-7), 118.9 (C-8), 109.9 (C-5), 78.7
(C-2), 43.6 (C-3).
Experimental
General procedures for synthesis of flavanones
Flavanone (1) and 6-hydroxyflavanone (2) were synthe-
sized as previously described by Zeraik et al. [17], with
minor modifications. In a 30-mL vial, the appropriated
acetophenone (2.5 mmol) and lithium hydroxide monohy-
drate (0.5 mmol) were dissolved in methanol (10 mL), and
the mixture was stirred at 5 °C for 10 min followed by the
addition of benzaldehyde (2.70 mmol). The reaction mix-
ture was stirred at room temperature and monitored by
TLC using hexanes/ethyl acetate (9:1) as the mobile phase.
The reaction was quenched after 6 days by pouring into
100 mL of stirring ice cold water, and a stick mass was
observed in the aqueous solution after quenching. Thus, the
product was extracted by ethyl acetate (3 9 100 mL),
dried over sodium sulfate and concentrated under reduced
pressure. The crude products were purified by flash chro-
matography using hexanes/ethyl acetate as the solvent
system in increasing the order of polarity. ( )-Flavanone
(1) and its hydroxylated derivative (2) were identified by
1H and 13C NMR spectra data obtained from a Varian
DRX-500 spectrometer (11.7 T). Chemical shifts (d) were
expressed in ppm. Coupling constants (J) were expressed
in Hz, and splitting patterns are described as follows:
s = singlet; d = doublet; m = multiplet; dd = doublet of
doublets (Fig. 1).
Thermal behavior
TG/DTG curves were obtained from a SDT 2960, from TA
Instruments, respectively. The TG/DTG curves were
obtained using a sample of about 2 mg in an a-alumina
crucible with heating rates of 5, 10 and 20 °C min-1 under
nitrogen and oxygen purge gases with flow of
100 mL min-1
.
Kinetic methodology
The method described below to use as kinetic methodology
was widely exposed in other work carried out [12–15]. In
this work, the kinetic parameters are obtained using an
isoconversional method on approximation to the tempera-
ture integral based on the convergent of a Jacobi fraction,
proposed by Capela et al. [16]. Thus, it should be noted that
this is not a self-plagiarism but is aimed at the use of a
standard methodology. Therefore, the use of methodology
standard analysis allows to reproduce the results obtained
by other research groups without the need to complicate the
understanding of extraction procedures and analysis. In
addition, to use this kinetic methodology, we use the ‘‘least
square method’’ and Origin software.
( )-Flavanone (1). ( )-2-phenylchroman-4-one. It was
obtained as a pale yellow solid in 88 %. 1H NMR (500 MHz,
DMSO-d6) d = 7.85 (dd; J = 8.0, 1.5 Hz, H-5), 7.40 (m;
H-6–H-8 and H-30–H-50), 6.96 (dd; J = 7.0, 1.5 Hz, H-20
and H-60), 5.40 (dd; J = 13.0, 3.0 Hz, H-2), 3.00 (dd;
J = 17.0, 13.0 Hz, H-3b), 2.81 (dd; J = 17.0, 3.0 Hz,
H-3a). 13C NMR (125 MHz, DMSO-d6) d = 191.9 (C-4),
161.5 (C-9), 138.7 (C-10), 136.1 (C-7), 128.8 (C-30 and
C-50), 128.7 (C-40), 127.0 (C-20 and C-60), 126.1 (C-5), 121.6
(C-10), 120.9 (C-6), 118.1 (C-8), 79.5 (C-2), 44.6 (C-3).
( )-6-Hydroxyflavanone (2). ( )-6-hydroxy-2-phenyl
chroman-4-one. It was obtained as an intense yellow solid
in 84 %. 1H NMR (500 MHz, DMSO-d6) d = 9.40 (s;
6-OH), 7.52 (dd; J = 8.5, 1.5 Hz, H-20 and H-60), 7.40 (m;
H-30–H-50), 7.12 (d; J = 3.0 Hz, H-5), 7.04 (dd; J = 9.0,
3.0 Hz, H-7), 6.95 (d; J = 9.0, H-8), 5.54 (dd; J = 13.0,
Capela and Ribeiro et al. approached that it is given by
the following equation:
1
Z
expðÀzÞ
expðÀxÞ
x3 þ 14x2 þ 46x þ 24
dz ¼
z2
x
x4 þ 16x3 þ 72x2 þ 96x þ 24
x
ð1Þ
A characteristic experimental curve presents the con-
versional fraction, a, as a function of the temperature for a
given heating rate, b. For each fixed value of a, there are
corresponding values Ta for temperature, values Ea for
activation energy and values Aa for the pre-exponential
factor.
