Thermal study of chalcones Thermal decomposition of chalcone and its hydroxylated derivatives Marcelo Kobelnik1 • Leonardo Miziara Barboza Ferreira2 • Luis Octavio Regasini3 • Luiz Antonio Dutra2 • Vanderlan da Silva Bolzani2 • Clóvis Augusto Ribeiro2 Received: 22 August 2017 / Accepted: 30 December 2017 / Published online: 23 January 2018 � Akadémiai Kiadó, Budapest, Hungary 2018 Abstract Chalcones a-b-unsaturated ketones are found in large plant species. Synthesis of chalcones and its three analogues hydroxy group at 20, 30 and 40 positions (2–4) was carried out. The studies of thermal behavior were made by thermogravimetry (TG) and differential scanning calorimetry, both under oxygen and nitrogen purge gases. In addition, the kinetic evaluation was carried out under heating rates of 5, 10 and 20 �C min-1 with sample mass of 2 mg in open crucibles. The kinetic results obtained by TG analysis showed that the thermal behavior under oxygen shows that the functional hydroxy group substitution affects the thermal behavior of each molecule, with a gradual increase in the thermal decomposition. The activation energy (Ea/kJ mol-1) showed under a nitrogen purge gas that the hydroxy group at 30 position (30-hy-chalcone compound) has a different kinetic behavior, while the chalcone under oxygen showed a low activation energy when compared with the other hydroxy groups. Keywords Chalcones � Thermal analysis � Kinetic behavior Introduction Chalcones are a-b-unsaturated ketones found in large plant species besides being a precursor for the biosynthetic route of flavonoids. Natural and synthetic chalcones have attracted attention in recent years due to the diversity of biological activities exhibited as anti-inflammatory [1], anticancer [2], antifungal [3] and leishmanicidal [4]. The synthetic preparation of chalcones involves Claisen–Sch- midt condensation reaction between ketone and aldehyde under acid or basic catalysis. Several other catalysis methods such as basic alumina, zinc chloride and Lewis acid such as BF3 and AlCl3 had been used [5]. The a-b-unsaturated bond conjugated to carbonyl acts as an electrophile and reacts with nucleophile allowing a cyclization via Michael’s addition [6]. Structures contain- ing hydroxy group in an ortho position might cyclize into flavanones and aurones. Thus, research studies the report of flavanones and aurones synthesis from chalcones and their biological activities [7, 8]. Due to their versatile structure, several synthetic routes were reported using chalcones as a precursor for getting heterocycles such as pyrazoline, oxiran, pyran, oxopyrimidine, isoxazoline [9], derivatives of pyridine [10], derivatives of benzheteroazepine [11]. Besides this, the chalcones or 1,3-diphenyl-2-propen-1- ones are considered as precursors of flavonoids and iso- flavonoids, which in turn affect the taste of foods, and they are also responsible for the color of flowers, fruits and leaves [12–15]. Furthermore, others interest in compounds 1–4 are correlated to their broad medicinal relevance, which includes antitumor (1 and 2) [16–18], antibacterial (3) [19], anti-inflammatory (1) [20] and antiprotozoal (1 and 4) [21, 22]. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10973-017-6956-2) contains supplementary material, which is available to authorized users. & Marcelo Kobelnik mkobelnik@gmail.com 1 Centro Universitário do Norte Paulista, UNORP, São José do Rio Preto, SP, Brazil 2 Instituto de Quı́mica, Unesp – Univ. Estadual Paulista, C.P. 355, Araraquara, SP 14800-900, Brazil 3 Departamento de Quı́mica e Ciências Ambientais, Instituto de Biociências, Letras e Ciências Exatas, Unesp – Univ. Estadual Paulista, São José do Rio Preto, SP, Brazil 123 Journal of Thermal Analysis and Calorimetry (2018) 132:425–431 https://doi.org/10.1007/s10973-017-6956-2(0123456789().,-volV)(0123456789().,-volV) http://orcid.org/0000-0001-6879-3172 https://doi.org/10.1007/s10973-017-6956-2 http://crossmark.crossref.org/dialog/?doi=10.1007/s10973-017-6956-2&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1007/s10973-017-6956-2&domain=pdf https://doi.org/10.1007/s10973-017-6956-2 Another application of these compounds relates to its in food technologies, which owing to the demand and inter- esting for new food antioxidants, it becomes very inter- esting to explore others molecules for the design, synthesis and characterization of new antioxidant agents in order to collaborate