before identification and estimation. Where a complete analysis of the impurity concentrates is not possible by Chromatography, as with the analysis of the iso-octane samples described, gasliquid chromatography enables the scope of existing analytical methods to be extended to lower concentration levels. ACKNOWLEDGMENT
The authors thank the Chairman of The British Petroleum Co., Ltd., for permission to publish this paper and the Chairman of the Section on Reference Fuel Analysis of Committee D 2 ASTilI for permission to include the results of the analyses of the isooctane samples. They also wish to thank L. M. E. Collyer, who carried out the mass spectrometric analyses. LITERATURE CITED
(1) Auvazov, B. V., T’yakhurev, D. A,, J . A p p l . Chem. (U.S.S.R.) 26, 467 (1953). (2) Bradford, B. TI’,, Harvey, D., Chalkley, C. B., J . Inst. Petroleum 41, 80 I 1 955). \ - - - - / .
(3) Brooks, D. B., Cleaton, R. B., Carter, F. R., J. Research Y a t l . Bur Standards 19, 319 (1937). ( 4 ) Callear, A . B., Cvetanovic, R. J., Can. J . Chem. 33, 1256 (1955). (5) Cropper, F. R., Heywood, A., Nature 172, 1101 (1953). Zbid., 174, 1063 (1954). Davidson, W. H. T., Chemistry & Industry 1954, 1356. Dijkstra, G., Keppler, 6. G., Schols, J. A , , Rec. trav. chim. 74, 805 (1955). (9) Glasgow, A. R., Jr., Streiff, A. J.,
Table VII.
Complete Analysis of Impure and Certified Samples
Certified Sample Impure Sample iii$ Composition, Composition, /O molar ratio Impurities 70 molar ratio Lower boiling 0.165 2,3-Dimethylpentane 80 Xi1 Nil =!=O 02 3-Methylhexane 11 2-Methylhexane 9 0,022 CTSaphthene 90 Higher boiling 0 022 CTKaphthene 90 (methylcyclohexane) 10.01 (methylcyclohexane) 10 10 =to,01 Paraffins Paraffins 0.022 Total 0.187 iro.01 dz0.03 hIole
Willingham, C. B., Rossini, F. D., Proc. Am. Petroleum Inst. 26, 111,
127 (1946). Goldschmeidt. G.. Monatsh. Chem. 2, 433 (1881).‘ ’ Griffiths. J. H.. James. D.. Phillios. C . S. G,, Anklyst 77,‘897’( 1952): ’ Griffiths, J. H., Phillips, C. S. G., J. Chem. SOC.1954, 3446. James. A. T.. Biochem. J. (London) 52, 242 (1952). James, A. T., Mjg. Chemist 26, 5 (1955). James, A. T., Research (London)8, 8 (1955). James, A. T., Martin, A. J. P., Analyst 77, 915 (1952). James, A. T., Martin, A. J. P., Biochern. J . (London) 50, 679 (1952). Zbid., 63, 144 (1956). James, A. T., Martin, A. J. P., Brit. iMed. Bull. 10, 170 (1954). James, -4.T., Martin, il. J. P., J. Avvl. Cheni. (London) . 6 ., 105 (igi6). (21) James, A. T., Martin, A. J. P., Smith, G., Biochem. J . (London) 52, 238 (1952). (22) James, D., Phillips, C. S. G., J . Chem. SOC.1953, 1600.
(23) Lichtenfels, D. H., Fleck, S. A., Burow, F. H., ANAL.CHEM.27, 1510 (1955). (24) Littlewood, A. B., Phillips, C. S.G., Price, D. T., J. Chem. SOC.1955, 1480. (25) Martin, A. E., Smart, J., Nature 175, 422 (1955). (26) Martin, A. J. P., Synge, R. L. M., Biochem. J . (London) 35, 1358 (1941). (27) Patton, H., Lewis, J., Kaye, W., ANAL.CHEM.27, 170 (1955). (28) Phillips, C. S. G., Discussions Faraday SOC.7, 241 (1949). (29) Ray, N. H., J. A p p l . Chem. (London) 4, 21 (1954). (30) Ibid., p. 82. (31) . . Scott, R. P. K., Nature 176, 793 (1955). (32) Sullivan, L. J., Lotz, J. R., Willingham, C. B., ANAL.CHEM.28, 495 (1956). RECEIVED for review June 11, 1956. AC cepted November 30,1956. Group Session on Analytical Research, Division of Refining, American Petroleum Institute, Montreal, Canada, May 1956.