Usually, the kinetic parameters are made under non-
isothermal conditions, defined by
O
O
O
O
1
Z
AE
expðÀzÞ
b ¼
dz
ð2Þ
HO
RgðaÞ
z2
1
2
E=RT
where b = dT/dt is a constant heating rate (T is the tem-
perature and t is the time), g(a) is the integral form of the
Fig. 1 Structures of ( )-flavanone (1) and ( )-6-hydroxyflavanone
(2)
123

Synthesis and evaluation of the thermal behavior of flavonoids
reaction model as function of the extent of reaction a, A is
the pre-exponential factor, E is the activation energy and
R is the gas constant.
Oxygen purge gas
100
80
60
40
20
0
(6–Hydroxyflavanone)
2.7
2.0
1.3
0.6
–0.1
(Flavanone)
The evaluation of the integral on the right side of
Eq. (2), known as temperature integral, is required, but this
integral does not have an exact analytical solution. Thus, it
is convenient to approximate the integral of temperature for
some function that yields suitable estimates to these kinetic
parameters.
Replacing the integral in Eq. (2) by the approximation
given in Eq. (1) is obtained the following expression for
heating rate b as function of the xa = 103/RTa:
0
100
200
300
400
500
600
700
Temperature/°C
expðBa À EazaÞ
Ea3z3a þ 14Ea2za2 þ 46Eaza þ 24
Fig. 2 TG/DTG curves of flavanone and 6-hydroxyflavanone with
mass sample of 2 mg in an oxygen purge gas and heating rate of
20 °C min-1
b ¼
xa
Ea4z4a þ 16Ea3za3 þ 72Ea2za2 þ 96Eaza þ 24
ð3Þ
2.5
where the activation energy is in kJ mol-1 and the
Nitrogen purge gas
(6–Hydroxyflavanone)
100
parameter Ba is defined as:
ꢀ
2.0
1.5
ꢁ
103Aa
80
(Flavanone)
Ba ¼ ln
ð4Þ
RgðaÞ
60
1.0
The estimates of the Ea and Ba can be obtained by the
nonlinear fitting of Eq. (3) to the b values as function of xa.
Once the g(a) function has been determined for each
conversional fraction a, the estimation of the Arrhenius
pre-exponential factor can be obtained from Eq. (4) and is
given by the following equation:
40
0.5
20
0
0.0
–0.5
350
0
50
100
150
200
250
300
Temperature/°C
À
Á
R
103
^
^
Aa ¼
exp Ba gðaÞ
ð5Þ
Fig. 3 TG/DTG curves of flavanone and 6-hydroxyflavanone with
mass sample of 2 mg in a nitrogen purge gas, heating rate of
20 °C min-1, in alumina crucible
Results and discussion
535 °C, with a mass loss of 97.34 % and with formation of
a low quantity carbonaceous residue. For the thermal
decomposition of 6-hydroxyflavanone, there are two mass
losses between 190–217 and 217–328 °C with a loss of
2.02 and 85.34 %, respectively.
TG/DTG curves of the flavanone and 6-hydroxyflavanone
are reported in Figs. 2 and 3 in oxygen and nitrogen purge
gases.
The analysis of the flavanone under an oxygen purge gas
shows only one mass loss of 98.01 % between 150 and
276 °C, which was attributed to thermal decomposition.
From the analysis of 6-hydroxyflavanone under oxygen, we
can see that this compound exhibited a mass loss of
40.03 % between 180 and 314 °C, which was also attrib-
uted to thermal decomposition. Moreover, this compound
shows one further stage of thermal decomposition, with a
mass loss of 53.66 %, which occurred in overlapping
reactions up to 590 °C. Furthermore, the 6-hydroxyfla-
vanone showed a higher thermal stability that the fla-
vanone, which is due to the presence of the hydroxy group
in the aromatic ring.
