Effects of Pressure on the Esterification of Long-chain Fatty Acids

Transcription

1 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) 29 3 ORIGINAL Effects of Pressure on the Esterification of Long-chain Fatty Acids Yoichi TAGUCHI*1, Angelo S. BALETA*2, and Akihiro OISHI*1 National Institute of Materials and Chemical Research (1-1, Higashi, Tsukuba-shi, Ibaraki-ken, 305 ) Industrial Technology Development Institute (Santos Ave, Bicutan, Taguig, Metro Manila 1604, Philippines) Abstract : The esterification of long-chain fatty acids such as lauric, stearic, oleic, elaidic, and linoleic acids in excess alcohol was accelerated under high pressure. Activation volume in the esterification of lauric acid with methyl, ethyl, i-propyl, n-butyl, i-butyl, and s-butyl alcohols was from -13 ` -21 cm3/mol. The esterification of unsaturated fatty oleic, elaidic, and linoleic acids with methyl alcohol was accelerated by greater compression and activation volume ranged from -27 `-33 cm3/mol. With dichloromethane as solvent in the esterification of long-chain fatty acids with methyl alcohol, the reactions were greatly accelerated at high pressure with apparent activation volume from 0.4 MPa to 200 MPa and as much as -80 cm3/mol. The effects of pressure on reactions in dichloromethane were virtually the same in all cases except for the reaction of linoleic acid. Key words pressure effect, esterification, long-chain fatty acid, activation volume, dichloromethane 1 Introduction Many organic reactions are known to be influenced with pressure1), and some of them are dramatically accelerated under high pressure. We have also reported the synthesis of heterocyclic compounds, such as 1,3-diazetizine-2,4-diones2),,6-lactams3), and quinazoline derivatives4), are strongly promoted under high pressure. These results show the possibility that under high pressure some organic reactions proceed more easily and more selectively than at ordinary pressure. On the other hand, high pressure treatment on food is receiving increasing attention5). And also in the field of oils and fats chemistry, pressure effects of lipid oxidation6) and micellization7) have been reported. Oils and fats from vegetables or animals are important natural resources because they are renewable and served stably, and long-chain fatty acids produced from oils and fats, such as lauric, oleic, and stearic acids, and their esters are useful compounds on various industries8). As the esterification of long-chain fatty acids needs purification and high energy7), another method of esterification which is highly selective and energy-saving is desired. Therefore, esterification by enzyme has been researched extensively9), however, the application of high pressure has also possibility of highly selective and energy-saving esterification. In this report, esterification of long-chain fatty acids such as lauric, stearic, oleic, elaidic, and linoleic acids with methanol was studied under high pressure. The effect of pressure on the esterification of lauric acid with various alcohols was also studied. Corresponding author : Yoichi TAGUCHI 4 5

2 29 4 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) 2 Experiment 2.1 Materials Lauric acid, stearic acid, oleic acid, elaidic acid, methyl laurate, methyl stearate, methyl oleate, methyl elaidate, ethyl laurate were reagent grade chemicals from Tokyo Kasei Kogyo Ltd., i-propyl laurate, n-butyl laurate, i-butyl laurate, and s-butyl laurate for standards were prepared from lauric acid and excess corresponding alcohols in the presence of sulfuric acid. 2.2 Apparatus The apparatus used for the high-pressure reaction was the same with that described previously10). 2.3 Measurement GLC was carried out using a Shimazu GC-14 chromatograph with a capillary column (Frontier Lab Ultra ALLOY-1). Products were analyzed at a temperature increasing from 100 Ž to 250 Ž at a rate of 5 Ž/min. 2.4 Esterification of long-chain fatty acid with excess alcohol The typical procedure was as follows. A homogeneous mixture of lauric acid (0.5 mmol) and methanol (25 mmol) in a sealed Teflon tube was compressed to 800 MPa, heated at 100 C, and maintained for 20 h in a high-pressure apparatus. The resulting mixture was poured to 10 ml of chloroform, and analyzed by GLC. The yield was determined to be 98% by using hexadecane as a standard. 3 Results and Discussion Table 1 shows the esterification of lauric acid with excess alcohols in an autoclave. These results demonstrated that higher temperature or longer time are needed for complete esterification. Table 2 shows the esterification of lauric acid under 800 MPa. When the esterification of lauric acid with methyl alcohol at 100 Ž, the reaction proceeded completely to give methyl laurate in 98% yield (Run 4). The similar reaction at ordinary pressure gave only 33% of methyl laurate (Table 1 : Run 1). The esterification of lauric acid with ethyl, n-butyl, i - butyl, and s-butyl alcohols were accelerated under 800 MPa. These results show that the esterification of lauric acid with excess alcohols is promoted by compression. The esterification of lauric acid with t-butyl alcohol did not proceed even under 800 MPa at 100 Ž. Figure 1 and 2 show the effect of pressure on the esterification of lauric acid with various Table 1 Esterification of Lauric Acid in a Sealed Tube). 4 6

