J. Serb. Chem. Soc. 71 (12) 1263–1268 (2006) JSCS–3518
UDC 547–732:547.657–326 Preliminary communication
PRELIMINARY COMMUNICATION
Microwave-assisted preparation of naphthenic acid esters VERA ]IRIN-NOVTAa*#, KSENIJA KUHAJDAa#, SLAVKO KEVRE[ANb#, JULIJAN KANDRA^b#, LJUBICA GRBOVI]a# and DJURA VUJI]a aDepartment
of Chemistry, Faculty of Sciences, University of Novi Sad, Trg D. Obradovi}a 3, 21000 Novi Sad and bFaculty of Agriculture, University of Novi Sad, Trg D. Obradovi}a 8, 21000 Novi Sad, Serbia (e-mail:
[email protected]) (Received 31 October 2005, revised 21 March 2006) Abstract: The synthesis of esters of natural petroleum acids of the naphthenic type assisted with microwave irradiation under the conditions of acid catalysis was carried out with various alcohols: methanol, ethanol, n-butanol and tert-butyl alcohol. Microwave dielectric heating of the reaction mixture in an unmodified microwave oven with activation of the naphthenic acids with sulfuric and p-toluenesulfonic acid afforded the esters of the naphthenic acids. Depending on the catalyst and the steric and nucleophilic properties of the alcohols, the yield of naphthenic esters ranged from 31.25 % to 88.90 %. As a consequence of microwave dielectric heating, the esterification time was reduced from 6–10 h to 5 min. Keywords: naphtenic acids, methyl-, ethyl-, n-butyl- and tert-butyl esters, microwave dielectric heating. INTRODUCTION
Interest in using the microwave irradiation technique in organic synthesis has been growing recently, which is evident from the constantly increasing number of published reports on the employment of this technique.1–5 In a large number of studies, use was made of conventional domestic microwave ovens6,7 but nowadays more studies are made in modified microwave ovens with a multi-mode system with reflux and temperature measuring facilities.8 The reaction of organic acids with alcohols is a very important reaction but, because of its reversibility, the yields are often very poor. Hence, catalyzed esterification reactions have permanently been an active area of research. Microwave activation of acid-catalyzed esterification has come in the focus of researchers as it appears that a number of known reactions give good yields in a reaction time shortened by several hundreds of times.6,8–10 *
Corresponding author.
#
Serbian Chemical Society active member.
doi: 10.2298/JSC0612263C
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The aim of this investigation was to study the acid-catalyzed esterification of natural petroleum acids with a number of different alcohols under the additional catalytic action of microwave radiation. Esters of natural petroleum acids are important because of their structural analysis and further derivatization, as the obtained esters represent very convenient intermediates.11 In addition, these esters themselves have very diverse applications as synthetic lubricants,12 dispersive agents,13 plastificators,14,15 degreasing agents in the process of leather processing,16 etc. Acid-catalyzed esterification of natural petroleum acids with absolute methanol or ethanol using conventional heating lasts about six hours, giving a yield of about 60 %. In view of the structural diversity of the mixture of natural petroleum acids, such an outcome of the reaction may result in the loss of some acid structures in the mixture. Hence, it is important to seek for more efficient esterification methods. In this study, esterification was performed using natural petroleum acids of the naphthenic type, isolated by alkaline extraction from the Vojvodina naphthenic crude oil "Velebit". Structurally, they are cycloalkanecarboxylic acids, mainly with mono- and bi-cyclic structure.17 EXPERIMENTAL General methods The naphthenic acids were isolated by alkaline extraction18,19 from the commercial fraction of atmospheric gas oil of the Vojvodina crude oil "Velebit". The acids were purified by triple alkaline extraction. The alcohols (methanol, ethanol, n-butanol and tert-butyl alcohol) were dried over calcium oxide and distilled. The acids and esters were dried over anhydrous Na2SO4. The combination of sulfuric acid or p-toluenesulfonic acid (p-TsOH) with microwave (MW) radiation (360 W) was used as the catalyst. The same alcohols were also used to perform acid-catalyzed esterification using sulfuric acid and conventional heating. The employed microwave oven was a 600-W Siemens domestic appliance with the mono-mode technique. The esterifications were carried out on a small scale of 100 mg of acid in glass vessels with Teflon stoppers. The yield was determined on an HPLC Agilent 1100 Instrument; column Spherisorb ODS 2; 5 mm, 250 mm, id 5 mm; mobile phase: 50.0 % water and 50.0 % acetonitrile, detection at 210 nm. IR spectra were taken on a Specord 75 IR spectrophotometer and the band positions are given in cm-1. 13C-NMR spectra were recorded in CDCl as a solvent on a Bruker AC 250 E instrument and 3 the chemical shifts are expressed in ppm downfield from tetramethylsilane. Quantitative 13C-NMR spectra with gated decoupling of the protons without NOE were measured with a pulse program for inverse gated decoupling, whereby the delay time d1 was 20 s and the flip time of the 90° pulse was 6 ms. The observed frequency range was fixed at 200 ppm, operating frequency at 62.9 MHz, and irradiation frequency at 4000 Hz. The number of scans was 12800 and time-domain accumulation was 32 K. Quantitative analysis was based on the intensity of each signal region (CH+CH2, CH3, C=O, C(CH3)3) by the integrator when CCl4 was added directly to the solvent (about 10 %). The regions of characteristic C-signals (expressed in ppm values) and their percentual participation in comparison to the total number of C-atoms were extracted from the quantitative 13C-NMR spectra. TLC experiments were carried out on a DC Alufolien Kieselgel 60 F254 (E. Merck), using a benzene:ethyl acetate 18:0.25 mixture as developer and detection with a 50 % aqueous solution of H2SO4.
