DRYING OILS AND RESINS Ultraviolet Absorption Study of Esters of the Acids of Drying Oils THEODORE F. BRADLEY AND DAVID RICHARDSON American Cyanamid Company, Stamford, Conn.
the monomeric distillates. This increased absorption is evidence of either increased unsaturation or increased complexity of molecular structure for these products. The decreased iodine values and other chemical evidence make ring formation the most acceptable explanation. Changes in the absorption spectra of linseed and tung oils upon heat bodying are presented; they indicate t h a t considerable amounts of unreacted acids are still present in these oils as the gel point is approached. In the case of linseed oil, there is a net increase in the amount of conjugated acids present in the first stages of heat treatment, followed by a gradual decrease.
Ultraviolet absorption methods have been applied to a series of samples prepared in the course of a fundamental study of the mechanism of the polymerization of some drying oils. Partial interpretation of the absorption curves has been accomplished through studies of analogous and related compounds. I t is shown that absorption spectra can be particularly useful in cases where conjugated double bonds are important. Some evidence can be deduced w-ith regard to the presence of cyclic or aromatic molecules in a given sample. The distillation of heat-polymerized methyl esters of the acids from a drying oil yields residues which absorb ultraviolet light more strongly than
HE application of ultraviolet absorption methods to studies of drying oils is not new, but i t is difficult, on the basis of previously published results (6, 10, 11, 16), to derive a clear picture of the value and limitations of these methods'. As a means of establishing these factors, the present study was made in conjunction with the work reported in two previous papers of this series (1, 2 ) * . Many of the spectroscopic results support the hypotheses presented in these papers. I n addition to a series of oils and related products, the absorption curves for a number of hydrocarbons have been investigated to provide a basis for the interpretation of the spectra of the oils in question.
where d L
T
= =
incident light optical density (log transmitted light length. in cm.. of solution of concentration c. in grams per 100 cc., through which the light passes
The horizontal scale is expressed as frequency in cm.-l (wave number)-i. e., reciprocal of the wave length in cm. (Figure 2). The region of the spectrum used in this work is 4000 to 2222 A. (25,000 to 45,000 em.-'). The long wave length limit is set arbitrarily; the short wave length limit is set by the strong absorption of the gelatin of the spectroscopic plates used in this work (Eastman, type 11-0).
Instruments and Methods
Interpretation of Absorption Curves
For the ultraviolet absorption measurements, a large Hilger, type E 492, quartz-prism spectrograph was used in conjunction with a Hilger Spekker photometer (Figure 1). Specially purified hexane or cyclohexane was used as the solvent. The solutions were examined in an adjustable micrometer absorption cell with quartz end plates. The operation of the photometer mas made automrttic through the application of a system of motors and relays which make successive density settings, time the exposures, and move the plate between exposures. The absorption curves presented here are plotted in terms of the logarithm of the extinction coefficient k , defined as
To illustrate the types of molecular structural changes which can be correlated with changes in the ultraviolet absorption spectrum, the curves (Figure 3) for hexane, cyclohexane, 3-hexene (S), cyclohexene, dimethylbutadiene ( 8 ) , lj3-cyclohexadiene ( 9 ) , p-cymene, and a-eleostearic acid (6) are presented. (Where no reference is given, the curve was measured by the present authors.) The curves for the series hexane, 3-hexene, and dimethylbutadiene or for the series cyclohexane, cyclohexene, and 1,3-cyclohexadiene illustrate the fact that general absorption increases with increasing unsaturation. The greatly increased absorption in the region of 43,500 cm.-l for dimethylbutadiene is characteristic of two conjugated double bonds in a straight-chain aliphatic compound. The three equally intense bands near 37,000 cm.-l for a-eleostearic acid indicate three conjugated double bonds. The greater absorptions of cyclohexane compared to those of hexane and of cyclohexene compared to those of 3-hexene suggest that for these, and possibly other compounds, cyclization increases the general absorption. The increased absorption in the 43,500 cm.+ region for 3-hexene
L
=
d/Lc
1 Just before this paper went t o press, J. P. Kass advised us of some rel a t e d spectroscopic investigations reported b y E. S. Miller and other of his associates in P r o c . Am. Soc. Bid. Chem., 32, 106 (19381, a n d in Oil and S o a p , 15, 62 (1938). We are glad t o direct attention t o this work a n d regret t h a t t escaped our notice until now. 2 Other papers in this series appeared in 1937, 1938, and 1!339.