Characterization of Long-chain Fatty Acids by infra red Spectroscopy RUDD A. MEIKLEJOHN’, ROBERT J. MEYER2, SANFORD M. ARONOVIC, HENRY A. SCHUETTE, and VILLIERS W. MELOCH Department of Chemistry, University of Wisconsin, Madison, Wis.
b Fatty acids may b e readily identified by infrared spectrophotometry of the solid compounds. Examination of the spectra of soaps as well as of the free acids by the potassium bromide disk technique has definite advantages, chief among which is the revelation of the “rule of two” relationship between the so-called band progressions and chain lengths. Infrared spectra of synthetic solid fatty acids of exceptionally high purity from C ~ to O C36 are presented, as well as the band progressions for soaps
from C3 to C36, to demonstrate this relationship and provide a catalog of spectra for direct identification.
T
HE EFFECTIVE USE of infrared spectroscopy for positive identification of the fatty acids has been subject to several difficulties. The liquid state offers few unique bands. I n the solid state the spectra contain many bands, but slight differences in preparatory techniques result in spectral discrep-
ancies (6). Gore and Kaight (4) have distinguished the shorter acids by measurement of the ratio of intensities of C-H stretching bands due to CHa and CH3 groups, but this technique becomes impractical for Cls and longer chains because of the smaller difference betmeen homologs. I n 1952 Jones, McKay, and Sinclair 1 Present address, Minnesota Mining and Manufacturing Co., Minneapoli8, Minn. 2 Present address, College of Pharmacy, University of Wisconsin, ;Cladison, Wis.
VOL. 29, NO. 3, MARCH 1957
32$
(6, 9) showed that the fatty acids had unique infrared spectra as Nujol mulls, and could be qualitatively distinguished from each other. They also called attention to a remarkable spectral feature in the µn region: a series of uniformly spaced bands whose number increased with chain length. The term "band progression" was applied to such a series of bands. Brown, Sheppard, and Simpson (1, 2) showed that these bands are attributable to CH2wagging. Findings in this laboratory indicate that comparison of the spectra of crystalline films may possess certain drawbacks. Because of the tendency of fatty acids to occur in several polymorphic forms and the frequent coexistence of ti?-o or more of these fornis in a single film, different film preparations of the same acid may show spectral discrepancies if the solidified capillary film technique is used. A second difficulty is caused by orientation of the solid film and partial polarization of the radiation within a spectrophotometer. I n this case significant intensity changes in several bands can be observed merely by repositioning a single film preparation within a cell holder. Such spectral differences as may occur raise doubt about the validity of an identification. The present authors have, therefore, sought to overcome these difficulties by adopting a reproducible sample-handling technique, and basing identification not merely on the ability to obtain an identical spectrum, but on the correlation of a prominent spectral feature with the chain length. Correlation of chain length with the number of bands in the band progression series of solid mulls of fatty acids was first made by Jones and his coworkers (6, 9). They recognized three bands in lauric acid (C12),nine bands in heneicosanoic acid (CZl),and generalized for the series C16 to Col that the number of apparent progression bands increases by one for each additional methylene group. Primas and Gunthard ( 7 ) have shown that the number of bands due to CHs wagging in compounds of the type R'CO(CH2CHr),COR" bears an approximate relationship of one band for each four consecutive methylene groups. In this laboratory, i t has been observed for fatty acid chains longer than twelve carbons that one band is, in fact, present for every two carbons in the chain. The band progression region of the soaps may be defined as 7.43 to 8.47 microns (Figure 4), and the bands crowd together when they become more numerous. Childers and Struthers (3) have prepared the sodium soaps of some shortchain fatty acids. The spectra had sharp, unique peaks, and the preparation of the soaps opened the way to reproducible spectra. The samples were prepared as quantitative mulls, and
