August, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
drying oils. It is hoped that this useful technology may now not only be applied to further our knowledge of structure of oils in general, but will eventually be employed on a commercial basis for the production of new and improved materials. The data presented here are but a brief introduction to possible applications of such a high-vacuum process.
Acknowledgment The author wishes to express appreciation for the assistanoe given by members of the staff of Distillatian Products, Inc., for carrying out some of the chemical analyses and also to Joseph J. Mattiello of the Hilo Varnish Company for many helpful suggestions.
Literature Cited (1) BrZnsted and Hevesy, Phil. Mag., 43, 31 (1922). (2) Farmer and Van den Heuvel, J . SOC.Chem. Ind., 57, 24 (1938); Moore, Biochem. J . , 31, 138 (1937). (3) Hickman, IND. ENQ.CHDM.,29, 1107 (1937). (4) Hoffman and Green, Oil & Soap, 16, 236 (1939).
1043
(5) Imperial Chemical Industries, Ltd., Brit. Patent 422,941 (Jan. 22, 1935); Imperial Chem. Ind., Ltd., and Hill and Walker, Ibid., 428,864 (May 16, 1935); Fawcett and Walker, Ibid., 442,000 (Jan. 31, 1936), U. 8. Patent 2,128,354 (Aug. 30, 1938); Hickman (to Eastman Kodak Co.), U. 8. Patent 2,126,466 (Aug. 9, 1938). (6) Krafft et al., Ber., 28, 2583 (1895), 29, 1316, 2240 (1896), 32, 1623 (1899), 36, 4339 (1903); Fischer and Harries, Ibid., 35, 2158 (1902); Erdman, Ibid., 36, 3456 (1903); von Rechenberg, J. prakt. Chem., 80, 547 (1909); Houben, Ber., 52, 1460 (1919); Walden, 2. anorg. Chem., 112, 225 (1920); Waterman and Rijks, 2. deut. Ole- u. Fett-Id., 46, 177 (1926). (7) Mennies et al., J . A m . Chem. Soc., 43, 2309, 2314 (1921); Coulson, Analyst, 57, 757 (1932). (8) Oosterhof, van Vlodrop, and Waterman, U. S. Patent 2,065,728 (Dec. 29, 1936). (9) Waterman et al., Chem. Weekblad, 24, 268 (1927), 26, 469 (1929); Burch, Proc. Roy. SOC.(London), A123, 271 (1929); Washburn, Bur. Standards J . Research, 2, 467 (1929); Hickman, J . Optical Soc. A m . , 18, 69 (1929); Hickman and Sanford, J. Phys. Chem., 34, 637 (1930). (10) Waterman and Oosterhof, Rec. trav. chim., 52, 895 (1933). PRFSENTED before t h e Division of P a i n t a n d Varnish Chemistry at t h e 99th Meeting of t h e American Chemical Society, Cincinnati, Ohio.
Reaction Mechanism of the Acid Hydrolysis of Fatty Oils J
J
J
TZENG-JIUEQ SUEN AND TSUN-PU CHIEN The Tung Li Oil Works, Chungking, China
NE of the rapid methods for obtaining free fatty acids
0
from a fatty oil is to heat the oil first with a few per cent of concentrated sulfuric acid for a short time and then steam it to bring about hydrolysis (1, 9). Recently large amounts of free fatty acids were desired in this plant, and this acid hydrolysis process was tried. The results on the rate of hydrolysis indicated that a study of the reaction kinetics would prove of value. I n studying the rate of hydrolysis of an ester which is partially miscible with water in aqueous solution, Lowenhers (6, 8) showed that the reaction is taking place practically in the water phase. If an acid is used as the catalyst, a constant amount of ester is changed per unit of time; i. e., the reaction is apparently of zero order. I n his study of the kinetics of soapmaking, however, Smith (6) showed that the saponification proceeds in the soap phase. Reaction a t the interface is of no significant magnitude except a t the beginning of the saponification in which no soap is present initially. Lascaray’s work on the hydrolysis of tallow with alkaline catalysts (9) agrees with Smith’s results, H e concluded that the bulk of the reaction proceeds in the oil phase, with the help of the dissolved water, The activity of the alkaline catalysts was explained by their power to increase the solubility of water in the oil phase.
