FEBRUARY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
(8) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. VII, p. 84,New York and London, Longmans, Green and Go., 1925. (9) Parravano, N., and Caglioti, V., Gazz. chim. ital., 64, 429-50 (1934).
(10) Stutz, G. F. A., Jr., and Pfund, A. H., IND.ENG.CHEM.,19,
.
,
51-3 ._ 11927).
(11) Thomas, A. W., and co-workers, J. Am. Chem. Soc., 54, 841
(1932) : 56, 794 (1934) : 57, 4 (1935). (12) Thornton, W., Jr., “Titanium,” A. C. S. Monograph 33, pp. 140-53, New York, Chemical Catalog Go., 1927.
Antioxidants and the Autoxidation of Fats
217
(13) Washburn, E. W., “Introduction to Principles of Physical Chemistry,” 2nd ed., New York, McGraw-Hill Book Co., 1921. (14) Weiser, H. B., “Inorganic Colloid Chemistry,” Vol. 11, pp. 257-75, New Y o r k , John Wiley &a Sons, 1935. (15) Werner, A., “New Ideas on Inorganic Chemistry,” by E. P.
Hedley, New Y o r k and London, Longmans, Green and Co., 1911.. ~ ~ (16) Work, L. T., and Tuwiner, S. B., IND. ENQ.CHEM.,26, 1263-8 (1934).
RECEIVED August
10, 1936.
VIII. Autoxidation of Oleic Acid, Methyl Oleate, and Oleyl Alcohol‘ L. A. HAMILTON* AND H. S. OLCOTT University of Iowa, Iowa City, Iowa
ATURAL fats and oils manifest two kinds of changes Previous workers have examined the oxidation of oleic acid by isolation of the end products after prolonged exin the presence of air or oxygen. The more highly posure to oxygen (6, 6,18,20). The numerous compounds unsaturated or drying oils absorb oxygen and polywhich have been detected and isolated indicate not only that merize to form stable films. Fats which are less unsaturated the reactions are complex, but also that some methods which absorb oxygen more slowly and in due time exhibit the phenomena of rancidity. Purified oleic acid acquires a have been used to accelerate the rate of oxygen absorption rancid odor on exposure to air and light indicating, as Powick actually change the course of the reaction so that different (14) pointed out, “that the oleic acid radical is the point of end products are formed. The experiments to be described were an attempt to exattack in the development of rancidity, and that a study of the chemistry of rancidity should begin with a study of the plore the mechanism of the uncatalyzed oxidation of oleic acid and two closely related compounds, methyl oleate and oxidation of oleic acid.” oleyl alcohol, by following and correlating the changes Despite the paucity of experimental data, several theories which are amenable to reasonably accurate measurement. have been advanced to account for the various transitory and These include the absorption of oxygen, the decrease in unend moducts of the reaction between oleic acid and oxygen. saturation, the evolution of water and carbon dioxide, and Bull-(,$) reviewed some of these. It is generally agreed that o n e m o l e c u l e of oxygen is initially absorbed at the double bond. The actual nature of The course of oxidation of oleic acid, methyl oleate, and oleyl the ”compound formed is unalcohol was studied by an apparatus and methods which permit the certain and was recently dissimultaneous measurement of the oxygen absorbed and of its districussed by Morrell and Davis bution among the transitory and final products of oxidation, including (11). From this point to the water, carbon dioxide, and carboxyl, hydroxyl, peroxide, and aldehyde end of the complicated series of r e a c t i o n s , the mechanism compounds. is u n k n o w n . Any proposed In the initial reactions a t the double bond, each molecule of theory must take into account methyl oleate and oleic acid absorbs approximately four, and each the presence of peroxides, hymolecule of oleyl alcohol approximately five, atoms of oxygen. Simuldroxyl groups, water, carbon taneously each of the three compounds loses one molecule of water. d io x i d e , e p i h y d r i n aldehyde (responsible for the Kreis test), The peroxide level in the early stages of oxidation is higher in oleyl and the numerous aldehydes alcohol and methyl oleate than in oleic acid ; conversely, the hyand acids which have been found droxyl content of oxidizing oleic acid is higher than that of methyl in rancid fats.
N
1 Previous papers in this series rtppearedin J . Biol. Chem., 90, 141 (1931): J . A m , Chem. SOC.,66, 2492 (1934); IND. ENQ.CHEM.,27, 724 (1936); Oil & Soap, 13, 98, 127 (1936); J . Am. Chem. Soc., 68, 1627, 2204 (1936). 2 Present address, Fooony-Vacuum Oil Company, Psulsboro, N. J.
oleate or the extra hydroxyl of oleyl alcohol. The destruction of the double bond occurs faster than would be expected i f the reaction proceeded at a unimolecular rate, presumably because of secondary reactions, which become more prominent as the oxidation progresses. A tentative explanation of these observations is proposed.
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218
the formation of peroxides, aldehydes, and hydroxyl and carboxyl derivatives.
