Composition and Analysis of Dehvdrated Castor Oil J
GEORGE W. P R I E S T AND J. D . VON MIKUSCH Woburn Degreasing Company of New Jersey, Harrison, N. J.
I
N T H E dehydration of castor oil, one hydroxyl group and
a neighboring hydrogen are removed from the carbon chain of the ricinoleic acid to form a new double bond. Ricinoleic acid already contains one double bond, and dehydration may lead to the formation of two new products, one of them containing two double bonds in isolated position, the other containing two conjugated double bonds. The two possible reactions may be written as follows:
rH---&Hl
H
H H H H H
I
I
I I
I
I
CHS(CH~)~C=C-C-C=C-(CH~)~COOH+ HSO
(1)
H
(9,12-linoleic acid)
H H H H H CH,(CH2),~-~-~-b=b-(CHl)rCOOH I l l --+
H H H H H
+
CH,(CHe),~-~=~-~=~--(CH2)rCOOH H20
I
(2)
H
(9,ll-linoleic acid)
D e t e r m i n a t i o n of Conjugated Portion
It has been generally assumed that the second one of these two reactions is predominating in the dehydration process over the first one and that from 75 to 90 per cent of the product consists of 9,ll-linoleic acid. This assumption apparently originated with the patents (26, 27) and numerous publications (24, 25) by Scheiber in which he claims that 90 per cent of the acids in the commercially produced triglyceride consists of the conjugated compound. At the time this claim was made by Scheiber, no analytical method was available to determine quantitatively the percentage of conjugated double bonds present in an oil, although qualitative tests were used to demonstrate their presence, such as coagulation with stannic chloride and the increase of iodine value (Wijs) upon prolonged exposure up to 10 days. Although such tests showed beyond doubt that appreciable amounts of the conjugated isomer were formed, they did not represent quantitative analytical methods as
would have been required either to prove or disprove Scheiber's contention that reaction 2 accounted almost exclusively for the process. When Boeseken and Hoevers ( 5 ) reacted the main fraction of the distilled ethyl esters of dehydrated ricinoleic acid with excess maleic anhydride a t 135" C. for one hour, they found that enough of the anhydride had disappeared to account for 7 5 per cent of conjugated double bonds in the mixture. This was apparently determined in only one instance. It is understandable that this result was then assumed by others to indicate that there is generally a preferential formation of conjugated systems in the dehydration of castor oil and its fatty acids, although Smit (28), in his extensive studies on linoleic acids, complains of low yields of the 9,11 isomer. Only since 1936, when first Kaufmann and Baltes (18) and then Ellis and Jones (11) worked out the practical application of the diene synthesis to the analysis of oils, have quantitative methods for determining the proportion of conjugated acid present been available. A study has since been conducted in this laboratory with the new methods and their application to dehydrated castor oils. I n the first phase we were concerned with the selection of one of these two methods. The Kaufmann method was found, as previously reported @ I ) , to show less dependence of the results on the weight of sample analyzed than the Ellis method. Furthermore, the diene values given by the Ellis method were generally several points higher than those found by the Kaufmann method, and in some cases the former could be proved definitely to be in error. Diene values of 3.9 and 4.2, for instance, resulted with a sample of thoroughly hydrogenated castor oil with an iodine value of 1.2 which could not possibly contain the indicated amount of conjugation. The conclusion that in the Ellis method the maleic anhydride reacts with other groups, such as hydroxyl groups, has since been confirmed by Bruce and Denley (8) and by Bickford, Dollear, and Markley ( 3 ) . The latter authors (4) investigated the effect of hydroxyl groups on the diene value determination by the Ellis method and by the Kaufmann reflux and sealed-tube methods in toluene solution. The original Kaufmann method ( I @ , however, prescribes the use of maleic anhydride in acetone solution and with this method the same sample of hydrogenated castor oil, which showed a diene value of 3.9 and 4.2 with the Ellis method, gave only 0.23 (21). This and similar results indicated that with the original Kaufmann method, the tendency of maleic anhydride to react with the hydroxy group is very small, if not absent altogether. However, even if the diene value of 3.2, which Dollear, Bickford, and Markley found on castor oil in toluene solution, is due to an unavoidable effect of its hydroxyl number of 166, i t is likely that the
1314
OCTOBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
less than 10 per cent of these hydroxyl groups remaining in dehydrated castor oil will not affect the results appreciably. The original form of the Kaufmann method was therefore selected for the analysis of dehydrated castor oil, but a few variations in laboratory detail were found advisable. Since this method, as now used in this laboratory, is believed to be best suited to dehydrated castor oil, it is given fully a t the end of this paper, although some of i t is merely a repetition of the original procedure by Kaufmann and Baltes. This procedure was used for the analysis of dehydrated castor oils made by various dehydrating processes and by various manufacturers. Among the oils analyzed were some made by the Scheiber process in Europe-i. e., by way of the fatty acids which are esterified after the dehydration process-and others made directly from the oil. Numerous determinations were carried out on both plant batches and laboratory samples of dehydrated castor oil made directly from the oil by catalysis (99). Some of the results obtained with representative samples of each of the better known dehydrated castor oils now on the market are tabulated together with other analytical data. Unfortunately no sample of the dehydrated castor oil made under the recent patent of Muenzel (do), in which it is also claimed that all ricinoleic acid is converted into 9,ll-linoleic acid (octadecadienoic acid) was available. Instead, a laboratory sample made with 4 per cent tungstic acid following the procedure described in the patent has been included in the table to take its place.