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molecular change has taken place. Also, triglycerides and methyl or ethyl esters of the same acids have similar spectra. The addition of a hydroxyl group to an aliphatic compound has no effect on the spectrum. Upon polymerization, changes in amounts of conjugated or unsaturated material present are indicated, but no evidence as to the size of the molecule or particle size can he deduced from the curves. The foregoing summary of the molecular features indicated by ultraviolet (U. V.) methods, as well as those not indicated in this manner, emphasizes the fact that the characteristics of ultraviolet ahsorution suectra are determined largely by the unsaturaied parts of the molecule. Table I summarizes these observations. D r y i n g Oil Acids The esters of stearic, palmitic, or other completely saturated acids show hut slight general absorption in the near-ultraviolct region of the spectrum. Therefore, the presence of saturated acids in an oil can be inferred only from the fact that the absorption of the absorbing components is decreased by dilution. On the other hand, very small proportions of more strongly absorbing impurities can readily be detected in samples of supposedly saturated materials. This fact is illustrated by comparing the curve for tripalmitin (Figure 4) with the one for tristearin. The markedly increased absorption of the latter can be associated with its measured iodine value of 0.38 as compared with the iodine value of 0 for the tripalmitin. Oleic esters (Figure 4) and those of other acids with but one carbon-to-carbon double bond do have more general absorp lion than the saturated acids hut have no discrete bands in the ultraviolet region covered by this investigation. This increased ahsorption represents the long wave length beginning of a broad absorption band, the peak of which is to be found in the far-ultraviolet region. Any bauds found in the nearultraviolet spectrum of samples of an ester of oleic acid indicate the presence of less saturated impurities. AU the samples which are supposed to be high in 9,1% linoleic acid (e. g., methyl ester of Neo-Fat 3R, Figure 4) show a poorly defined band at 37,000 em.-'. Whether or riot this band is a property characteristie of this acid is not now evident. The intensity of this band does not appear in
shoukl probably he interpreted as indicative of the presence of a small percentage of a conjugated diene (3). The curve for p-cymene is an example of the spectrum of a simple disubstituted aromatic compound characterized by narrow bands in the region of 35,000 to 40,000 cm.-'. It is well known thut iodine values arc not reIiable measures of the number of double bonds present in an oil containing conjugated systems. To the spectrograph, however, conjugation is the most easily measured characteristic of an oil; consequently, spectral data, combined with iodine values, indicate the distribution of unsaturation between conjugated and nonconjugated acids. In the case of a sample which shows no spectroscopic evidence of conjugation hut which does have increased general absorption in conjunction with lowered iodine values, the surmise is that the sample is cyclic in structure. This effect is even more marked in cases where the sample is bicyclic. It must be remembered in all this work, hop-ever. thnt a trace of some highly absorbent compound present in a sample may easily lead to misinterpretation of the absorption curves. Before the ultraviolet absorption spectra can be employed to the fullest extent as an analytical tool and before one ban derive the maximum amount of valid information from the present data, it is believed that a larger number of pure compounds will have to he examined so that the neoessary background of fundamental data is available for reference. In contrast to the foregoing list of molecular characteristics which are detectable by ultraviolet absorption methods, some of the factors which do not appreciably affect the spectrum must be considered. The length or branching of carbonto-carbon chains bas little effect on absorption except as related to double bonds (9). For examole. a C,, acid would have a snectcum not
spectrum except as regards the conjugation or nonconjugation of two or more double bonds. The difference in the spectrum of an acid and that of its ester is smdl, provided no other
"CLIO" "SED
FIGURE 2. THE
RELATION BETWEEN WAVELENOTHS A N D WAVEXTUMBEM IN THE SPECTRUZf I-SED I N T H I S I N V E S T r C h X o A
REGION OF
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TABLE I MOL. CHARACTERISTICS EVIDENT FROM U. V. DATA
GENERAL EFFECT ON U. V. A4BSORPTIONCURVES
Degree of saturation (io- Absorption decreased with hydrogenadine No.) tion or loss of double bonds Pair of conjugated double Broad, intense absorption band with bonds max. near 43,500 cm.-l Three conjugated double Broad, intense absorption band -7ith bonds max. near 37,000 cm. -1 Cyclization Simple cyclic compounds have more general absorption than corresponding straight-chain aliphatic material (and may give appearance of increased unsatn. where this does not exist) Aromatic ring Several narrow bands between 35,000 and 40,000 cm. -1
3.0
2.5
2.0
I. 5
1.0
MOL. CHARACTERISTICS
z c Y
N O T EVIDENT FROM U. V.