VOL. 29, NO'. 2
regulator I caused the elevator t o rise until the whole system was at atmospheric pressure. The gases in A were then circulated through absorption train J for 15 minutes, or 10 minutes after the dystem ceased to deApparatus crease in volume. This removed any carbon dioxide or water which may have been present a t the &art of a run. The pump An apparatus was designed3with the folloWing objectives: was stopped, and the absorption train was separated from the to keep a rapidly stirred sample under oxygen, at constant rest of the system by closing K and M . The absorption tubes were then r e m o v e d , temperature and presweighed, and replaced sure, and to measure in the system. q u a n t i t a t i v e 1y the About an hour was oxygen absorbed and required for the system to attain temperathe water and carbon ture equilibrium. The dioxide evolved (Figvolume of oxygen abure 1). Separate runs sorbed a t any time were discontinued at could be read directly from the b u r e t after different intervals afthe carbon dioxide and ter active oxygen abw a t e r h a d been resorption had begun, moved. Usually the and the residue was a b s o r p t i o n was recorded two or three a n a l y z e d for iodine times a day, circulatnumber, p e r o x i d e s , ing the gases through aldehydes, acetyl J for an hour before number, acid v a l u e , taking each reading. and s a p o n i f i c a t i o n The buret could be refilled with oxygen as number. required, by closing &, Reaction flask A was filling with oxygen as a 500-cc., round-botd i r e c t e d above, and tom flask with two side readjusting t h e presarms, equipped with sure before reopening a mechanical stirrer Q. In effect, the samworking through a tall ple in A at 80" C. was m e r c u r y seal which absorbing oxygen from was fitted to the reaca reservoir a t room t i o n flask through a temperature; t h e resg r o u n d glass joint. ervoir could have any FIQURE 1. DIAGRAM OF APPARATUS A was immersed to desired capacity but the level indicated by its volume would be line N in a constant-temperature bath maintained at 80" C. and read with the accuracy attainable with a 50-cc. or 100-cc. buret. was connected by a piece of thick-walled rubber tubing to the buret system consisting of one 50-cc. and one 100-cc. buret enAfter the sample had oxidized for the desired length of time and the oxygen absorption had been recorded, A was removed cased in glass noncirculating water jackets. By properly adfrom the bath and the oxidized material was poured into a test justing stopcocks, either could be used alone, or both could be tube from which samples for analysis were immediately weighed used together. The volume of gas in the burets was adjusted by (by difference). a mercury column, the height of which was determined by the level of the mercury in bottle G carried on elevator H . H could be made t o rise 3.5 cm. per minute by means of an electric motor Methods (not shown), through a system of reducing gears and pulleys. The motor was automatically controlled, through a relay, by Carbon dioxide and water were determined gravimetrically mercury-nichrome contact I . A slight decrease in internal presafter absorption in ascarite and dehydrite tubes. A series sure broke this contact and started the motor. The elevator of six special glass-stoppered U-tubes was used; 1, 2, 5, and then rose until the internal pressure was restored to its initial 6 were filled with dehydrite and 3 and 4 with ascarite. The level. Carbon dioxide and water were removed by repeated circulaincrease in weight of tubes 5 and 6 was added to that of tion of the gases in A , t ough a system of six absorption tubes tubes 3 and 4 to calculate the carbon dioxide evolved. Tube at J . Circulating ump was a small automobile gasoline pump 6 showed no appreciable change in weight. Although any sold under the tra& name "Autopulse." It was similar in prinvolatile fatty acids would also be absorbed in the ascarite, ciple to a diaphragm pump and was driven by an electric vibrator attached to the diaphragm and operating on a 6-volt d. c. curno attempt was made to differentiate between them and rent. The pump was made airtight by sealing in the gaskets carbon dioxide. I n the early stages of the oxidation, the with litharge and coating the outside with picein. amounts absorbed were so small as to be relatively unimAt t.he start of a run, the mercury level was adjusted t o the top portant. The assumption that the gain in weight of the deof one buret. About 10 cc. of the material t o be studied were weighed accurately into flask A , the mercury seal and stirrer were hydrite tubes was due entirely to water may not be justified, insert,ed and aligned, and the flask was connected into the system a point which is now under investigation. The data presented by means of short pieces of rubber tubing. Sto cocks P and Q do, however, have a relative significance. were opened, R was closed, and oxygen (freed ofcarbon dioxide Peroxides were determined as directed by French, Olcott, and water) was passed for 10 minutes through P and Q into A and out through the unfilled mercury seal. The current of oxyand Mattill ( 7 ) . Usually 0.2- to 0.3-gram samples were used. gen was then stopped, the mercury seal was filled, P and S were The 0.002 N to 0.007 N sodium thiosulfate used for the closed, and R was opened. The mercury level in the buret was titrations was standardized daily against a dilute potassium lowered by means of elevator H until the system was under about dichromate solution. The difference between duplicate de3 em. of negative pressure and was kept at this point for 10 minutes. If there was no change in the mercury level in the buret, terminations was usually less than 5 per cent. the system was airtight and the mercury column was adjusted to Aldehydes were determined by the bisulfite method of Lea its previous level; Q was then closed, P opened, and the buret (10). The combined bisulfite, after dissociation from the filled with oxygen through P by lowering the mercury bottle and aldehyde, was titrated with approximately 0.02 N rather than elevator H . P was closed, Q and S were opened, and pressure 0.002 N iodine as directed. This decreased slightly the ac8 Adapted from thoae desoribed by Almquist and Branch [ J . An. C h e n . curacy of the determination but gave a better end point. Soc., 54, 2293 (1932)l and by Brigge [J.Dairy Research, 3, 61 (1931)l.