Indian Journal of Chemical Technology Vol. 15, January 2008, pp. 75-78
Esterification of carboxylic acids by acid activated Kaolinite clay Dilip Konwar*a, Pradip K Gogoi*b, Pranjal Gogoia, Geetika Borahb, Ruby Baruahb, Neelakshi Hazarikab & Rituraj Borgohainb a
Organic Chemistry Division, Regional Research Laboratory, Jorhat 785006, India b Department of Chemistry, Dibrugarh University, Dibrugarh 786004, India Email:
[email protected];
[email protected]
Received 19 March 2007; revised received 3 September 2007; accepted 11 October 2007 Esterification of carboxylic acid was carried out by using acid activated kaolinite clay. This heterogeneous catalyst, activated kaolinite clay has been found to be a mild solid catalyst for the esterification of carboxylic acid in good yields. The catalyst is recoverable and can be recycled after activation. Keywords: Carboxylic acid, Alcohols, Kaolinite, Esterification
In recent years, there has been tremendous upsurge of interest in the use of different heterogeneous and environment friendly catalysts for various organic transformations1. Such catalysts can help to minimize waste production, render the synthetic process more attractive from both the environment and process economic point of view. They can be easily separated from the reaction products by simple filtration and can be quantitatively recovered in the active form. As they can be recycled, the process becomes less expensive and at the same time the contamination of the products is avoided, as in the case of other Lewis acid catalyst, by trace amount of metals. Protection-deprotection of functional groups is a common and very essential process in multistep organic synthesis2. The carboxylic groups can be protected by converting them to their corresponding esters, which is usually performed by reacting them with a suitable alcohol in the presence of different catalysts3. Some of the catalysts involved for this transformations are alkyl chloroformate and Et3N4, phenyl dichlorophosphate5, DCC and an aminopyridine6, SiO2/NaHSO47, amberlyst 158, USY zeolite9, MoO3/ZrO210, MgSO4/H2SO411, salycilic resin/FeCl312, SiO213, celite/CsF14, dowex 50WX215, beta zeolite16 etc. Most of the reported methods suffer from some
drawbacks, which include the use of transition metal10, long reaction time8, high temperature16, formation of byproduct14 etc (Table 1). Of late, the esterification of high molecular fatty acids has gained significance due to its importance in the conversion of biomass like Jathropha oil into biodiesel, which has a high flash point. By esterification, the flash point can be substantially lowered to use it as a conventional fuel in combination with diesel. Due to the strong catalytic activity as Bronsted acid, activated Kaolinite clay can be used in fine organic synthesis17. Unlike acid activated Montmorillonite, which is frequently used as acid catalyst, it is not common to find report about the use of acid activated Kaolinite clay. Wide and frequent use of Montmorillonite is due to its higher swelling and better cation exchange capacity compared to Kaolinite. Nevertheless, it is possible to obtain modified-Kaolinite after special treatments and then it is suitable for different applications similar to modified Montmorillonite uses, e.g., gas retentions18. In continuation of the search for the development of environment friendly methodologies19, here a mild and clean method for the esterification of carboxylic acid in xylene in presence of microwave radiation as well as in refluxing condition is reported in this paper. Experimental Procedure Material and Methods
Reagents were procured from Aldrich Chemical Co., SD fine chemicals, India, and were used as such without purification. The progress of the reactions were monitored by thin layer chromatography (TLC) in hexane and ethyl acetate. Column chromatography was performed using silica gel (100−200 mesh) by applying standard chromatographic techniques. Melting points were determined in a capillary tube and are uncorrected. 1H NMR and 13C NMR spectra were recorded on a Bruker 300 MHz Spectrometer using TMS as initial standard. IR spectra were recorded on FTIR-system-2000 Perkin-Elmer spectrometer. Mass spectra were recorded on ESQUIRE 3000 Mass Spectrometer. XRD analysis of random powder samples were done on Jeol X-ray diffractometer, model ZDX-11P3A, using Cu Kα radiation. All the yields reported refer to isolated product.
INDIAN J. CHEM. TECHNOL., JANUARY 2008
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Table⎯1 Various catalyst/reagents used for esterification and their merits/demerits Sl. No
Catalyst/reagent
Experimental conditions 4
Remarks
o
1
Alkyl chloroformate/Et3N
CH2Cl2, 0 C
2 3
1,2-Benzisoxazol-3-yl diphenyl phosphate5 SiO2/NaHSO47
The method has limitation with sterically hindered acids, formation of acid anhydride and carbonate byproducts. 1,2-benzisoxazol-3-ol was formed as byproduct.
MeOH, rt, 5h
React with a wide range of aliphatic carboxylic acids and alcohol, selective formation of esters, poor yield.
4 5
Amberlite 158 USY zeolite9
MeOH, rt, 15 h MeOH, 130oC, 0.5h
Required long reaction time. The catalyst can be recycled but high temperature was required in this method.