3 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) 29 5 Table 2 Esterification of Lauric Acid under 800 MPaa). Fig. 2 Pressure Effect on the Esterification of Fig. 1 Pressure Effect on the Esterification of Lauric Acid (1). Lauric acid 0.5 mmol, alcohol 25 mmol, 100 Ž, 20 h Laurie Acid. Lauric acid 0.5 mmol, alcohol 25 mmol, 100 Ž, 20 h alcohols at 100 Ž for 20 h. All reactions were accelerated by compression. Similar curves were observed between n-butyl and i-butyl alcohol, and between i-propyl and s-butyl alcohol. Secondary alcohols such as i-propyl and s-butyl alcohol had less reactivity than primary alcohol such as n-butyl and i-butyl alcohol under high pressure. This tendency is agreed with the reactivity of alcohol at ordinary pressure. These esterifications are regarded as being a first order reaction with respect to lauric acid, because a large excess of alcohols were used. Therefore, the apparent reaction velocity constant (k) is given by Eq. 1, and calculated from the data given in Fig. 1 and 2. The activation volume (PIT ) which shows the extent of pressure effect can be estimated by Eq. 2. Table 3 shows the apparent activation volumes on the esterification of lauric acid with various alcohols in the range from 200 MPa to 400 MPa. Obtained values suggest these reactions have moderate pressure effect. ( 1 ) 4 7

4 29 6 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) Table 3 Activation Volume on the Esterification of Lauric Acida). a : Concentration of lauric acid a() : Initial concentration of lauric acid x : Yield of laurate ( 2 ) Figure 3 shows the pressure effect on the esterification of C18 fatty acid with methyl alcohol. On the reaction of unsaturated fatty acids such as oleic, elaidic, and linoleic acids with methyl alcohol, approximately the same curves were obtained. It means that these reactions have almost the same reactivity and the same pressure effect. Stearic acid which is a saturated fatty acid demonstrated different effect of pressure from unsaturated fatty acids. The yield of methyl ester decreased by compression of 800 MPa. The reason of this difference was considered to be difference of solubility in methyl alcohol under 800 MPa. Unsaturated fatty acids can be easily dissolved, but a large amount of stearic acid remained as solid in methyl alcohol at room temperature. When THF is used as a solvent for absolute solvation, the yield of methyl stearate increased with pressure. Fig. 3 Pressure Effect on the Esterification of C18 Fatty Acid with Methyl Alcohol. Fatty acid 0.5 mmol, Me0H 50 mmol, 100 Ž, 20 h. a) Stearic acid 0.5 mmol, Me0H 1.5 mmol, THF 2 ml. 4 8