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Duplicate determinations on samples of rancid oleic acid weighing'from 0.2 to 0.4 gram usually agreed within 3 to 5 per cent. Acetyl numbers were determined by the method of West, Hoagland, and Curtis (21). It was found necessary to neutralize the slight amount of acid in the commercial butyl alcohol before it was used to dilute the samples. The blank determinations did not adequately correct for this source of error. Duplicate determinations of the acetyl number usually checked within 1 to 2 per cent, but occasionally greater variance was encountered. The acetyl number used is that of West and is defined as "the milligrams of acetyl taken up per gram of substance.'' Free carboxyl was usually calculated from the blank runs in the hydroxyl and total carboxyl (saponification value) determinations. I n both cases, the samples were heated for one hour on the steam bath. In later runs, free carboxyl was determined by dissolving weighed samples in 50 cc. of 95 per cent alcohol and titrating with 0.1 N alcoholic sodium hydroxide. As noted below, all methods gave theoretical results with pure oleic acid, but the oxidized samples showed an increase in acid value when the latter was determined after heating with either pyridine or alcohol. Thus, true free acid was obtained only when the sample was not heated before titration. Saponification values were determined as directed by Jamieson (9) with the following departures: Samples weighed 0.4 to 1.0 gram; 0.3 N alcoholic potassium hydroxide was used for the saponification; 0.1 N hydrochloric acid was used for the titrations. Iodine numbers were determined with an iodine monobromide reagent in carbon tetrachloride (15) for runs 1 to 47. The excess iodine was titrated with 0.1 N sodium thiosulfate. For runs 48 and beyond, a pyridine sulfate dibromide reagent in glacial acetic acid (Rosenmund-Kuhnhenn), prepared as directed by Yasuda (23),was used. With samples weighing 80 to 100 mg., 20 cc. of the reagent, and 0.007 N sodium thiosulfate, triplicate determinations usually agreed within 1 per cent. Before each run the reaction flask was washed with hot water and soap, rinsed with distilled water, washed with 20 per cent hot aqueous sodium hydroxide, and again rinsed thoroughly with distilled water. It was then dried in an oven a t 120" C.
Preparation of Materials OLEICACID. Oleic acid was prepared from olive oil by combining the methods of Skellon (19) and Raymond (f6). After saponification, the soaps were repeatedly extracted with ether to remove the unsaponifiable constituents of olive oil. The fatty acids obtained by acidification were subjected to a lead salt precipitation to remove the saturated acids. This was followed by several crystallizations as the lithium soaps, to remove the acids more unsaturated than oleic, and by fractional distillation of the methyl esters of the acids obtained from the insoluble lithium soaps. The fatty acids recovered from the middle fractions had the theoretical iodine number of pure oleic acid, but they still contained small amounts of other acids. The traces of more unsaturated acids were removed by repeated crystallization of the acids from acetone a t -40" to -80" C. The recrystallized oleic acid had the following constants : iodine number, 88.3 (theoretical, 90.2) ; acid value, 199.4 (theoretical, 198.2) ; peroxide value, 0; acetyl number, 0. These values indicated that the sample contained 97.9 per cent oleic acid contaminated with 2.1 per cent saturated acids. Palmitic and stearic acids are practically unaffected by oxygen a t 80" C., over a period of days. For this reason, the oleic acid was used without further attempts a t purification, and
219
a correction was applied to the calculations, as described below, for the saturated acids present. METHYLOLEATE. Methyl oleate was prepared by the method of Skellon (19) and used for the present studies before the practice of recrystallizing oleic acid from acetone was adopted. The methyl esters of the acids recovered from the insoluble lithium soaps were repeatedly fractionated under reduced pressure, the higher and lower boiling fractions being discarded each time. The final fraction (200 cc. from 950 cc.) had an iodine number of 86.2 (theoretical, 85.8); saponification value, 188 (theoretical, 189); and zero acid, peroxide, and acetyl values. OLEYLALCOHOL.Oleyl alcohol was prepared from olive oil as directed in Organic Syntheses (12). The fraction which distilled a t 148" to 165" C. (0.45 mm.) had the following constants: iodine number, 89.6 (theoretical, 94.8) ; acetyl number, 161.0 (theoretical, 160.5). The sample thus contained 94.8 per cent of oleyl alcohol. The impurity appeared to be palmityl alcohol which, under the conditions used, should not be appreciably oxidized. DIHYDROXYSTEARIC ACID. Dihydroxystearic acid (high melting isomer) was prepared by oxidation of sodium oleate with alkaline potassium permanganate a t room temperature (17). After recrystallization from hot ethyl alcohol, it melted at 134" C. (copper bar). Its constants were: iodine number, 0.0; acetyl number, 282 (theoretical, 272); acid value, 177.2 (theoretical, 177.0). OXIDOOLEIC ACID. This compound was prepared by the action of perbenzoic acid in chloroform on oleic acid a t 0" C. for 2 days. The solvent was removed and the oxide separated from the benzoic acid by solution in petroleum ether. After two recrystallizations from methyl alcohol the oxide melted a t 54" C. (copper bar); other investigators report melting points of 57" to 58" C. (6, IS, 16). Its constants were: acid value, 187.5 (theoretical, 188); iodine number, 0.0. When oxidooleic acid was refluxed for 4 hours with the pyridine-acetic anhydride mixture used for determining acetyl value, an amount of acetic anhydride equivalent to 0.7 mole per mole was taken up. The usual length of time for the acetyl determination is 45 minutes, and the expected amount for oxidooleic acid, 2 moles, from which it is concluded that the oxide oxygen is determined only in part by the acetyl value analyses.