6
MoO3/ZrO210
MeOH, 85oC, 6 h
7
MgSO4/H2SO411
t-BuOH, CH2Cl2, rt, 18 h
8 9
Salycilic resin/FeCl312 SiO213
10
Celite/CsF14
C5H11OH, PhH, ∆, 123 min i-OctOH, neat, MW 175W, 5 min CH2=CHCH2Br, MeCN, ∆, 1.5 h
11
Dowex® 50WX215
Butyl formate-octane, 100oC, 100 min
12
Beta zeolite16
DMC, 150oC, 4 h
The catalyst can be recycled but transition metals like Mo and Zr were used. Formation of t-butyl esters in good yields, requires long reaction time and use of strong H2SO4. Excess amount of alcohol is required for esterification. Synthesis of 2,4-dichlorophenoxyacetic acid esters with high yields in short reaction time. Esterification of chiral α-substituted carboxylic acids proceeded in excellent yields with retention of configuration. During the chemoselective esterification of phenolic carboxylic acids, a slight amount of dialkylated product was formed as byproduct. This method was used for the esterification of α, ωdicarboxylic acids to the corresponding monoesters in high yield, 4-6% diester is also formed during the reaction. Required high temperature.
Kaolinite from Dergaon, Assam, India (long. 92°52’E, lat 26°4’N) was purified by reported method20 and characterized by XRD, IR spectral methods and elemental analysis. Preparation of Kaolinite catalyst
Kaolinite was slurried with water and then mixed with con. H2SO4 (98%) at acid/clay ratio of 0.3, followed by heating on a water bath for 16 h with constant stirring. At the end of the reaction time the activation was stopped by adding a large amount of distilled water (five times of the reaction mixture volume), the resulting slurry was repeatedly centrifuged in hot deionized water until free of sulphate ion. The acid activated Kaolinite was dried at 105°C for 2 h and subsequently ground to a particle size of 120-160 μm range. The XRD pattern of kaolinite clay shows sharp peaks at d = 7.14, 4.35, 3.57, 2.28, 1.67 and 1.54 A°. The FTIR spectrum of the clay shows peaks at 3654, 1106, 1020, 918, 794 and 754 cm-1. Chemical composition of kaolinite clay was found to be: 45.74% SiO2, 38.48% Al2O3, 0.334% CaO, 6.97% MgO and 4.08% Fe2O3.
General procedure for the synthesis of esters
To a mixture of carboxylic acid (1 mmol) and alcohol (1.5 mmol) in xylene (5 mL) was added modified Kaolinite clay (0.4 g; two times of the substrate in grams). The mixture was subjected to microwave radiation for 6 min (model Samsung, M 1 630N, 600 watts). After cooling the mixture to room temperature, the catalyst was filtered off and washed with the solvent (10 mL). The unreacted starting compounds were removed by washing the filtrate with 5% NaOH followed by water (3×15 mL). The organic layer was dried over anhydrous Na2SO4 and the solvent was removed under reduced pressure. The resulting residue was extracted with diethyl ether (15 mL) and the crude product was purified by preparative TLC over silica gel (1:15 EtOAc- Hexane as a solvent system) to afford pure esters. Alternatively, esterification can also be achieved by refluxing the above mixture in presence of the catalyst for 9 h accompanied by azeotropic removal of water using Dean-Stark apparatus. Spectral data of the synthesized esters
Ethyl benzoate (Sl No. 1): I.R (CHCl3) 1722, 1224 cm-1; 1H NMR (300 MHz, CDCl3): 1.4 (t, 3H), 4.4
NOTES
(q, 2H), 7.4-8 (m, 5H); 13C NMR (75 MHz, CDCl3): 14.4, 60.3, 128.2, 129.5, 130.3, 166; HRMS calcd. for C9H10O2 (M+) 150.18, found 150.21. Methyl cinnamate (Sl No. 2): I.R (CHCl3) 1717, 1230 cm-1; 1H NMR (300 MHz, CDCl3): 3.8 (s, 3H), 6.45 (d, 1H), 7.4-7.6 (m, 6H); 13C NMR (75 MHz, CDCl3): 51.5, 118.7, 128.1, 128.9, 130.6, 134, 14, 144.8, 167.3; HRMS calcd. for C10H10O2 (M+) 162.19, found 162.20. Diethyl malonate (Sl No. 3):I.R (CHCl3) 1731, 1195 cm-1; 1H NMR (300 MHz, CDCl3): 1.32(t, 6H), 3.34 (s, 2H), 4.2-4.4 (q, 4H); 13C NMR (75 MHz, CDCl3): 14.7, 61.5, 41.5, 166.9; HRMS calcd for C7H12O4 (M+) 160.07, found 160.26. Phenyl benzoate (Sl No. 4): I.R (CHCl3) 1725, 1261 cm-1; 1H NMR (300 MHz, CDCl3): 7.2-7.7 (m, 8H), 8.1-8.3 (m, 2H); 13C NMR (75 MHz, CDCl3): 121.1, 125.6, 128, 129, 129.8, 130, 133, 151.2, 164.7; HRMS calcd for C13H10O2 (M+) 198.22, found 198.21. Methyl (4-methoxy)phenylacetate (Sl No. 5): I.R (CHCl3) 1727, 1185 cm-1; 1H NMR (300 MHz, CDCl3): 3.5 (s, 2H), 3.7 (s, 3H), 3.8 (s, 3H), 7-7.3 (m, 4H); 13C NMR (75 MHz, CDCl3): 40.2, 51.7, 55.2, 114.1, 126.2, 130, 158.5, 172.2; HRMS calcd for C11H12O3 (M+) 180.20, found 180.11. Dodecanoic acid p-tolyl ester (Sl No. 6): I.R (CHCl3) 1738, 1185 cm-1; 1H NMR (300 MHz, CDCl3): 0.9 (t, 3H), 1.3-1.3 (m, 4H), 1.6-1.8 (m, 14H), 2.3-2.4 (m, 5H), 6.9-7.1 (m, 4H); 13C NMR (75 MHz, CDCl3): 14.1, 21.3, 22.8, 25.13, 29.6, 32.7, 34.2, 121, 129.8, 133.2, 149.6, 171.7; HRMS calcd for C19H30O2, (M+) 290.45, found 290.41.