in food sciences [23–28]. Therefore, the aim of the present study was to describe thermal characterization of the chalcone (1) and it is three analogues with hydroxy group at 20, 30 and 40 position (2–4), as seen in Fig. 1. The solid compounds were characterized by thermogravimetry (TG) and differential scanning calorimetry (DSC) at three heating rates and also under nitrogen and oxygen purge gases. In addition, TG curves were also used to obtain the kinetic information about the thermal decomposition stage. Experimental Synthesis of chalcones and their derivatives (Fig. 1: 1–4) Chalcone (1) and its hydroxylated analogues (2–4) (Fig. 1) were synthesized by Claisen–Schmidt aldol condensation using protocols reported in previous studies from our group [29–31]. In a 30-mL vial, the appropriated acetophenone (2.5 mmol) and lithium hydroxide monohydrate (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 hexane/ethyl acetate (3:1) as the mobile phase. The reaction was quenched after 24 h by pouring into 100 mL of ice-cold water. In the synthesis of these com- pounds as seen in Fig. 1, from 1 to 4, 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. On the other hand, the synthesis of 2 and 3 led to precipitation after quenching with cold water, and it was then filtered and crystallized with hot acetone: ethanol (1:1). For compounds 1–4, the crude products were purified by flash chromatography using hexanes: ethyl acetate as the solvent system in increasing order of polarity. Characterization by nuclear magnetic resonance (NMR) Chalcone (1) and its hydroxylated derivatives (2–4) were identified by 1H and 13C NMR spectra data obtained from 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; br s = broad singlet, d = doublet; t = triplet; m = multiplet; dd = doublet of doublets. These data were extensively detailed in supporting information. Characterization by thermal analysis TG/DTG curves were obtained from a SDT 2960 model (TA Instruments). The evaluation of these compounds by TG analysis were carried out with sample masses around 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. In addition, the kinetic study was performed using the three heating rates, which have been extensively used in previous studies [32–36]. More- over, the use of this standard in kinetic analysis is very useful for future comparisons, besides allowing the repro- ducing of results obtained by us and also by other research groups. The kinetic methodology used in this work was proposed by Capela and Ribeiro, which is an isoconver- sional method and is based on approximation to the integral temperature on the convergent of a Jacobi fraction [37–40]. The DSC curves were carried out using a calorimeter SDT 2910 model, from TA Instruments, with heating rates of 20 �C min-1 from 30 to 400 �C under nitrogen and oxygen purge gases, with a flow of 50 mL min-1. Results and discussion The TG/DTG evaluations of the chalcone and its hydrox- ylated derivatives under nitrogen and oxygen purge gases with mass of 2 mg and heating rate of 20 �C min-1 are shown in Fig. 2a–d, respectively. The chalcone (Fig. 2a) shows that the initial thermal decomposition begins to take place after 105 �C going to 250 �C for both purge gases. Also it is possible to see that in oxygen, during the mass loss, there was a decrease in the O 1 2 3 4 O O OOH HO HO Fig. 1 Structures of chalcone (1) and its hydroxylated derivatives (2–4) 426 M. Kobelnik et al. 123 temperatures, which is minor than that in nitrogen. The opposite effect under nitrogen gas can also be seen with 30- hy-chalcone (Fig. 2c), where the reaction occurs at an interval of thermal decomposition from 150 to 326 �C. These observed effects can be suggested as a relation of heat transfer and thermal conductivity of the gases with the samples. The analyses with 20-hy-chalcone (Fig. 2b) show that there is a similarity of thermal behavior of this sample under both purge gases. The DTG curve showed that the thermal decomposition stage occurs between 148 and 271 �C, without carbonaceous residues at the end of the reaction. For the analysis of 40-hy-chalcone (Fig. 2d), it is possible to see that the thermal decomposition is also similar, but at the end of the reaction there was a difference in behavior. Other curves were made to verify this behavior, but the results were the same, being, therefore, as suggestion a possible reaction between the sample and the oxygen. For