Calculations The data were computed on a molar basis. I n the calculations, corrections were made for the small amounts of saturated compounds and, more important, for the increased weight of the samples after oxidation, as illustrated in the following sample calculation: A 8.8256-gram sample of oleic acid 1 (288 grams equal 1 mole, corrected) absorbed 97.6 cc. of oxygen (at normal temperature and pressure) in 34 hours; 0.0335 gram of water and 0.0064 gram of carbon dioxide were given off. The oxygen, carbon dioxide, and water in moles are multiplied by the factor 288/8.8256 to give the required answers in moles per mole of starting material: 0.1421 for oxygen, 0.0607 for water, and 0.00475 for carbon dioxide. For the other determinations, it was first necessary to correct the weight of the oxidized nonvolatile product, Y , back to a corresponding weight of starting material, X . Y and X are related by the following equations: Y
=
X
+ O2 (grams absorbed) - HzO (grams produced)
therefore
-
COz (grams produced)
Y = 8.9261 grams
Thus, 288 grams (1 mole) would give rise to 288Y/X grams, or 291.3 grams of nonvolatile oxidation product. This
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Of these values the apparent total carboxyl (determined from the saponification value) and the apparent free acid (determined on a sample after heating with pyridine or alcohol) probably have little significance. The latter value is a combination of the true free carboxyl plus an indefinite amount of carboxyl formed from thermal decomposition of the peroxides, an amount which varies with the temperature and time of heating. The true carboxyl value is obtained by titrating a t room temperature in alcohol. Unfortunatelv the effect of heati& the rancid sample in pyridine or alcohol was n o t recognized previous to the work on oleyl alcohol; consequently the true acid values of methyl oleate and oleic acid, after oxidation, are not available. The apparent total carboxyl is a combination of true free acid, esterified acid, a c i d f o r m e d from the destruction of the peroxide, and any carbon d i o x i d e which might be liberated by the saponification p r o ce d u r e. As yet no m e t h o d has been devised to measure the carboxyl and hydroxyl combined as ester in the r a n c i d mixture, or the oxygen present as TIME IN HOURS oxide linkages. FIQURE 3
FIQURE2
value is used as the “molecular weight” of the oxidized material in order to reduce all the calculations to a molar basis. For example, 0.2755 gram of oxidized material required 5.4 cc. of 0.00716 N sodium thiosulfate. Moles of peroxide per mole of starting material therefore kquals (5.4 X 0.00716 X 291.3)/(0.2755 x 2 X lOOO), or 0.0205. H y d r o x y l , aldehyde, and free and total carboxyl are calculated on a similar basis. Iodine numbers were used to calculate the OF OXIDATION,IN MOLES OF PRODUCT PER MOLEOF TABLEI. PRODUCTS amount of destruction of the double bond as ORIQINALSUBSTANCE the oxidation proceeds. An iodine number of Free Free CarCar80.7 indicated a sample containing (80.7 X Time Double boxyl, boxyl 100)/90.2 or 89.7 per cent oleic acid. Thereof 0 2 Bond Free Total PyriAlco-’ Oxygen Absorp- DePerAlde- HyCar- dine, hol Abfore, 291.5 grams (corresponding to 1 mole) of tiona stroyed oxide Ha0 Cot hyde droxyl boxyl Heated Heaied sorbed oxidized material would contain 0.897 X 291.3, Xr. Methvl . ~“~ . Oleate No.4-3 or 261.3 grams of oleic acid. In other words, 8 0.045 0.082 0,020 0.003 0.013 20.5 of the original 282 grams would have been 20.5 0.232 0.216 0.061 0.023 0.043 36 0.428 0.300 0.305 0.070 0.074 destroyed. The required ratio of moles of double 46.5 0.612 0.242 0.340 0.122 0.071 52.5b 0.638 0.262 0.586 0.134 0.051 bond destroyed per mole of starting material 59.5 0.690 0.220 0.631 0.156 0.078 was therefore 20.5/282, or 0.0727. 63.5b 0.726 0.226 0.630 0.178 0.066 0.855 0.149 0.704 0.390 0.021 76b These calculations fail to take into account 93.5 0.925 0.083 1.122 0.319 0.026 154.5 0.944 0.036 2.065 0.427 0.014 volatile products other than carbon dioxide and water. There is also the possibility that some of Oleic Acid No. 1 8.5 0.023 0.027 0.022 0.000 0.009 0,033 1.039 1.010 1.010 0.040 these two latter constituents may have been so 14.8 0.085 0.055 0.026 0.003 0 010 0 072 1082 0 987 0.982 0.168 tenaciously held in the oxidizing material that 38 0.316 0 100 0 169 0 038 0‘027 0’352 1’148 0’972 0 965 0.567 0.602 0:090 01518 01113 01066 0:490 1:565 01974 01965 1.040 61 they were not quantitatively recovered. It is Oleyl Alcohol No. 1 felt, however, that the quantitative results are 0,092 0 112 0 062 0.013 0.020 1 148 0 118 0 044 0.01SC 0.229 12.5 significant indications of the trend of the reac21.8 0.166 0:172 0:126 0.026 0.015 11112 0:202 0:090 0.0540 0.458 tions. 0.280 0.269 0.248 0.056 0.027 1.145 0.318 0.141 O.10Zc 0.712 33 ~