In addition, Fig. 4a, b shows the DSC analysis carried
out under oxygen purge gas with heating rate of
20 °C min-1 (Vh). The melting point (Tmp) of these com-
pounds can seen in the first endothermic peak, which are
ascribed in 78.5 °C to flavanone and 215 °C to 6-hydrox-
yflavanone (both are better seen in Figure B). The DSC
curves of flavanone show an event between 210 and
265 °C, which is in agreement with mass loss observed in
the TG curve, while the 6-hydroxyflavanone does not show
event during thermal decomposition in interval of 250 and
450 °C. This fact probably indicates that the exothermic
and endothermic reactions occur in equilibrium during
thermal decomposition. For 6-hydroxyflavanone, the DSC
curve shows a sharp exothermic peak between 468 and
The thermal decomposition of flavanone under a nitro-
gen purge gas (Fig. 3) occurs in only one stage from 150 to
123

L. M. B. Ferreira et al.
300
200
100
0
3
20 °C
Nitrogen purge gas
100
80
60
40
20
0
5 °C
10 °C
T
2
mp
T
B
mp
1
(Flavanone)
T
mp
A
0
(6–Hydroxyflavanone)
T
mp
Vh = 20 °C min–1
–1
0
50
100
150
200
250
300
350
–100
0
100
200
300
400
500
600
Temperature/°C
Temperature/°C
Fig. 6 TG/DTG curves of flavanone with mass sample of 2 mg in a
nitrogen purge gas, heating rates of 5, 10 and 20 °C min-1, in
alumina crucible
Fig. 4 DSC curves of flavanone and 6-hydroxyflavanone with mass
,
sample of 2 mg in an oxygen purge gas, heating rate of 20 °C min-1
in aluminum crucible
505 °C, which was attributed to the combustion of the
residual material.
1.5
Oxygen purge gas
Figures 5–8 show the TG/DTG curves of the flavonoids
that were used to obtain the kinetic parameters. As can be
seen from these curves, and also as expected, there is a
displacement in the decomposition temperature with the
increase in the heating rate. For analysis under both purge
gases, the kinetic analysis was carried out for the main
stage of thermal decomposition, namely in the temperature
ranges indicated in Table 1, which were used in each
heating rate to obtain the kinetic data.
20 °C
100
5 °C
1.0
10 °C
80
0.5
60
0.0
40
20
–0.5
400
0
50
100
150
200
250
300
350
Kinetic parameters
Temperature/°C
Figure 9 shows the plot of the heating rates (b) in function
of 1000/RTa with the adjustment of the variation for acti-
vation energy (Ea). It can be observed from this figure that
the corresponding activation energy (Ea) to heating rates of
Fig. 7 TG/DTG curves of 6-hydroxyflavanone with mass sample of
,
2 mg in an oxygen purge gas, heating rates of 5, 10 and 20 °C min-1
in alumina crucible
3
Oxygen purge gas
20 °C
2.0
20 °C
Nitrogen atmosphere
100
100
80
60
40
20
0
5 °C
10 °C
1.5
1.0
0.5
0.0
–0.5
5 °C
80
60
40
20
0
2
10 °C
1
0
–1
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
400
Temperature/°C
Temperature/°C
Fig. 5 TG/DTG curves of flavanone with mass sample of 2 mg in an
oxygen purge gas, heating rates of 5, 10 and 20 °C min-1, in alumina
crucible
Fig. 8 TG/DTG curves of 6-hydroxyflavanone with mass sample of
2 mg in a nitrogen purge gas and heating rates of 5, 10 and
20 °C min-1
123

Synthesis and evaluation of the thermal behavior of flavonoids
Table 1 Ea/kJ mol-1 and correlation coefficient (r) for the thermal decomposition stages
Compound
Gas purge and stage
evaluated
Temperature ranges
(DTG curves)
Ea/kJ mol-1a
ra
Flavanone
(Nitrogen)
(5 °C) 130–240 °C
(10 °C) 139–262 °C
(20 °C) 155–281 °C
(5 °C) 130–249 °C
(10 °C) 141–258 °C
(20 °C) 160–279 °C
(5 °C) 174–303 °C
(10 °C) 187–320 °C
(20 °C) 192–337 °C
(5 °C) 178–322 °C
(10 °C) 190–345 °C
(20 °C) 212–356 °C
71.69 0.02
0.99974
first decomposition stage
(Oxygen)
77.16 0.01
89.53 0.02
74.71 0.12
0.98925
0.99969
0.99723
first decomposition stage
6-Hydroxyflavanone
(Nitrogen)
1st decomposition stage
(Oxygen)
first decomposition stage
a
Average
r = 0.99992
20
15
also to reproduce the kinetic data [12–16, 18–21]. The
values of the correlation coefficient (r) and activation
energy (Ea) are listed in Table 1. The relations between the
activation energy (Ea) versus conversion degree (a) value
are shown in Fig. 10.