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Heptadecanoic acid p-tolyl ester (Sl No. 7): I.R (CHCl3) 1740, 1205 cm-1; 1H NMR (300 MHz, CDCl3): 0.9 (t, 3H), 1.3-1.3 (m, 4H), 1.6-1.8 (m, 26H), 2.3-2.5 (m, 5H), 6.9-7.0 (m, 4H); 13C NMR (75 MHz, CDCl3): 14.7, 21.2, 22.8, 25.1, 29.5, 29.6, 32.7, 34.2, 121.3, 129.2, 133.2, 149.1; HRMS calcd for) C25H42O2 (M+) 374.61, found 374.267. Octadecanoic acid p-tolyl ester (Sl No. 8): I.R (CHCl3) 1741, 1195 cm-1; 1H NMR (300 MHz, CDCl3): 0.9 (t, 3H), 1.3-1.37 (m, 4H), 1.6-1.8 (m, 24H), 2.3-2.5 (m, 5H), 6.9-7.1 (m, 4H); 13C NMR (75 MHz, CDCl3): 14, 21.9, 22.8, 25.1, 29.7, 29.6, 32.4, 34.2, 121.5, 129, 133, 149.4; HRMS calcd for C24H40O2 (M+) 360.58, found 360.55 Results and Discussion R'-COOH + R"-OH
Kaolinite
R'-COOR" Xylene Microwave/ Refluxed
R' = Alkyl, Aryl R" = Aryl, Alkyl
Scheme 1 As shown in Table 2, various type of aliphatic (entry 6−8), aromatic (entry 1, 4, 5) and conjugated carboxylic acids (entry 2) are smoothly converted to their corresponding aromatic as well as aliphatic esters under the influence of Kaolinite clay in presence of microwave as well as at refluxed condition. During the reaction, other functionality,
Table 2⎯Modified Kaolinite clay catalyzed preparation of esters. Sl. No.
Carboxylic Acid
Alcohol
1
Ph - COOH
C2H5OH
2 3
Ph -CH = CH -COOH CH 2 (CO O H)2
CH3OH 2C2H5OH
Ph - COOH
C6H5OH
4 5
COOH
40 (36)
Melting Point Found/reported (oC) liquid
CH 2(COOEt)2
43 (32) 35 (28)
39/3621 liquid
P h -O-CO -P h
46 (37)
69-71/7021
44 (38)
liquid
Ph -COOEt Ph -CH = CH -COO -Me
CH3OH
CH3O
Yields a, b (%)
Product
COOMe CH3O
6
C11H23 COOH
pCH3-C6H4OH
C11H23 COOC6H4- pCH3
34 (30)
liquid
7
C17H35 COOH
pCH3-C6H4OH
C17H35 COOC6H4- pCH3
30 (26)
liquid
8
C16H33COOH
pCH3-C6H4OH
C16H33 COOC6H4- pCH3
30 (23)
liquid
a
: isolated yield; b: Figures in parenthesis indicate % yields in reflux condition.
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INDIAN J. CHEM. TECHNOL., JANUARY 2008
such as alkyl, ether and conjugated double bonds remain intact. However, yields of esterification were better in microwave radiation process than the refluxed process and aromatic acids responded better compared to aliphatic acids. Thus, a mild and efficient method has been developed for the esterification of aliphatic and aromatic carboxylic acids to their corresponding aliphatic as well as aromatic esters by using modified kaolinite clay in good yields. This method is free from hazardous and expensive reagents, contamination and the product can be separated by simple filtration. Acknowledgements Authors thank CDRI, Lucknow for providing elemental analysis, FAB-Mass spectra and 1H NMR spectra. We also thank Ms. Priyanka Sharma for isolation and purification of Kaolinite clay from crude samples. PG thanks CSIR, New Delhi for a grant of JRF fellowship.
5 6 7 8 9 10 11 12 13 14 15 16 17 18
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