42.8
Results The complete data obtained on methyl oleate, oleic acid, and oleyl alcohol are presented in Table I. Figures 2, 3, and 4 show the manner in which the values change as the oxidation proceeds after the end of the induction period.
~
0.354 0.254 0.353 0.123 0.050
1.182 0.473 0.191 0.1320 0.842
Each entry represents a separate run’ the time of oxy en absorption was. measured from the end of the induction Deriod, whhher this was proyonged by the addition of an IihibitG Or not b Methyl oleate No. 4-2 plus 0.0002 per cent Inhibit01 concentrate W5-10. The induction period of these samples waa appreciably longer than those of the unprotected runs. T h e nnnfmmitv of the data indicates that Inhlbitols do not affect the progress of the autoxidation after the end of the induction period (8), e Alcohol cold. 0
___
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221
Table I1 shows the ratio of each of the forms of oxygen to gases obtained by exhaustion of very rancid cottonseed oil the moles of double bond destroyed. The results for oleyl samples. Hydrogen was not measured in the present experialcohol show a recovery of 90, 75, 84, and 90 per cent of the ments. oxygen absorbed, accounted for as oxidation products. The iodine number of a rancid sample of oleic acid was deterUntil from 5 to 10 per cent of the double bond has been mined. The peroxides in the sample were then destroyed with destroyed, the actual amount of oxygen absorbed is small, potassium iodide as in the peroxide determination, the rancid and the portions measured by analysis of the oxidation prodacids were recovered and dried, and the iodine number was ucts are proportionately smaller. This low concentration of redetermined. The iodine values were identical before and the various forms of oxygen largely increases the error of the after destruction of the peroxides. Apparently the peroxides determinations, since conditions prevent the use of larger in rancid fatty acids do not influence the determination of the samples, probably accounting for the high peroxide values obiodine number, which confirms Yamaguchi's results (WW) tained in the earliest stages of oxidation. The oxygen not and is contrary to the suggestion of Andrews (1) that peroxides accounted for is probably a measure of ester and oxide linkinterfere with the determination of a correct iodine value for ages. rancid fats. Figure 5 shows the results obtained when the oxidation of Possibly the method used for determining aldehydes might methyl oleate is continued until the double bond is almost also estimate any ketones present. The aldehyde (carbonyl) completely destroyed. The fact, that the values for the peroxide, aldehyde, and hydroxyl increase and then decrease, has been assumed to TABLE11. RATIOOF VARIOUSFORMS OF OXYQENTO DOUBLE BONDDESTROYED mean that these are intermediate products in the reaction. However, the decrease in hydroxyl Free Double Carvalue may be due to ester formation. Bond boxyl Destroyed 0 2 Alcohoi, Free The change in iodine number with time for per Mole Absorbed/ H20/ Peroxide/ COz/ Aldehyde/ Cold/ R droxyl/ (D.B.D.) D . B . D . D . R . D . D . B . D . D . B . D . D . B . D . D.B.D. 6.B.D several samples of methyl oleate, oleic acid, and oleyl alcohol is presented in Table 111. Carbon Methyl Oleate No. 4-3 dioxide and water were removed from the reac0.045 2.75 0.44 1.81 0.064 0.282 ... 0.175 0.232 2.02 0.26 0.92 0.100 0.185 ... 0.159 tion flask as formed except in the runs on methyl 0 428 2.16 0.82 0.70 0.164 0.172 ... 0.226 oleate samples 1 and 2. The iodine numbers 0.612 1.92 0.56 0.39 0.200 0.116 ... 0.244 0.63Sa 1.96 0.92 0.412 0.210 0.079 ... have been corrected for the increased weight 0.690 2.04 0.92 0.32 0,226 0.113 ... 0 266 of nonvolatile oxidation products so as to give 0.726" 1.91 0.87 0.31 0.245 0,091 ... ... 0.855" 1.95 0.71 0.175 0.456 0.025 ... a true measure of double bond not destroyed. 0.925 2.18 1.22 0.089 0,346 0.028 ... 0:ois 0.944 2.48 2.06 0.038 0,453 0.014 ... 0.038 The time is measured from the moment oxygen a b s o r p t i o n b e g i n s , whether the induction Oleyl Alcohol No. 1 period was prolonged by an inhibitor or not, as 0.0915 2.50 0.196 0.665 0.68 1.23 0.144 0.222 0.306 0.220 0.166 2.76 0.090 0.76 1.04 0.156 indicated. 0.280 2.54 0.364 0.240 0.88 0.96 0.186 0,097 The values of k, the reaction constant, 0.354 0.372 0.265 2.38 1.00 0.72 0.348 0.141 l k = -1nt
a a--2
Oleic Acid
0.023 1.74 0.98 0.085 1.97 0.90 0.073 1.96 0.89 0.316 1.80 0.54 0.602 1.73 0.86 a bee footnote be Table I.