both analyses, there was the presence of car- bonaceous residues, and the mass losses were 83.75 and 92.72% under oxygen and nitrogen purge gases. For all samples seen in Fig. 2, it is possible to observe that there is an inflection in the TG/DTG curves between the initial temperature and before the thermal decomposi- tion. This effect was attributed to the melting of the sam- ples, which can be seen in the DSC curves in Figs. 3 and 4, which are associated with the same temperature as the TG/ DTG curves. The DSC curves were carried out under nitrogen and oxygen purge gases, respectively, and the values of the melting points are indicated in the curves. The difference observed between the melting point analyses made with oxygen and nitrogen is attributed to the arrangement of samples in the crucible and also to the fact that samples analyzed are not a single crystal but are small different sized crystals. For a better understanding and also for a comparison between these hydroxylated derivatives, the TG/DTG curves in oxygen and nitrogen purge gases were overlap- ping, as it is possible; see in Figs. 5 and 6, respectively. 110 90 70 50 30 10 3 2 1 0 2 1 0 1.5 1.0 0.5 0.0 2.0 1.5 1.0 0.5 0.0 –0.5 90 70 50 30 10 90 70 50 50 30 10 90 70 50 100 150 200 250 300 350 400 30 10 0 –10 Temperature/°C M as s/ % D er iv . m as s/ % /° C N 2 N 2 N 2 N 2 O 2 O 2 O 2 O 2 Δ (a) (b) (c) (d) Fig. 2 TG/DTG curves of chalcone (a), 20-hydroxychalcone (b), 30- hydroxychalcone (c) and 40-hydroxychalcone (d), with mass sample around 2 mg in a-alumina crucible, under oxygen and nitrogen purge gases and heating rate of 20 �C min-1 50 10 20 –10 0 100 150 200 250 300 350 4000 Temperature/°C chalcone 91.31 °C 58.55 °C 123.31 °C 175.31 °C 2-hy-chalcone 3-hy-chalcone 4-hy-chalcone H ea t f lo w /m W E nd o E xo Fig. 3 DSC curves of chalcone and their hydroxylated derivatives compounds, with mass sample around 2 mg in aluminum crucible, under nitrogen purge gas and heating rate of 20 �C min-1 Thermal study of chalcones 427 123 The thermal behavior in oxygen shows that the func- tional hydroxy group substitution present at each position in the chalcone affects the thermal conductivity of the molecule, because there was a gradual increase in the thermal decomposition temperature. In addition, the ther- mal decomposition at 30 and 40-hy-chalcone shows a dif- ference in carbonaceous residues generated at the end of the reaction. However, this fact is not seen for the chalcone and 20-hy-chalcone. For nitrogen analysis, the thermal behavior is similar to that seen in oxygen purge gas, except that there were no carbonaceous residues generated at the end of the thermal decomposition. Kinetic evaluation Figure 7 shows the overlapping of the three TG curves with heating rates of 5, 10 and 20 �C min-1 of chalcone as a set of curves to obtain the kinetic data. The other compounds were not placed because they follow the same tendency of displacement of the heating rates. Moreover, in others works, we have used a pattern of at least three TG curves for kinetic behavior analysis, as suggested by the ICTAC committee [40–45]. In a similar way, for all curves (even for those not shown), the result shows that there were no changes to the thermal behavior, and therefore the profiles of curves remained the same, that is, without apparent overlapping of thermal decomposition reactions. Table 1 shows the average of the values of activation energy (with a coefficient variation), the linear correlation obtained for compounds and the temperature intervals in DTG curves that were used for kinetic evaluation. The low activation energy values obtained for these compounds indicate the dependence on temperature, which implies that low temperatures are required for thermal decomposition. 