Ea = 88.17443 1.18552
50 %
β
The thermal decomposition of these samples can be
understood as an interaction of oxygen and nitrogen purge
gases. We can then see that for the 6-hydroxyflavanone
under oxygen, there is a decrease in activation energy,
whereas for analysis under nitrogen, there was little
increase. This fact indicates that the reaction between the
6-hydroxyflavanone and the oxygen gas favors the decrease
in activation energy, because the reaction among the
sample and oxygen favors the decrease in activation
energy. However, the nitrogen purge gas, as is an inert gas,
the chemical reaction is improbable, resulting in a reaction
between the own molecules of the sample. This fact can be
evidenced by TG curves, which demonstrate that the
thermal stability under a nitrogen purge gas is higher than
under the oxygen purge gas.
10
5
0.212
0.216
0.220
0.224
0.228
RTα/1000
Fig. 9 Data of dispersion of b versus degree conversion (a—50 %)
of the flavanone under a nitrogen purge gas for the decomposition
stage
150
6–Hydroxyflavanone – oxygen
6–Hydroxyflavanone – nitrogen
Flavanone – nitrogen
Flavanone – oxygen
100
50
0
With the results obtained for flavanone under oxygen
and nitrogen purge gas, it is possible to observe that the
activation energy has the same tendencies of the plots to
maintain the same contour and be parallel to each other,
which demonstrates that the kinetic reaction only has one
behavior. Moreover, as the flavanone does not have the
hydroxyl group in the aromatic ring, the reactivity with
oxygen and nitrogen was minimized, and therefore, the
suggestion of a high reactivity of the 6-hydroxyflavanone
with oxygen purge gas may be true. In addition, the
intermolecular interactions can be more high between
molecules of 6-hydroxyflavanone and therefore also pro-
motes greater stability of the particles, while for flavanone
0.0
0.2
0.4
0.6
0.8
1.0
α
Fig. 10 Calculated Ea/kJ mol-1 as a function of a for flavanone and
6-hydroxyflavanone for the decomposition stage
5, 10 and 20 °C was obtained in DTG curves (5–95 %). In
addition, the kinetic parameters can be conventionally
expressed with the extent of triplet TG curves, because
three or more curves contribute to the measured data and
123

L. M. B. Ferreira et al.
7. Toumi ML, Merzoug S, Boutefnouchet A, Tahraoui A, Ouali K,
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hyperglycaemic state and attenuates embryopathies in pregnant
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the hydroxyl group.
8. Bandgar BP, Gawande SS, Bodade RG, Gawande NM, Khobra-
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Conclusions
In conclusion, this work showed the comparisons of ther-
mal behavior of two flavanones by TG analysis which can
be seen by their thermal properties. The thermal decom-
position of the flavanone under a nitrogen purge gas
showed that this compound has a homogeneous degrada-
tion process which can provide a good evaluation of the
kinetic behavior. However, for the 6-hydroxyflavanone, it
appears likely that this compound contains contributions
from more than a single rate process. However, these
compounds need to be evaluated in detail to understand the
degradation mechanism to provide new information on the
products of thermal decomposition. The kinetic character-
istics for much thermal decomposition are markedly
influenced by the several variables and may involve several
degradation processes and therefore the mechanisms of the
thermal decomposition [18, 19]. In this paper, the kinetic
evaluation indicates that the hydroxyl group in the aromatic
ring can affect the kinetic behavior. Thus, the uses of
complementary observations with others types of mole-
cules may be useful to indicate a course to a better
understanding the kinetic behavior.
channels. Naunyn-Schmiedeberg’s
2004;370:290–8.
Arch
Pharmacol.
11. Szliska E, Kostrzewa-Suslow E, Bronikowska J, Jaworska D,
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J Therm Anal Calorim. 2009;97:493–6.
13. Souza JL, Kobelnik M, Ribeiro CA, Capela JMV. Kinetics study
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