are calculated as for a first-order reaction. Figure 6 shows the log of iodine number (corrected) plotted against time. The double bond is, in all cases, destroyed more rapidly than would be expected if the reaction proceeded a t a unimolecular rate, in which case IC would be constant, and the plot of log iodine number against time would be a straight line. Several other experiments were berformed which have a bearing on the interpretation of the data. A sample of dihydroxystearic acid (melting a t 134" C.) was placed in a flask with oxygen a t 80' C. The very slow rate of oxygen absorption and peroxide formation was comparable to that exhibited by saturated fatty acids. The solubility of dihydroxystearic acid in oleic acid was found to be less than 5 per cent. A sample of oxidooleic acid was similarly treated. A 3gram sample absorbed about 2 cc. of oxygen in 3 days a t 80" C. Thus it appears that neither of the above products is readily autoxidizable, and therefore probably neither occurs in the main line of the autoxidation reaction. Several attempts were made to identify carbon monoxide in the reaction flask. The gases, given off when methyl oleate was allowed to autoxidize until most of the double bond was destroyed, were bubbled slowly through a dilute solution of hemoglobin. No carbon monoxide hemoglobin could be detected by spectroscopic means. French, Olcott, and Mattill (7) detected small amounts of hydrogen over an oxidizing lard-cod liver oil mixture, and Andrews ( 1 ) found appreciable amounts of hydrogen in the
1.180 0.650 0.298 0,314 0.150
0.000 0.039 0.065 0.122 0.188
0.382 0.122 0.085 0.085 0.110
... ... ... ... ...
1.44 0.85 1.34 1.11 0.81
IN IODINE NUMBER WITH TIME TABLE111. DECREASE
Time of 01 COT. AbsorpIodine tionR No. k Hr. ----Methyl Oleate No. 10 81.0 .... 38 71.2 0.0034 61 50.5 0.0077 70 29.8 0.0143 91 16.4 0.0176 92 9.9 0.0292 ---Methyl 0 9 21 2Sb 24 C 25 28b 48b 52b 61
Oleate No. 286.2 82.4 0.0054 68.4 0.0114 61.3 0.0122 61.0 0.0143 58.5 0.0154 54.5 0.0145 27, 0.0242 24.8 0.0242 19.4 0.0244
-Methyl 0 22 53 58 74
Oleate No.4-2d86.0 70.4 0: ooio 33.7 0.0177 25.4 0.0210 13.5 0.0250
a b d
....
Time of On Absorption Hr. -Methyl
0 8 21 36 47 59 154 144
Cor. Iodine NO.
k
Oleate No. 4-387.4 0 6664 81.6 66.7 0.0114 52.5 0.0141 36.2 0.0187 19.0 0.0186 5.0 0.0186 3.3 0,0226
-Oleic 0 8.5 14.8 38 61
Acid No. -1 88.3 86.3 81.5 61.8 35.2
0:662b 0.0053 0,0094 0.0161
-Oleyl 0 12.5 21.8 33 42.8
Alcohol89.6 81.4 74.6 64.1 57.8
0.0076 0.0084 0.0097 0.0102
....
See footnote 0 Table I. Inhibited Inhibited by hydroq+none, &naphthol. Inhibited with Inhibit01 W6-10, a oonoentrate from wheat germ oil.