330
ANALYTICAL CHEMISTRY
IO0 80
60 40
20 0 80 60 40
20 0
3
80
60
k 4 0
@ 2c
$ 0 I80 I-
5 6C
O4C
@z2c
kf
C
0c 6C 4c 2c
C
8C
6C 4c 2C
C 3
Figure 1,
4
5
6
7 8 9 10 I 1 12 WAVE LENGM IN MICRONS
I3
I4
6
Spectra of normal aliphatic acids in potassium bromide matrix 0.01 mmole of fatty acid In 12-mm. diameter circular disk
both qualitative and quantitative determinations were shown to be practicable. The sodium, barium, and silver soaps have been prepared in this laboratory, and excellent spectra have been obtained for their potassium bromide disks. More of the progression bands are apparent for a soap than for the corresponding acid, and this is due to the removal of an interfering peak a t 7.65 microns attributable to the C-0-H grouping. The progression bands of different soaps of the same acid show no alteration in position. The work reported here proposes that the spectrum of an unknown free acid be obtained by the potassium bromide pellet technique, and then compared with a
reference spectrum for identification. If doubt as to the identity persists, the soap should be prepared, mixed into a potassium bromide disk, and the progression bands in the spectrum counted. An acid with an odd number of carbons may be distinguished from the next longer even-numbered acid which has the same number of progression bands by the shift in wave length of the entire set. Comparison of the progression bands enables identification of a straight-ohain fatty acid even if a reference spectrum is not available. Branched acids give similar bands which are nonuniformly spaced, so that the straight and branched types may be readily distinguished. A group of spec-
FREQUENCY IN CM' I oc
4ooom
200
150;)
1ooo9008x)
m
0c 6C 4c
2c C
0c
6C 4c
2c C
0
2
8C
6C
k 4 C
2
cn
z 2 c
a
K
+
O a0
i-
6 60 O40
3
20 0 80 60
about 1 mm. thick and 12 mm. in diameter. Preparation of Soaps. BARIUM. Several milligrams of the free acid was dissolved in about 3 ml. of methanol. One milliliter of a 1% solution of barium chloride dihydrate in methanol was added. Concentrated aqueous ammonia (28%) then was added dropwise to the hot methanolic solution until no additional precipitate was formed. The precipitate was centrifuged, washed twice with 3-ml. portions of hot methanol, and dried a t 110' C. The barium soaps of the Csto Ca6acids were made. SODIUM.Aqueous sodium hydroxide, LY, rvas used to titrate an ethanolic solution of the acid to a phenolphthalein end point. The solution was then evaporated to dryness a t 110' C. The SOdium soaps of only the Ca, C4, and CS acids were prepared. SILVER. Silver nitrate (1.7 grams) rras dissolved in 5 ml. of distilled water, and 80 ml. of absolute ethyl alcohol mas added, Concentrated aqueous ammonia was added drop\\-ise until the brown turbidity which first formed just disappeared, and the solution mas made up to 100 ml. with ethyl alcohol. The silver solution was added dropwise to the hot methanolic solution of the acid until no additional precipitate formed. The white precipitate was centrifuged, washed several times with hot methanol, and dried in a vacuum oven a t 60' C. The silver soaps of the CS to Czoacids were made.
40 APPARATUS
20
The spectra shown Fere obtained by use of the double-beam Baird Associates Model B infrared spectrophotometer. The reference used in all cases was a polished rock salt plate. All spectra reproduced herein are the originals. Figure 4 is a montage of actual curves obtained by use of a calcium fluoride prism, but all other work was accomplished with a sodium chloride prism.
0
80 60 40
20 0
Figure 2.