~
.
Contrary to the common belief that dehydrated castor oil consists mostly of the triglyceride of 9,ll-linoleic acid, i t is found to contain more of the 9,12-linoleic acid than of the former. The diene values of commercial oils, as well as of samples made by various methods in the laboratory, range from 15 to 22, indicating a content of 17.3 to 25.4 per cent of the 9,11 isomer i n the glycerides. From these diene values i t is calculated that only one fourth to one third of the dehydration reaction leads to the formation of conjugated double bonds. Analytical data are listed for a number of commercial dehydrated castor oils, including both raw oils and oils processed to higher viscosjties. The latter have lower acetyl values than the raw oils; however, i t is shown that the designation “completely dehydrated” for the bodied oils, as opposed to the raw oils, is not justified. An analysis of the distilled fatty acids, which are found to contain a somewhat greater proportion of conjugated double bonds than the oils, is included. The application of the customary analytical methods to dehydrated castor oil is discussed, and some changes are recommended.
1315
The surprising results of the diene value determinations were as follows: (a) The conjugated portion (9,ll-linoleic acid) in the oil is by no means predominating but actually greatly exceeded by the isolated portion (9,12-1inoleic acid). ( b ) All oils tested showed diene values in approximately the same range although in some cases these oils had been made by entirely different methods; the values for commercially dehydrated castor oil of good quality averaged from 15 to 22 and those for the commercially distilled fatty acids from 20 to 28, although in each case somewhat lower values were occasionally encountered. Since the theoretical diene value for 9,ll-linoleic acid is 90.7 and that of its triglyceride is 86.6, a diene value of 15 to 22 indicates the presence of 16.5 to 24.3 per cent of the acid, or of 17.3 to 25.4 per cent of its glycerides. Diene values of 20 to 28, as found in the distilled fatty acid sample, accordingly show that they contain about 22 to 30.9 per cent of the conjugated isomer. Although the values given for the oil have been determined on the raw oil samples, wherever these were available, the possibility must be taken into consideration that the conjugated portion originally present may have been slightly higher and some of these double bonds may have been lost by polymerization during the dehydration process. This, however, could account for only a few points a t most because the low viscosity of F to G on the Gardner-Holdt scale, as obtained in the case of good-grade raw oils, shows that the oil is only slightly polymerized. Furthermore, if any polymerization with loss of diene value took place during the dehydration process, the total iodine value of the product should also be lower than that calculated from the decrease in acetyl value. Actually, as will be shown below, the Hanus iodine number which measures the total unsaturation of dehydrated castor oil checks fairly closely with the calculated value. I n the case of raw dehydrated castor oil, the diene value as determined by the Kaufmann method must therefore be accepted as evidence of practically the total share of reaction 2 in the dehydration process. However, although the diene values of the bodied oils-. g., G and H of Table I-are lower than those of the raw oils, they are higher than would be expected from the decrease in iodine value during bodying. This permits the conclusion that new conjugated double bonds are formed in bodying, a phenomenon which has long been believed to take place in the bodying of nonconjugated drying oils and recently proved in the case of linseed oil by means of a spectroscopic method by Bradley and Richardson (6). As a confirmation of the correctness of our results with the diene value determination, a recent publication by Forbes and Neville ( I d ) may be quoted where a diene value of 16.66 (Kaufmann method) was found for a sample of castor oil which had been dehydrated with 16 per cent of kaolin. A diene value of 16.66 indicates that 19.2 per cent of the total acids present were conjugated. A study by Brod, France, and Evans (7) on the polymerization of the ethyl esters of dehydrated ricinoleic acid, in which the diene value of the middle portion of the distilled esters was found to be 28 (Ellis method), further confirms our conclusions. Since the theoretical value of the ethyl ester of 9,ll-linoleic acid (molecular weight, 308) is 82.4, a value of 28 indicates that 34 per cent of this compound was present in the mixture of pure esters. This would correspond to about 30 per cent of 9,ll-linoleic acid in the distilled fatty acid mixture from dehydrated castor oil, since these also contain the linoleic, oleic, and saturated fatty acids from the original castor oil, amounting to about 12 per cent (IO). Their value, therefore, is in agreement with the results obtained by us on the commercial fatty acids which, as stated, were found to contain between 22 and 30.9 per cent of the conjugated isomer.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1316
SECTION OF Casma
OIL
The value found by Brod and collaborators, therefore, does not eonlirm the figure given by Boeseken (6)quoted ahove, although the same procedure for the dehydration of the acid and for the preparation of the esters was apparently used in both cases.