Experimental The apparatus is shown in Figure 1. A 500-cc. three-necked flask was fitted with a mercury-sealed electrical stirrer, E, directly driven by stirring motor M , with a reflux condenser, C , a thermometer, TI, and a sam ling tube, Y. The various oils and fats studied were obtained 8om the local market. The oil t o be hydrolyzed was first charged into the flask and heated with
stirring t o 115’ C.; 3 or 4 per cent by weight of strong sulfuric acid was added with further heating at 115’ C. for 10 minutes. It was then allowed t o cool while the apparatus was being transferred to a thermostat, B, the temperature of which was meas-
~~
~
Fatty oils were hydrolyzed by the sulfuric acid method, and the reaction course was followed by determining the acid number of the samples at various time intervals. By plotting the logarithm of the difference of saponification number and acid number, of the oils hydrolyzed, against time, straight lines were obtained. Since this is practically the same as plotting the logarithm of the concentration of unhydrolyzed oil in the oil phase against time, the reaction appears to be first order. Hydrolysis with different oil-water ratios yielded the same results. By assuming that the reaction takes place in the oil phase, all the data can be satisfactorily interpreted. Some of Lewlrowitsch’s data on hydrolysis of oils with hydrochloric acid as the catalyst can be interpreted in the same way.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ured by thermometer 2'2 and maintained at 100" + 0.1" C. When the oil reached 100" C., a measured amount of boiling water was added to the flask, and the stirrer set to operation. The total volume of the liquids in the flask amounted to about 300 cc. The speed of the stirrer was kept as constant as possible so that the reaction mixture formed a uniform and fine emulsion. Any water or oil vaporized was caught in the reflux condenser and ran back to the flask. At definite time intervals after the stirrer was set in motion, samples were withdrawn from the flask through the sampling tube, the manipulation of which is obvious. The oil layer of each sample, after repeated washin with warm water, was analyzed for its acid value by the U S U ~method (4). The saponification numbers of the oils and fats were also determined.
Keglecting the small difference between the specific gravity of the ester and that of the water-insoluble reaction products, C,is evidently proportional to the quantity (S - A ) , where S is the saponification number (milligrams of potassium hydroxide required for complete saponification of one gram of a fat or oil) and A is the acid number (milligrams of potassium hydroxide required t o neutralize. the free fatty acids in one grain of a fat or oil). Calling z = S - A,
;;
The results are shown in Tables I and I1 and Figure 2. As in most of the experimental studies on chemical kinetics, the zero time is difficult to fix. I n order to compare the results of hydrolysis with different oil-water ratios, the experimental data in Table I1 were slightly shifted by graphical interpolation in such a way that the acid number a t t = 0.5 becomes 41.7. OF VARIOUS OILSAT 100" C. TABLE I. HYDROLYSIS
Oil
H&OI concn. HtSOi, % of bil" Vol. oil/vol. water Saponification No. A at t = 0.5 hr. = 1 hr. = 2 hr. = 4 hr. = 6 hr.
Rapeseed 86.5 3 1/1,5 186.6 40.2 57.1 81.4 120.4 143.8
157.7 146.4 129.5
= 8 hr.
z a t t = 0 . 5 hr.
1 hr.
105.2 66.2
= 2 hr. = 4 hr. 5
6 hr.
42.8 28.9 2,166 2.112
= 8 hr.
Log
2
a t t = 0.5 hr. = 1 hr. = 2 hr. = 4 hr. = 6 hr. = 8 hr.