5 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) 29 7 Table 4 shows activation volumes on the esterification of C18 fatty acids with methyl alcohol. The esterification of unsaturated fatty acids had larger negative activation volume than lauric acid (Table 2). These values showed that these reactions were influenced appreciably with pressure. Although the esterification of stearic acid with methyl alcohol had positive activation volume (Run 4), similar negative activation volume to the esterification of lauric acid was obtained in THF solvent (Run 5). Figure 4 shows effect of pressure on the esterification of stearic acid with methyl alcohol in various solvents. The yield of the reaction in acetone decreased as pressure increased. The reactions in THF and dichloromethane were accelerated by compression. The results of Fig. 4 revealed that dichloromethane was a very excellent solvent under high pressure, so pressure effect on the esterification of C18 fatty acids with methyl alcohol was investigated in dichloromethane (Fig. 5). The esterification of stearic, oleic, and elaidic acids had almost the same tendency relating to pressure. The esterification of linoleic acid with methanol in dichloromethane gave interesting curve different from the other fatty acids. Though the Table 4 Activation Volume on Methanolysis of C18 Fatty Acidsa). Fig. 4 Effect of Pressure on Methyl Esterification of Stearic Acid in Various Solvent. Stearic acid 0.2 mmol, Me0H 3 mmol, solvent 2 ml, 100 Ž, 20 h 4 9

6 29 8 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) Fig. 5 Effect of Pressure on the Methyl Esterification of C18 Fatty Acids in CH2C12. Fatty acid 0.2 mmol, MeOH 3 mmol, CH2C12 2 ml, 100 Ž, 20 h Table 5 Activation Volume on Methanolysis of Various Fatty Acids in CH2C12a). cause of the difference of pressure effect is not clear, the state of packing of linoleic acid under high pressure may be different from other acids. Table 5 shows activation volumes on the esterification of C18 fatty acid in dichloromethane. The values obtained are higher negative values than those of Diels-Alder reaction or Menshutkin reaction which are well known to be accelerated largely under high pressure. These results show that the esterification of fatty acids in dichloromethane are strongly accelerated by compression. 4 Summary Esterification of long-chain fatty acid with excess alcohol was accelerated under high pressure and moderate range of negative activation volumes was estimated. Esterification of unsaturated fatty acid was promoted by compression more greatly than that ofsaturated fatty acid. When dichloromethane was used as a solvent on the esterification of long-chain 5 0

7 J. Jpn. Oil Chem. Soc. Vol. 46, No. 3 (1997) 29 9 fatty acid, very large negative activation volumes were estimated, and similar pressure effect was observed on the esterification of long-chain fatty acid except for linoleic acid. It is considered that these results show the possibility of highly selective esterification under mild conditions by using high pressure technique. (Received Aug. 22, 1966; Accepted Oct. 14, 1966) Reference 1) N. S. Isascs, Tetrahedron, 47, 8463 (1991) ; K. Matsumoto, A. Sera, Synthesis, 1985, ) Y. Taguchi, A. Oishi, T. Tsuchiya, I. Shibuya, Nippon Kagaku Kaishi, 1994, ) Y. Taguchi, T. Tsuchiya, A. Oishi, I. Shibuya, Bull. Chem. Soc. Jpn., 69, 1667 (1996). 4) A. Oishi, M. Yasumoto, M. Goto, T. Tsuchiya, I. Shibuya, Y. Taguchi, Heterocycles, 38, 2073 (1994). 5) "Use of high pressure in food" ed by R. Hayashi, Sanei Shuppan, Kyoto (1987). 6) P. B. Cheah, D. A. Ledward, J. Am. Oil Chem. Soc., 72, 1059 (1995). 7) Y. Moroi, S. Kawasaki, Y. Kai, Y. Murata, M. Tanaka, J. Jpn. Oil Chem. Soc., 45, 627 (1996). 8) "Shibousan no Kagaku" ed by K. Inaba, J. Hirano, Saiwai Shobou, Tokyo (1981). 9) T. Yamane, J. Jpn. Oil Chem. Soc., 44, 62 (1995). 10) M. Yasumoto, N. Asou, Y. Taguchi, T. Tsuchiya, I. Shibuya, K. Yonemoto,Kagaku Gijyutu Kenkyusyo Hokoku, 86, 163 (1991). 5 1

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