Determination of Total Unsaturation
As i n l h e case of tung oil and other compounds containing conjugated double bonds, it could be expected that dehydrated castor oil will he only partially saturated with halogen in the customary treatment with Wijs solution. Gelber and Boeseken (14) have already found that 9,ll-linoleic acid gives a n instantaneous Wijs number of 91, which shows that i t behaves similarly to other conjugated systems in this respect. They observed that in the case of both eleostearic and 9,ll-linoleic acid the theoretical iodine value is obtained only upon treatment with large excess and on prolonged standing with the reagent for several days. Excess of reagent alone, however, is not sufficient to obtain theoretical values for tung nil, as shown in great detail by Ho, Wan, and Wen (16). Even with over 400 per cent excess of Wijs solution, only two of the three double bonds in eleostearic acid are attacked if the time is short and the temperature low (19). It could similarly be expected that in the case of dehydrated castor oil not all of the conjugated portion would take up halogen under the ordinary Wijs procedure, even if a large excess of reagent was used. The fact that the iodine value of dehydrated castor oil increases steadilv on Drolonzed exmsure durina- 10 days or more (24) confirms this. If Wijs reagent reacts primarily with only one of the two double bonds in 9,ll-linoleic acid, it is obvious that tbis method alone is not suited for studying the dehydration of castor oil. Suppose that in comparing two dehydrated castor oils made with different catalysts, one is found to have a Wijs iodine value that is 10 points lower than the other. The lower value may then be due to one of two causes. Either the catalyst used in this case was less efficient than the other and the dehydration was not so complete, or the dehydration
VOL. 32, NO. 10
DEnYnaaTloN PLANT proceeded to a greater extent according to reaction 2 and formed more conjugated double bonds than in the other case. Since the two oils may differ from each other as a result of both nf these causes, the conclusions that can be drawn by comparing Wijs numbers alone are exactly nil; i. e., they do not answer the question either as to the total number of new double bonds formed or as to the quantity of conjugated ones alone. Obviously it is necessary to use a method with dehydrated castor oil which will react as nearly as possible with all of the double bonds present, isolated as well as conjugated. A few tests with the Kaufmann iodine (17) and the RosenmundKuhnhenn methods (e$) showed that both of these, like the Wiis method. eave values which did not indicate the total unsaturation. The total unsaturation. on the other hand, can be determined accurately by a' quantitative hydrogenation and possibly by the method of Rossmann ($3). Furthermore, it was found in this laboratory (19) that standard Hanus solution (g), when applied in excess of a t least 400 per cent for one hour a t 20" C., will give reproducible constants which correspond fairly closely to the total unsaturation as calculated from the drop in hydroxyl value during dehydration. To demonstrate this, the increase of iodine value expected from the decrease of acetyl value is calculated for sample A (Table I). The castor oil used had an acetyl value of 145.5 and an iodine value of 85.0. The hydroxyl value, as calculated by . I
~
hydroxyl value
=
acetyl value
1
~
- 0.00075 acetyl value
was therefore 163.1. The dehydrated oil had an acetyl value of 13.5-i. e., a hydroxyl value of 13.7. The decresse in hydroxyl value then was 149.4, which expresses milligrams of potassium hydroxide equivalent to the hydroxyl groups in 1 gram of oil and corresponds to 47.8 mg. of water removed during dehydration. One gram of the castor nil originally added 0.850 gram of iodine. After dehydration 1 - 0.0478, or
OCTOBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.9522 gram of oil should add the same 0.850 gram of iodine plus an additional (0.0478 x 126.9 X 2)/18, or 0.674 grami. e., a total of 1.524 grams; therefore, the “total” iodine value Bhould be 1.524/(0.9522 x loo), or 160.1. The Hanus value found was 155.5, within 4 points of the ealculated total. In most cases where a similar calculation was carried out, the Hanus number was found to lie close to, although usually a few points below, the theoretical unsaturstion. It should be borne in mind that in oils with only isolated double bonds, such as linseed, the Hanus numbers are a few points below the theoretical. It appears, therefore, that the Hauus number determination represents a rapid method for determining, a t least approximately, the total unsaturation present, irrespective of the arrangement of the bonds in the isolated or conjugated position. Although the Wijs uumher alone, on the other hand, is of no value in comparing dehydrated castor oils, it may be u s e ful if determined together with the Hanus number. In order to obtain constant and reproducible results with Wijs solution on dehydrated castor oil, it is necessary to use about 400 per cent excess of reagent, just as in the Hanus number determination. This was shown by Forbes and Neville (IS), who also noticed that the iodine values thus obtained were considerably lower than those calculated from the amount of water formed in the dehydration of castor oil (12). Therefore, if the Wijs number is determined on dehydrated castor oil, using about five times as much reagent as the oil will consume, constant values are obtained which lie about 10 to 15 points below the Hanus number. Since oils of the linseed type, which contain only isolated double bonds, always give a Hanus number which is several points lower than the Wijs number. i t is evident that the
1317
and other condensation reactions also muse a reduction of the acetyl value. Any conchision as to the completeness of the dehydration derived from this value is therefore subject to the condition that such side reactions have been avoided during the dehydration of the oil. Among the various methods available for the determination of the acetyl value, the Andr4Cook procedure mommended by the American Oil Chemists’ Society (1) gives the most reproducible results. Unlike most other methods i t is not materially disturbed by the presence of any free acids in dehydrated castor oil, 88 shown by the low value obtaiued with the distilled fatty acids in column I , Table I.
DEHYDRATED C A ~ TOIL ~ RFILMSHOWINQ FROSTINO, TYPICAL WHENTWNQOIL AND OTHEB Oils WITB Comwnnmn DOWBLE Boxins ARE DRIEDWITAOUT DRIERS
It had been noticed that in dehydrating eastor oil, even with the strongest catslysts, the acetyl value does not drop to zero even on greatly prolonged treatment. A residual value is instead approached, which in no case has been found to be below 10 for raw dehydrated castor oil. All analyses which were made on oils dehydrated by different methods and by various manufacturers (including those of the samples listed in Table I) corroborate this observation. The acetyl value of high-quality raw oil was found to average from 12 to 20. Since most other raw drying oils have also definite acetyl values, frequently in the neighborhood of 10, this residual acetyl value of raw dehydrated castor oil seems immaterial. This is especially true since the oil is not customarily used in the raw state but is first bodied to a higher viscosity. In the bodying processes used commercially by the manufacturers TABLE I. Oil.
Acid value 88p”ni6astio” value Iodine vdue ( H e n u ) Iodine value (Wiia) Diene value (Ksufmaon) Acetyl due VisoositY (Gerdnei) Color (HaIliad Speoific ersvity (25’ C.) Hersbromide d u e Moisture. %
A 1.8
190.4 155.5
141.2 21.4
13.5 F-0 1-2L 0.9305 0.0 0.0
ANALYSIS OF
DEHYDRATED CASToR OILS AND C
B 2.0 190.0 147.1
4.4 192.7 143.8
15.4 14.3
15.5 24.8
1 0.935
Z3L
...
H
... ...
...
K
... ... ...
D 13.1 190.9 157.0 145.8 21.9 16.1
E F
2L
... ...
I . .
E 17.0 194.2 152.9
...
17.8
19.1 P 1-ZL
... ...
...
FA-
F
ACIDS
a
X
1.56
3.4 191.9
4.8 191.6
1j.i:6 17.6 10.6
1ii:s
iii:i
192.0
M 2L 0.9365
.... 0.0
12.7 6.7 23 ZL-2 0.9505
....
0.0
12.7 9.2 22-23 IL-l
... ... ...
I 195.9 197 3 105.2 153.0 26.5
1.2