Spectra of normal aliphatic acids in potassium bromide matrix 0.01 rnrnole of fatty acid in 12-mm. diameter circular disk
tra of normal aliphatic acids from C ~to O CSsis presented-for reference in Figures 1, 2, and 3.
lower purity. Barium chloride dihydrate, silver nitrate, and anhydrous methanol used in the preparation of the soaps were all of reagent grade,
MATERIALS
PROCEDURES
The,Clo to C~P, normal aliphatic acids were prepared synthetically by Schuette and coworkers (8) a t the University of Wisconsin. The methods and intermediates used in the syntheses lvere selected so as to leave no doubt about the purity and structure. Estimated purity m-as 99.9% or better, as established by melting point data and cooling curve observations. The shorter acids, CS, C4, and Cg, used only in the form of soaps to extend the family of progression bands, were generally of
Preparation of Potassium Bromide Pellets. Freeze-dried potassium bromide (200 mg.) was mixed with 0.01 mmole of acid or soap. A small amount of carbon tetrachloride was added to the powders in an agate mortar and triturated to form a smooth paste. After evaporation of the carbon tetrachloride in 1 minute, trituration was resumed for 1 minute longer. The powder was pressed in a hardened tool steel die under a force of 10 tons for 2 minutes in vacuo to form a flat disk
RESULTS A N D DISCUSSION
The spectra presented in Figures 1, 2, and 3, obtained from potassium bromide disks, show greater detail than spectra usually afforded by solid films. An important reason for this difference is the minimization of polymorphism in the former. In addition, instrumental polarization of the infrared radiation can cause nonreproducible spectral variations when the specimen exhibits overall crystallographic orientation, as may frequently happen in solid films. The more nearly random orientation of sample in the potassium bromide disk minimizes the orientation-polarization effect. This technique, therefore, has the advantages of yielding sharper and more reproducible spectra and is recommended over the solid film technique for those fatty acids obtained as crystals. The 7.4- to 8.5-micron region deserves special attention. The number of bands VOL. 2 9 , NO. 3, MARCH 1957
331
I oc
4ooom
rn
1500
FREQUENCY IN CM." 1ooo900
7.65 microns is eliminated, and the rest of the progression bands are unmasked. The patterns exhibited by the progression bands are shown in Figures 4 and 5 . Their continuity in the spectra of the soaps from Ca to C30 is most striking. The family of curves obtained from the free acids is not identical n-ith, although it is similar to, the family of curves obtained from the soaps, revealing a n influence on all of the bands by the end group. Close esamination of the band progressions of the soaps as given in Figure 5 reyeals the following anomalies.
m
830
8C 60 40
20
0
w 8o 0
Z 60
g z
40 20
In the soaps below C16, weaker intermediate bands can be seen which are not regularly spaced. Their intensities gradually become relatively negligible in chains longer than (312. To use the rule of two, these intermediate bands must be disregarded. Their existence in the shorter acids indicates that one band is probably produced for each methylene group, but that limitations on resolution allow detection of only half of them in chains longer than c16. The band at the shortest 'ivave length is always closer to the second band than the spacing of the remainder of the progression bands in each set. This effect may be related to the extraordinary
s 80o 60
5 04 0 cc F
20
O 80 60 40
20
0
3 Figure 3.
4
5
6
0 II 12 7 8 9 WAVE LENGTH IN MICRONS
13
14
5
Y
Spectra of normal aliphatic acids in potassium bromide matrix 0.01 mmole o f fatty acid in 12-mm. diameter circular disk
in the progression series is approximately equal to half the number of carbons in the chains. By empirically defining the region as 7.43 to 8.47 microns, the following relationship exists for the straight-chain fatty acids with an even number of carbons, Number of bands in progression series = Number of carbons in chain 2
For the acids with a n odd number of carbons, Number of bands in progression series = Kumber of carbons in chain 1
+
2
This rule of two relationship is obvious from the spectra of the solid acids from Clo through C ~ Rbut , in the longer acids only the bands a t the long wave length end of this region remain distinct. That the shoulders on the broad band a t 7.65 microns are really members of the band progressions can be brought out more clearly by preparation of soaps of the acids. I n this way, the band at
332
ANALYTICAL CHEMISTRY
Figure 4.