100 200 300 400 500 6000 Temperature/°C –20 –40 0 H ea t f lo w /m W E nd o E xo chalcone 2-hy-chalcone 3-hy-chalcone 4-hy-chalcone 58.07 °C 90.84 °C 123.13 °C 175.83 °C Fig. 4 DSC curves of chalcone and their hydroxylated derivatives compounds, with mass sample around 2 mg in aluminum crucible, under oxygen purge gas and heating rate of 20 �C min-1 50 100 150 200 250 300 350 400 450 5000 Temperature/°C 50 90 70 30 10 –10 M as s/ % Δ 3 2 1 –1 0 D er iv . m as s/ % /° C chalcone 2-hy-chalcone 3-hy-chalcone 4-hy-chalcone 110 Fig. 5 TG/DTG curves of chalcone and their hydroxylated deriva- tives compounds, with mass sample of 2 mg in a-alumina crucible, under oxygen purge gas and a heating rate of 20 �C min-1 50 90 70 30 10 –10 M as s/ % Δ 110 50 100 150 200 250 300 350 400 450 5000 Temperature/°C 3 2 1 –1 0 D er iv . m as s/ % /° C chalcone 2-hy-chalcone 3-hy-chalcone 4-hy-chalcone Fig. 6 TG/DTG curves chalcone and their hydroxylated derivatives compounds, with mass sample of 2 mg in a-alumina crucible, under nitrogen purge gas and a heating rate of 20 �C min-1 50 100 150 200 250 3000 Temperature/°C 3 4 2 1 –1 0 D er iv . m as s/ % /° C 40 80 60 20 0 M as s/ % Δ 100 10 °C 5 °C 20 °C Fig. 7 TG/DTG curves of chalcone with sample mass around 2 mg in a-alumina crucible, under nitrogen purge gas and a heating rates of 5, 10 and 20 �C min-1 428 M. Kobelnik et al. 123 The relation between the activation energy versus con- version degree under nitrogen and oxygen purge gases is shown in Figs. 8 and 9, respectively. As can be seen in the nitrogen purge gas, the kinetic behaviors are linear, with a more pronounced difference for 30-hydroxychalcone, while for analysis under oxygen purge gas, the major difference occurs for chalcone. It is important to note that the kinetic behavior in oxygen follows a pattern of minor activation energy for chalcone to major activation energy for 40-hy- chalcone. The minor activation energy of chalcone under oxygen is probably due to the weak intermolecular interactions, while for the other molecules this interaction is similar, hence the motive of the closest activation energies. However, under nitrogen, the 30-hy-chalcone has major value of activation energy, which stands out from the others. This behavior is a probable indication that this molecule has an intermolecular interaction greater than the others. In previous papers about the synthesis and thermal characterization of flavanone and 60-hydroxyflavanone flavanones, it was possible to see that there was an alter- ation in the activation energy under nitrogen purge gas with Table 1 Ea (kJ mol-1), correlation coefficient (r) and temperature intervals of DTG curves for thermal decomposition Compounds Purge gases Temperature ranges (DTG curves) *Ea/kJ mol-1 *r Chalcone Nitrogen (5 �C) 121–225 �C (10 �C) 136–236 �C (20 �C) 148–257 �C 87.76 ± 0.04 0.99646 Oxygen (5 �C) 105–219 �C (10 �C) 114–230 �C (20 �C) 121–255 �C 55.72 ± 0.01 0.96343 20-Hydroxychalcone Nitrogen (5 �C) 137–243 �C (10 �C) 152–256 �C (20 �C) 162–272 �C 85.97 ± 0.02 0.99661 Oxygen (5 �C) 122–237 �C (10 �C) 150–262 �C (20 �C) 168–272 �C 81.56 ± 0.04 0.99646 30-Hydroxychalcone Nitrogen (5 �C) 181–290 �C (10 �C) 200–314 �C (20 �C) 211–330 �C 106.45 ± 0.02 0.99964 Oxygen (5 �C) 168–290 �C (10 �C) 187–309 �C (20 �C) 201–329 �C 85.76 ± 0.01 0.99694 40-Hydroxychalcone Nitrogen (5 �C) 186–299 �C (10 �C) 200–319 �C (20 �C) 221–337 �C 85.77 ± 0.01 0.99895 Oxygen (5 �C) 194–302 �C (10 �C) 203–315 �C (20 �C) 218–334 �C 91.20 ± 0.04 0.99547 *Average 60 0.0 0.2 0.4 0.6 0.8 1.0 120 100 80 140 chalcone - nitrogen 2-hy-chalcone - nitrogen 3-hy-chalcone - nitrogen 4-hy-chalcone - nitrogen α E a/ kJ m ol –1 Fig. 8 Calculated Ea/kJ mol-1 as a function of a for the transition phase stage under nitrogen purge gas 60 120 100 80 140 E a/ kJ m ol –1 0.0 0.2 0.4 0.6 0.8 1.0 α chalcone - oxygen 2-hy-chalcone - oxygen 3-hy-chalcone - oxygen 4-hy-chalcone - oxygen Fig. 9 Calculated Ea/kJ mol-1 as a function of a for the transition phase stage under oxygen purge gas Thermal study of chalcones 429 123 hydroxy group in flavanone, with the increase in the acti- vation energy to a higher value [26]. 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Wang H, Zhang J, Wang G, Xu R, Zhang P, Liu S, Song T. Characteristics and kinetic analysis of co-combustion of brown coal and anthracite. J Therm Anal Calorim. 2016;126:447–54. 45. Kobelnik M, Ribeiro CA, Dias DS, Crespi MS. Estudo do com- portamento cinético da decomposição térmica do composto de 2-metoxibenzalpiruvato de manganês no estado sólido. Rev Unorp. 2012;3:28. Thermal study of chalcones 431 123 https://doi.org/10.1007/s10973-017-6468-0 https://doi.org/10.1007/s10973-017-6468-0 https://doi.org/10.1007/s10973-017-6763-9 Thermal study of chalcones Thermal decomposition of chalcone and its hydroxylated derivatives Abstract Introduction Experimental Synthesis of chalcones and their derivatives (Fig. 1: 1--4) Characterization by nuclear magnetic resonance (NMR) Characterization by thermal analysis Results and discussion Kinetic evaluation Conclusions References