2.022 1.821 1.632
1.461
Rapeseed 86.3 4
1/1,5 186.6 44.6 59.5 87.2 127.4
151.1 163.0 142.0 127.1 99.4 59 2 35 5 23.6
2.15% 2.104 1.997 1.772 1.550 1.373
Peanut 86.5 4
1/1.5 192.0 57.6 79.1 113.0 151.8 169.5 173.5 134.4 112.9 79 0 40.2 22.5 18.5
2.128 2.05% 1.898 1.604 1.362 1.267
Tallow 86.5 4
1/1.5 197.0 42.6 51.1 79.5 118.2 143.0 138.7 154.4 145.3 117.4 78 8 54.0 38 3
2.189 2.162 2.070
1.896 1.732 1.583
Laid 86.5 4
1/1,5
197.4 53.9 74.5 111 . 4 152.9
172.9 181.2 143 5 122.9 86.0 44.5 24.5 16.2
2.157 2,089 1.934 1.648 1.389 1,209
TABLE 11. HYDROLYSIS O F RAPESEED OILa WITH DIFFEREST OIL-WATERRATIOSAT 100" C. ~ i Acid ~ S o . ,~with Vol. , Oil/F'ol. W a t e r as Follows: Hours 1/2 1/1.5 1/1 2/1 Average 41.7 41.7 41.7 41.7 41.7 0.5 57.8 58.1 58.0 57.5 57.9 1 85.5 85.9 86.5 86.0 2 86.0 4 126.5 126.2 126.8 126.0 126.4 6 150 2 150.5 1 5 0 . 4 149.5 130.2 S 162.7 1 6 2 . 8 162.7 162.5 162.7 a H&O4 concentration, 86.5%; H ~ S O is I 45% b y weight
z
144 rl
128 7 100 6 60 2 36.4 -711 9
hvelage Log x 2.161 2,109 2.002
1.780 1.561 1 378
of t h e oil
Hydrolysis Mechanism Consider the reaction of hydrolysis of a n-nter-insoluble ester: RCOOR'
+ HOH +RCOOH + R'OH
If the reaction takes place only in the oil phase, the reaction velocity equation will assume the folloTYing form :
where C,
C, C, k t
=
=
= =
concentration of catalyst in oil phaje concentration of ester in oil phase concentration of water in oil phase constant time
Obviously C, is constant in a given experiment. If we assume further that the concentration of water in the oil phase is constant-i. e., the oil is always saturated with water-the above equation becomes as shown in Equation 2 .
Vol. 33, No. 8
_-
=
1235
-In z = k3t where kr
=
+ k,
(3) (4)
constant of integration
As this apparently represents a reaction of the first order, a plot of log z against t vould result in a straight line. As Figure 2 shows, the experimental data check with the above assumption. The fact that during the later stages the experimental points do not fall on straight lines (Figure 2) is probably due to the fact that a t this time more than 85 per cent of the oil has been hydrolyzed, and the reverse reaction of esterification begins to shorn some disturbing effect. Moreover, at this stage the acid and saponification numbers begin to be almost the same in magnitude. A small error in either value will introduce a serious error in z. A heterogeneous reaction with oil and water phases can take place only in one or more of three alternative ways: in the water phase, in the oil phase, and a t the interface. The results in Table I1 show that the degree of hydrolysis is practically independent of oil-water ratio within the accuracy of experimental technique. If hydrolysis takes place a t the interface, a postulate assuming the same interfacial area per unit amount of oil \Yith diffeient oil-water ratios mould be required. This is certainly not true (7). Smith (6) also showed that to consider the saponification reaction of oils by alkaline solutions as an interface reaction is untenable in most cases. The identical results with different oil-water ratios do not contradict the assumption that the reaction takes place in the water phase. For a given quantity of oil, theamount of reaction in the water phase, if i t occurs a t all, will be proportional to the volume of water and to the FIGURE 1. APPARATUS FOR OIL concentration of HYDROLYSIS
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1941
1045