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INDUSTRIAL ANI) ENGINEERING CHEMISTRY
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atoms per molecule4 (Table 11). The extra free hydroxyl formed in the oxidation of oleyl alcohol increases almost as rapidly as with methyl oleate, suggesting that the difference in oxygen absorption per mole is probably not attributable to oxidation of the hydroxyl group of oleyl alcohol.6 In the case of methyl oleate, the ratio reaches five atoms per double bond destroyed only when oxidation is continued after the destruction of almost all the double bond. The peroxide level maintained with methyl oleate and oleyl alcohol is approximately twice that of oleic acid. Inversely, the free hydroxyl liberated by oleic acid is from two to four times that liberated by methyl oleate or oleyl alcohol. Oleic acid shows a slight decrease in free acid as the oxidation proceeds. On the other hand, oleyl alcohol shows an increase in free acid, which is roughly parallel to the free hydroxyl increase, but slightly greater in magnitude. The true free acid of methyl oleate is considerably below the values listed, since these were obtained by a hot titration; however, they probably lie slightly above the free hydroxyl values as in the case of oleyl alcohol. These differences between oleic acid on the one hand and methyl oleate and oleyl alcohol on the other, with regard to peroxide, hydroxyl, and free acid produced, might possibly be due to a reaction between a peroxide group and the free carboxyl group of oleic acid (or an oxidized derivative) in which a hydroxyl group is formed and the peroxide group destroyed. In the absence of appreciable amounts of free carboxyl (oleyl alcohol and methyl oleate), the stability of the peroxide complex is apparently greater.
value of a rancid sample of oleic acid was determined before and after acetylation. The fact that there was no change in this value suggests that none of the carbonyl was in equilibrium with an enol form. The effect of temperature and solvents upon the free acid value was determined. Pure oleic acid and pure dihydroxystearic acid gave theoretical acid values when samples were heated for one hour in pyridine, diluted with butyl alcohol, and titrated with alcoholic sodium hydroxide, and also when heated in 95 per cent ethyl alcoholfor 1to 3 hours and similarly titrated. The acid value of a sample of rancid oleyl alcohol was determined by dissolving a portion in 50 cc. of 95 per cent alcohol and titrating with 0.1 N sodium hydroxide (acid value, 9.73); by dissolving a similar portion in 5 cc. of pyridine, diluting with 30 cc. of butyl alcohol, and titrating with 0.1 N sodium hydroxide (acid value, 10.75); and by dissolving a third portion in 5 cc. of pyridine, heating for one hour on a steam bath, diluting with 30 cc. of butyl alcohol, and titrating with 0.1 N sodium hydroxide (acid value, 17.2.). The data presented on rancid oleic acid in Table I show that the acid value after heating in alcohol is slightly lower than that found after heating in pyridine. The higher values after heating are probably due to acid liberated by some thermal decomposition of peroxides.
Discussion A comparison of the data presented in Tables I and 11, and in Figures 2, 3, and 4,reveals a general similarity in the results obtained on the three compounds, methyl oleate, oleic acid, and oleyl alcohol. Several points of difference, however, indicate that the reactions are not identical. One difference lies in the amount of oxygen absorbed. Oleyl alcohol absorbs approximately five atoms of oxygen per molecule, whereas oleic acid and methyl oleate absorb about four
00
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FIGURE 5
Aldehydes are present from the start of oxidation. The values in Figure 5 suggest that the aldehydes are transient intermediate compounds, being formed and destroyed a t nearly equal rates. 4 From measurements a t room temperature of the gain in weight and t h e dearease in iodine number, Aas [Fettchem. Umschau, 42, 71 (1935); 43, 52 (1936)l concluded t h a t oleic acid absorbs 3 atoms of oxygen per double bond destroyed. 5 Although oleyl alcohol may be distilled in vacuo without decomposition, i t is possible, as suggested by H. N. Stephens, t h a t under t h e conditions of the experiment a small part of the oleyl alcohol may be dehydrated with the formation of a second double bond, the oxidation of which would account for the extra oxygen absorbed.