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(A) General procedure of the MW-assisted esterification Naphthenic acids (0.10 g, 0.4 mmol), dry alcohol (3 ml) and sulfuric acid or p-TsOH (0.06 mmol) were mixed in a glass vessel (10 ml) with a teflon stopper. The reaction was performed in the microwave oven at an engaged power of 360 W and reaction time of 5 min. The reaction was followed by TLC (benzene:ethyl acetate 18:0.25) at one-minute time intervals. The alcohol was removed in a rotating evaporator, the obtained product dissolved in ether (10 ml) and extracted with aqueous solution of sodium hydroxide (pH 9). The ether extract was washed with water to neutral and dried over anhydrous Na2SO4. Evaporation yielded crude esters of naphthenic acids. (B) General esterification procedure with conventional heating The same amounts of reactants as in Procedure (A) were mixed in a glass vessel and under constant stirring heated at reflux under anhydrous conditions (calcium chloride pipe) for 6–10 h. The rest of the procedure was identical to that described above. RESULTS AND DISCUSSION
The results of the acid-catalyzed esterification of naphthenic acids with different alcohols, obtained using the co-catalytic effect of microwave irradiation and conventinal heating, are presented in Table I. In view of the fact that the mixture of natural petroleum acids isolated from the Vojvodina crude oil "Velebit"18 which was used for esterification consisted of diverse structures, the esterification performed by the conventional acid-catalyzed procedure proceded slowly and gave low yields. The esterification carried out at reflux under magnetic stirring gave the best yield with ethanol of 66.0 % after 6 h, with methanol 62.15 % after 6 h, with n-butanol 58.50 % after 9 h, and with tert-butyl alcohol only 34.10 % after 10 h. The results presented in the Table I, obtained on a small scale of 100 mg of acids, indicate that the microwave dielectric heating acted as an effective promoter of the reaction. In the reaction with methanol, already in the first minute, 60.39 % and 50.31 % of napthenic acids had reacted in the presence of sulfuric acid and p-TsOH, respectively, which at the beginning represents an acceleration of 360 times compared with the conventional mode of heating. After 5 min, the esterification with methanol in the presence of sulfuric acid gave a yield of 86.14 % and in the same time, in the presence of p-TsOH the yield being 68.10 %. In this work, although a conventional microwave oven with a mono-mode system was used, with effective stirring of the reaction mixture a marked acceleration was achieved in the reactions with all four alcohols. A high yield in the reaction of tert-butyl alcohol with naphthenic acids in the presence of p-TsOH as catalyst (62.76 %) was obtained in 5 min. It is known that tert-butyl group sterically hinders nucleophilic attack of the alcohol, while H2SO4 under these conditions favors elimination, which may be the cause of the low yield of tert-butyl napthenate in the presence of H2SO4 as the acidic catalyst (31.25 % under microwave irradiation and 34.10 % with H2SO4 and conventional heating). In the IR spectrum of tert-butyl naphthenate, in addition to the band of an ester structure, the strong band at 1650 cm–1 is proof that elimination had occurred.