catalyst. With the same amount TABLE111. HYDROLYSIS OF OILS WITH HYDROCHLORIC ACIDAS CATALYST (9) of catalyst present, the product of Rapeseed Oil volume of water times concentration Whale Oil (S 195) Tallow (S = 200) Lard (S = 201) ~ i ~ ~ t(6’ , 185) of catalyst, and hence the rate of Hours A x Logs A Logs A x Log 2: A x Log x reaction, remains unaffected no matter 0 2.2 182.8 2.262 6.0 189.0 2.276 11.2 188.8 2.276 11.3 189.7 2.278 2 19.7 165-.3 2.218 26.7 168.3 2.226 43.4 156.6 2.195 14.5 186.5 2.270 how much water is used. But as 4 49.6 151 4 2.180 Lowenherz (6, 8) showed, the reac7 ii:i i6i:g 2:640 ioi:3 ii:7 1:972 iii:5 ii:5 i:&z 71.5 129:5 2.112 9 89.6 95.4 1,980 120.3 74.7 1.873 131.7 68.3 1.834 86.4 113.6 2,059 tion in the water phase will appear 12 107.2 77.8 1.891 142.7 52.3 1.718 153.2 46.8 1.670 101.2 99.8 1.999 14 120.1 64.9 1.812 155.4 39.6 1.598 167.0 33.0 1.518 116.8 84.2 1.926 as a reaction of zero order, the theo16 127.3 57.7 1.761 162.3 32.7 1.514 . . . . . . . . . 128.5 72.5 1.860 retical equation of which, however, 18 134.2 50.8 1.706 170.0 25.0 1.398 . . . . . . . . . 138.0 63.0 1.799 20 140.3 44.7 1,650 172.0 23.0 1.361 . . . . . . . . . 147.3 53.7 1.730 does not fit the experimental data. 22 144.0 41.0 1.612 . . . . . . . . . . . . . . . . . . 148.7 52.3 1.718 Assuming that the reaction takes 24 151.8 33.2 1.521 . . . . . . . . . . . . . . . . . . 155.0 46.0 1.663 place in the oil phase is the simplest way of interpreting the above fact. As-long as ‘there-is enough water assumption also satisfactorily correlates the data on reacpresent to saturate the oil, the rate of reaction is controlled tion kinetics. by what is going on in the oil phase. As shown above, this I n the hydrolysis of fatty oils, three reactions are going on simultaneously (3), 2.2
-
-
ORopeseed oil, 4 % o c i d X Rapeseed oil, 3 %acid 2.
1.S
x
-
0,
CHZOH
I
1.7
EZFOOCR L5
L3,
2
8
6
4
t -HOURS
0
Rapeseed oil
----f
A
HOH
+ RCOOH
AHaOH
The observed reaction rate is evidently the sum of these three reactions. The reaction mechanism described above is not restricted to the cases of hydrolysis with sulfuric acid as the catalyst. Lewkowitsch (8) carried out many experiments on the hydrolysis of various oils with hydrochloric acid as the catalyst. Since the experiments were not performed with close control, most of the data do not show a smooth reaction course. However, a few of the series do indicate that the reaction follows a regular course, which should not be overlooked. They are shown in Table I11 and Figure 3. Again, by plotting log 5 against t, straight lines are obtained.
Conclusions
2.0
1. The reaction of acid hydrolysis of fatty oils takes place in the oil phase. 2. The reaction appears to be first order with respect to the unhydrolyzed oil.
x 1.8
-r
1.6
Literature Cited
1.4
1.2
0
CHzOH
+ HOH
4
8
t2
1 6
20
24
t -HOURS FIGURE12 (Above). HYDROLYSIS O F OILS AT 100’ c. OF OILS WITH HYDROFIGURE 3 (Below). HYDROLYSIS CHLORIC ACIDAS CATALYST
(1) Groggins, P. H., “Unit Processes in Organic Syntheses”, 2nd ed., pp. 609-10, New York, McGraw-Hill Book Co., 1938. (2) Lascaray. L., Fette u. Seifen, 46, 628 (1939); Brit. Chem. Abstracts, B59,373 (1940). (3) Lewkowitsch, J., “Chemical Technology and Analysis of Oils, Fats, and Waxes”, 6th ed., Vol. 1,Chap. 11, New York, Macmillan Book Co., 1927. (4) Ibid., Chap. VI. (5) Lowenherz, 2.physik. Chem., 15, 389 (1894). (6) Smith, L. E., J . SOC.Chem. Ind.,51, 377T (1932). (7) Suen, T. J., 8c.D. thesis, Mass. Inst. Tech., 1937. (8) Taylor, H. S., “Treatise on Physical Chemistry”, 2nd ed., Vo’. 2, p. 1053, New York, D. Van Nostrand Co., 1931.