0.5
v
DATA
h
8
Except as related to double bonds in the chain Except as related to one another-i. e., conjugation Satd. aliphatic esters and corresponding triglycerides are indistinguishable Mol. weight or particle size Polymerization evident only by effects on characteristics listed in table above
0.0
0 I-
I -0.5 W
0
s
- 1.0 -
REMARKS
Carbon chain length or branching Positions of double bonds in carbon chain Acid, ester, or alcohol
1.5
-2.0
- 2.5 -3.0
- 3.5 W A V E NUMBER
x 10-3 ( C M T ~ )
3.0
2.5
all cases to be proportional to the amount of 9,12-1inoleic acid thought to be present. The absorption spectrum of 9,ll-linoleic acid (IO), Figure 4, is characterized by the broad, intense band a t 43,500 cm. already indicated as being associated with the chaintype aliphatic compounds containing two conjugated double bonds. This band is so intense when compared with other possible sources of absorption in this region that it serves as an accurate measure of the proportion of the acids of any oil which have two conjugated double bonds. The spectrum of ethyl linolenate (three nonconjugated double bonds) was recorded by van der Hulst (IO),Figure 4. The two weak absorption bands of moderate width shown as 31,800 and 33,200 cm.-’ are also seen in the spectra of raw linseed oil (Figure 5 ) , linseed methyl esters (Figure B), and soybean esters (Figure 10). These bands are certainly associated with the presence of linolenic acid, but their origin has not been settled. Edisbury (7) suggested that these bands
2.0
c z W 1.5
u
FIGURE 3 ( t o p ) . ABSORPTIOSc:URVES
Y
s
1. Hexane 2 . Cyclohexane 3. 3-Hexene ( 3 ) , contains some conjugated diene 4. Cyclohexene, not free of dienes 5. Dimethylbutadiene (8) 6 . 1.3-Cyclohexadiene (9) 7. a-Eleostearic acid ( 6 ) 8. p-Cymene
0 V
z 0.5 x
Y
0 -I 0.0
-
FOR VARIOUS H Y D R O C A R -
BOKS
1.0
FIGURE 4 (bottom). ABSORPTIOSC ~ R V EFOR S DRYING OIL ACIDS . ~ K DESTERS
0.5
Curve 1
- 1.0
2
3 4
5
WAVE NUMBER
x
1 0 - 3 (cM.-I)
6 7
Compound Iodine No. Tripalmitin 0 Tristearin 0 38 86 5 Methyl oleate (contains dienes) Methyl oleate (after treatment with maleic anhydride) 83 hIethyl ester of Sieo F a t 3R (large11 9,12-11noleate) 140 9,ll-Linoleic acid (f0) 163.4 E t h y l linolenate (IO) 246.9
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1.5
f.0
0.5
0.0
0.5
1.0
28
28
30
32
M
34
WAVE NUMBER
x
10-3
38
40
42
44
(cM?)
FIGURE 5. ABSORPTION CURVESFOR LINSEEDOILS(12) 1. 2. 3. 4.
Raw Heat-bodied No. 1 Heat-bodied No. 2 Heat-bodied No. 11
are due to the presence of small quantities of cyclized monomers formed from polyethylenic acids of the type of linolenic acid. The most striking absorption bands encountered in ultraviolet studies of the drying oils are the group of three intense, unresolved bands characteristic of the three conjugated double bonds of the eleostearic acids of tung oils (Figure 3) and also seen in the spectrum of oiticica oil (Figure 12). These bands, found a t 35,500, 37,000, and 38,300 cm.-’, were studied by Dingwall and Thomson (6); these authors can distinguish between a- and /3-eleostearic acids and assay mixtures of them on the basis of slight shifts in the exact positions of the bands. I n a mixture of raw oils the proportion of acid with three conjugated double bonds can readily be determined from a measurement of the absorption at 37,000 cm.-l. The significance of the weak band near 31,750 cm.-l in the curves for both tung and oiticica oils has not yet been determined. It may have an origin comparable to that of the similarly placed band associated with linolenic acid.
Linseed Oil The ultraviolet absorption spectra of a series of linseed oils which were heat-bodied in a vacuum a t 575’ F. are given in Figure 5. They represent stages in the heat-bodying of linseed oil and have been described in detail (1.2). The first effect of heat treatment seen in sample 1is the disappearance of the “linolenic” bands a t 31,500 and 33,000 cm.-l, accompanied by an increase in the band a t 43,500 cm.-l, which indicates a n increase in acids with two conjugated double bonds, This marked change in the spectrum is accompanied by corresponding changes in the physical and chemical properties, including a great decrease in hexabromide number, which also indicates disappearance of linolenic acid. This is good evidence in support of the hypothesis first presented by Scheiber (14) that, in acids containing two or more nonconjugated double bonds, there is a shift of double bonds, t o conjugated positions before polymerization occurs. The increased absorption with prolonged heating has been noted by other workers (4,6, 7, IS) in connection with the saponification of linseed oils. They do not state that i t represents an
FIGURE6 . ABSORPTION CURVES FOR LIXSEED ESTERS Distn. a t Curve Esters 1 Mm., O C. 1 Methyl, of linseed oil acids 155-68 2 Nonvolatile 24-hr. dimer 3 Firat 24-hr. monomer fraction