of soaps
Family of curves relating progression bands
guishable composite spectra. In such cases, the mixture of acids may be converted to a mixture of soaps, and the spectrum obtained. The irregular series of progression bands resulting then may be visually unscrambled into sets of uniformly spaced bands. The separated sets can be used to help identify the components. Verification is made by comparison of the remainder of the spectrum .rvith reference spectra. Figure 6 s h o w tJvo specific examples of this application. CONCLUSIONS
The use of the potassium bromide disk technique, in which melting of the crystallized fatty acid is avoided, results in sharper, more reproducible spectra. If an unknown fatty acid is not crystalline or is somewhat impure, preparation of the soap will still permit rapid evaluation of the chain length if progression bands are counted and the rule of two is applied. Even in half-and-half mixtures, the components can be identified by a visual unscrambling of the two sets of uniformly spaced bands. LITERATURE CITED
don A247, 35-58 (1954). ( 3 ) Childers, E., Struthers, G. K., hx.4~. CHEW.27, 737-41 (1955). (4) Gore, R. C., Waight, E. S., in "Determination of Organic Structures by Physical Methods," Braude, E. rl.,
WAVE LENGTH IN MICRONS
FREQUENCY IN CM.-'
FREQUENCY IN CM.-I Figure
5.
1400
Band progressions of soaps of fatty acids
All from barium soaps except for C3, C4, soaps were used
CB,for
1300
1200
I I00
which sodium
w *O
w
0 2
M
chemical reactivity of the a-methylene group, which n-ould be concomitant with a weaker C-H bond and/or a greater C-H distance, causing a methylene wagging slower than otherwise would be expected. The rule has utility even if used solely on an empirical basis, provided the above-mentioned disparities are taken into account. It may be difficult to decide, solely on the basis of the spectrum of a mixture, which component acids are present unless other information is available. More than one combination of mixed fatty acids might give nearly indistin-
st
2t 5z 60
5z
a I-
-l
60
a a
a
I-
5
40
W
v
40
V
K
2 20
1 '6
0 7
0
I
L
MIXTURE
0 9
WAVE LENGTH IN MICRONS
7
8
9
WAVE LENGTH IN MICRONS
Figure 6. Identification of half-and-half mixture of soaps by separation into uniformly spaced progression bands VOL. 29, NO. 3, MARCH 1957
333
Nachod, F. C., eds., 219, Academic Press, Xew Yorf; 1955. (5) Jones, R. N., McKay A. F., Sinclair, R. G., J. Am. dhem. Sac. 74, 2575-8 (1952).
(6) Jones, R. N., Sandorfy, C., in “Chemical Applications of Spectroscopy,’’ vol. IX of “Technique of Organic Chemistry,” A. Weissberger, ed.,
p. 295, Interscience, New York, 1956.
(7) Primas, H., Gunthard, H. H., Helv. Chim. Acta 36, 1659-70 (1953). (8) Schuette, H. A., others, Oil & Soap 16, 209 (1939); 17, 155 (1940); 20, 263 (1943); 22, 107, 238 (1945); 25, 64 (1948); 28, 361 (1951). (9) Sinclair, R. G., McKay, A. F., Jones,
R. N., J . Am. Chem. SOC.74, 25705 (1952).
RECEIVEDfor review June 25, 1956. Accepted November 7, 1956. Taken in art from a thesis submitted by Sanford . Aronovic in partial fulfillment of the requirements for the degree of doctor of philosophy.
&
Infrared Absorption of Aldehydic C- H Group Ortho-Substituted Benzaldehydes SHRAGA PINCHAS Weizmann Institute o f Science, Rehovoth, Israel
,Sixteen benzaldehydes, ten of which were substituted in the ortho position, were measured in the aldehydic C-H region in order to test the assumption that the increase in the C-H frequencies of benzaldehydes containing a proton acceptor in the ortho position is due to hydrogen bonding. All these aldehydes obey the observed regularities and fall into two groups. The first, containing unbonded benzaldehydes, absorbs at 2 7 2 0 to 2 7 4 5 and 2 8 1 2 to 2832 cm.-‘. The second, containing hydrogen-bonded aldehydic C-H groups, absorbs a t 2 7 4 7 to 2 7 6 5 and 2860 to 2900 cm.-*. The origin of the second band is discussed. The results verify the assumption that the cause of the rise in the C-H frequencies is a hydrogen bond affecting the aldehydic C-H group. Thus a clear-cut differentiation between an orthosubstituted and a nonsubstituted benzaldehyde is possible in many cases.