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TABLE I. Results of the esterification of naphthenic acids in the presence of H2SO4 and p-TsOH under MW irradiation and conventional heating Catalyst H2SO4 + MW* HPLC React. time yield** 60.39 % 1 min 86.14 % 5 min
Catalyst p-TsOH + MW HPLC React. yield ** time 50.31 % 1 min 68.10 % 5 min
Ethanol
88.08 %
5 min
80.61 %
5 min
n-Butanol
85.90 %
5 min
50.40 %
5 min
t-Butyl alcohol 31.25 %
5 min
62.76 %
5 min
Ester
Alcohol
1
Methanol
2 3 4 *MW
Catalyst Relative accelH2SO4 eration Conv. React. heat. time 360 72 62.15 % 6 h 72 66.00 % 6 h 108 58.50 % 9 h 120 34.10 % 10 h
radiation of 360 W; **Yield was determined by HPLC of the reaction mixture
As can be seen from the Table I, p-TsOH as the acidic catalyst in combination with MW irradiation gave a good conversion only in the esterification with ethanol (80.61 %, 5 min), whereas in the esterification with tert-butyl alcohol the yield was 62.76 % because p-TsOH does not catalyse the elimination reaction. Literature data indicate that the use of a more powerful MW source would accelerate the reaction and increase the degree of conversion, but this could only be attained under reflux, i.e., in an MW oven adapted for organic synthesis. The structures of the synthesized esters were confirmed by their IR and quantitative 13C-NMR spectra: IR (film): 2980–2930 (nas and nsCH2), 1740 (nC=O), 1430–1450 (dCH2, dasCH3), 1380 (nasC–O–C), 1260 (nC=O) cm–1; (1) Methyl naphthenate: 13C-NMR (CDCl3): d 14.0–14.5 (4.4 %, CH3), 19–43 (86.5 %, CH+CH2), 50.8–51.7 (5.7 %, OCH3), 172.8–178 (4.4 %, C=O); (2) Ethyl naphthenate: 13C-NMR (CDCl3): d 13.7–14.8 (10.7 %, CH3), 18.1–49 (8.9 %, CH+CH2), 59.3–60.7 (5.9 %, OCH2), 172.5–176.9 (4.5 %, C=O); (3) n-Butyl napthenate: 13C-NMR (CDCl3): d 13.0–14.5 (9.7 %, CH3), 18–48 (82.25 %, CH+CH2), 60.3–63.9 (4.76 %, OCH2), 172.8–176.3 (3.28 %, C=O); (4) tert-Butyl napthenate: 13C-NMR (CDCl3): d 14.0–14.38 (4.64 %, CH3), 18–46 (90.9 %, CH+CH2 + t-Bu), 172.5–174.1 (1.95 %, C = O). CONCLUSIONS
The above results represent a contribution to the new approach to accelerate esterification reactions. Esterification of naphthenic acids by conventional method lasts 6–10 h whereas in a microwave oven of 360 W, the reaction proceeds without conventional heating to high conversion already in the first minute (with methanol: 60.39% in the presence of H2SO4 and 50.31 % in the presence of p-TsOH), and to
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very high yields after 5 min (86.14 % and 68.10 % in the presence of H2SO4 and p-TsOH, respectively). Under MW irradiation for 5 min, the best yield of 88.08 % was obtained in the reaction of naphthenic acids in the presence of H2SO4. On the other hand, tert-butyl naphthenate in the presence of this acid as catalyst was formed in low yield because of dehydration, but in the presence of p-TsOH the yield after 5 min was 62.76 %. Acknowledgement: This research was supported by the Ministry of Science of Environmental Protection of the Republic of Serbia.
IZVOD
DOBIJAWE ESTARA NAFTENSKIH KISELINA POMO]U MIKROTALASNOG ZRA^EWA VERA ]IRIN-NOVTAa, KSENIJA KUHAJDAa, SLAVKO KEVRE[ANb, JULIJAN KANDRA^b, QUBICA GRBOVI]a i \URA VUJI]a aDepartman za hemiju, Prirodno-matemati~ki fakultet, Univerzitet u Novom Sadu, Trg D. Obradovi}a 3, 21000 Novi Sad i bPoqoprivredni fakultet, Univerzitet u Novom Sadu, Trg D. Obradovi}a 8, 21000 Novi Sad
Sinteza estara prirodnih naftnih kiselina naftenskog tipa uz pomo} mikrotalasnog zra~ewa u uslovima kisele katalize izvedena je sa razli~itim alkoholima: metanolom, etanolom, n-butanolom i tert-butil-alkoholom. Mikrotalasnim dielektri~nim zagrevawem reakcione sme{e u nemodifikovanoj mikrotalasnoj pe}nici i uz aktivaciju naftnih kiselina sumpornom i p-toluensulfonskom kiselinom sintetizovani su estri naftnih kiselina. U zavisnosti od katalizatora, zatim sternih i nukleofilnih osobina alkohola, prinos naftnih estara je varirao od 31,25 % do 88,90 %. Reakciono vreme esterifikacije smaweno je sa 6–10 sati na 5 minuta pri iskqu~ivo mikrotalasnom dielektri~nom zagrevawu. (Primqeno 31. oktobra 2005, revidirano 21. marta 2006)
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