FEBRUARY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
A slow production of carbon dioxide occurs throughout the reaction. Table I1 shows that the ratio of carbon dioxide to moles of double bond destroyed increases as the oxidation proceeds; the highest ratio obtained, however, even when the reaction was allowed to continue until oxygen absorption had practically ceased, was $ still less than 0.5 mole per double bond deo“ stroyed. The formation of carbon dioxide C 1.4 -0 therefore is part of a reaction in which only E half or less of the original material is involved . 1.2 The Darallelism between the curves for Der00 oxides, -double bond destroyed, and water,- especially in the data for oleyl alcohol, suggests that one mole of the unsaturated compound reacts with two moles of oxygen to give one mole of water and a compound containing a peroxide group and another oxygen atom, possibly in the form of an oxide linkage. Further reaction with oxygen would yield the split products which have been identified in rancid fats. Skellon (20) recovered 35 per cent of nonoic acid from a very rancid oleic acid, thus indicating that the oxide linkage suggested above would occur in that part of the chain between the double bond and the functional group, carbons 9 to 18 emerging as nonoic acid. Raymond (16) demonstrated that oleic acid in the presence of autoxidiziiig benzaldehyde is changed quantitatively to the oxidooleic acid, corresponding to the dihydroxystearic acid (melting point, 95’ C.). Pigulevskii and Petrov (15) showed that perbenzoic acid reacts quantitatively in the cold with oleic acid to give the same compound. Boeseken (8, 3) found that peracetic acid reacts quantitatively with oleic acid to give the monoacetate of dihydroxystearic acid (melting at 95’ C.). However, Skellon’s data (10)and the authors’ observations demonstrate that none of this isomer of dihydroxystearic acid is isolable from the oxidation products of the uncatalyzed reaction of oleic acid. It thus appears necessary to assume that, although aldehydes are formed and removed in the autoxidation of oleic acid, presumably by a peracid mechanism, these peracids do not react with unchanged oleic acid. If they did, the 95’ C. dihydroxystearic acid would be recovered along with or instead of the higher melting isomer. Ellis (6) isolated 16 to 20 per cent of oxidoelaidic from both oleic and elaidic acid preparations after their oxidation when this was catalyzed by cobalt elaidate. The oxide was formed only in small amounts in the absence of the catalyst. The 16 per cent of dihydroxystearic acid (melting a t 132’ C.) which Skellon (10)isolated from rancid oleic acid (viscous but fluid) after saponification, must have all been present in combined form since the free acid is practically insoluble in oleic acid. The data in Table 111 and the curve in Figure 6 indicate that the destruction of the double bond does not proceed a t a unimolecular rate. This is in sharp contrast to Yamaguchi’s data (22); he found that the destruction of the double bond of oleic acid and of several oils was strictly a reaction of the first order, and unaffected by secondary reactions. The probable explanation of this discrepancy lies in the difference in methods. The reaction flask used in this laboratory did not allow the escape of any oxidation products except carbon dioxide and water. Yamaguchi oxidized samples by bubbling oxygen through the liquid a t 100” C. Considerable amounts of volatile materials were collected. It would appear from the character of the results he obtained that all aldehydes formed were immediately distilled from the reaction mixture before oxidation could occur. The secondary reactions which would
O L E I C ACID X I
(
OLEYL ALCOHOL
*- I v
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METHY OLEATE # I
10
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I N HOURS
FIGURE 6
occur under ordinary circumstances were probably thus avoided. Before a satisfactory explanation of the manner in which the chain is broken can be advanced, it will be necessary to isolate and characterize the shorter chain products formed, from a sample which has been oxidized under controlled conditions such that the amount of the peroxide complex decomposed and the amounts of decomposition products formed from it may be calculated. Experiments in this direction are in progress. A formulation of the hypothetical reactions discussed above is reserved for future publications.
Acknowledgment The authors are indebted to Lever Brothers Company for a grant-in-aid, to H. N. Stephens of the University of Minnesota for several helpful suggestions, and to H. A. Mattill for his advice and encouragement.
Literature Cited Andrews, J. T. R., Oil & Soap, 12, 104 (1935). Boeseken, J., and Belinfante, A. H., Rec. trav. chim., 45, 914 (1926). Boeseken, J., and Elsen, G., Ibid., 48, 363 (1929). Bull, H. B., “Biochemistry of the Lipids,” pp. 54-7, Minneapolis, Burgess Publishing Co., 1935. Ellis, G. W., Biochem. J., 26, 791 (1932). Ibid., 30, 753 (1936). French, R. B., Olcott, H. S., and Mattill, H. A., IND. ENG. CHEM.,27, 724 (1935). Hamilton, L. A., and Oloott, H. S., Oil & Soap, 13, 127 (1936). Jamieson, G. S., “Vegetable Fats and Oils,” A. C. S. Monograph 58, New York, Chemical Catalog Co., 1932. Lea, C. H., IND. ENG.CHEM.,Anal. Ed., 6, 241 (1934). Morrell, R. S., and Davis, W. R., J . Soc. Chem. I n d . , 55, 237T (1936). , Org. Syntheses, 15, 51 (1935). Pigulevskii, G. V., and Petrov, M. A , J. Russ. Phus. Chem. Soc., 58, 1062 (1926). Powick, W. C., J. A g r . Research, 26, 323 (1923). Ralls, J. O., J . Am. Chem. SOC.,55, 2083 (1933). Raymond, E., J. chim. phys., 28, 480 (1931). Robinson, G. M., and Robinson, R., J. Chem. Soc., 127, 175 (1925). Scala, A., Staz. sper. agrar. ital., 30,613 (1897). Skellon, J. H., J . Soc. Chem. I n d . , 50, 131T (1931). Ibid., 50, 3821‘ (1931). West, E. S., Hoagland, C. L., and Curtis, G. H., J . Biol. Chem., 104, 627 (1934). Yamaguchi, B., J a p a n . Chem. Soc. Rev., 53, 1134 (1932); Rept. Aeronaut. Research I n s t . Tokyo I m p . Univ., 5 , 195, 287 (1930); 6, 219, 237 (1931). Yasuda, M., J . Biol. Chem., 94,401 (1931-32). ~~
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R ~ O B I V EOctober D 16, 1936. Presented in p a r t before the Division of Industrial and Engineering Chemistry a t the 92nd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 t o 11, 1938. From a thesis presented by L. A. Hamilton t o the Graduate College, University of Iowa,in partial fulfillment of the requirements for the Ph.D. degree, February, 1936.