A
the C-H stretching frequency of the formyl group in various substituted benzaldehydes is usually in the narrow region of 2720 to 2740 crn.-l, in the case of a number of orthosubstituted benzaldehydes it rises to about 2760 cm.-l (81). The other characteristic band for the formyl group a t about 2820 cm.-l also rises in these benzaldehydes to the region of 2860 to 2890 cm.-l. As this increase in frequency could be correlated with neither the electronic effects of the ortho substituents nor their steric effects and because this increase was observed only with benzaldehydes that n-ere substituted by a proton acceptor but not with o-tolualdehyde, it was tentatively assumed to be due t o some kind of hydrogen bonding between the polar hydrogen atom of the formyl group and the substituting proton acceptor. As LTHOUGH
334
ANALYTICAL CHEMISTRY
such a hydrogen bond involving a C-H group seemed somewhat uncommon and the number of the “anomalous” benzaldehydes already studied then was small (only five) it seemed worth while to study the infrared spectra in this region of more benzaldehydes, especially ortho-substituted ones, in order to find whether this assumption is consistent with more experimental data. It was also interesting to see if a frequency of about 2760 cm.-’ for the formyl C-H stretching can really be taken as a reliable proof of the presence of a proton-accepting atom or group in an ortho position, in an unknown substituted benzaldehyde. An additional number of benzaldehydes were therefore measured in the infrared region. EXPERIMENTAL
Most of the measurements were made with a Perkin-Elmer infrared spectrometer, Model 12 C. Some of them were made with a direct current PerkinElmer spectrometer, Model 12 B. Unless otherwise stated, measurements were carried out with a sodium chloride prism. The materials studied were commercial of the highest available purity, used without further purification, were obtained as a gift from various organic chemists, or were synthesized according to the literature. 2-Chloro-3- and 2-chloro-5-hydroxybenzaldehyde were synthesized by (quick) chlorination of m-hydroxybenzaldehyde in acetic acid (iyhich contained some water). Under these conditions, contrary to Lock and Hosaeus ( l 7 ) ,both isomers are formed, the latter in larger quantities. They were separated by dissolving the mixture in a sodium carbonate solution and precipitating with hydrochloric acid. 2-Chloro-3hydroxybenzaldehyde came out of the solution first and melted after crystallization from dilute acetic acid, a t 136’ C. According to Hodgson and Beard (11) it melts a t 139’. 2-Chloro-5-
hydroxybenzaldehyde (6-chloro-mhydroxybenzaldehyde) was then precipitated from the filtrate on addition of more acid and standing. After crystallization from dilute acetic acid it melted a t 113.5-114.5’. Hodgson and Beard (11) give 111’ as its melting point. 2-Nitro-5-fluorobenzaldehyde was synthesized by Jose Schwarcz (24). RESULTS
The results of the measurements of the infrared absorption in the formylic C-H region in the case of 16 benzaldehydes (ten of which can be described as ortho-substituted) are summarized in Table I. Apart from the measurements in this region, some materials were also studied in the whole 2.5- to 12-micron region, in order to detect possible irregularities in the spectra of the anomalous benzaldehydes. The main bands observed are presented in Table 11. Experimental conditions are the same as those of Table I. The results for salicylaldehyde are in good agreement with the values read from the curve given by Barnes and associates ( 5 ) for the 1050 to lSOO-cm.-l region and the value of 1666 cm.+ found for its C=O frequency in carbon tetrachloride solution agrees well with the value of 1661 cm.-1 given by Bellamy (6) for this frequency in chloroform solution. The value of 1698 cm.-’ for this frequency in 2-naphthaldehyde also is in accord with the value of 1702 cm.-‘ given by Hunsberger (18). Both 3-chlorobenzaldehyde and its 4isomer have been measured by Lecomte (16) in the region up to 1200 em.-’; the results are comparable. DISCUSSION
2730-Cm.-‘ Frequency. As can be seen from Table I, all the benzaldehydes in which no hydrogen bond that
