infrared Absorption Spectra Some Long-chainFatty Acids, Esters, and Alcohols 0. D. SHREVE
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R. IIEETIIER, E. I. du Pont de Sernours & Company, Inc., Philadelphia 46, Pa. AND
I*. B. KNIGHT AND DANIEL S W E K S , Eastern Regional Research Laboratory, Z'hiladelphia I8, Pa.
Infrared absorption spectra from 2 to 15 microns are given for a number of pure long-chain saturated and monounsaturated fatty acids, methyl esters, triglycerides, and alcohols. The four classes of compounds studied are readily distinguished by spectral features common to the members of each class. Within each class, trans compounds are readily distinguished from cis and/or saturated compounds, but the two last-named types can be differentiated onl? b! careful examination of their
spectra. Distinction between individual cis or trans or saturated compounds within a class also requires careful study of the spectra. Terniiudly uneaturated compounds are readily distinguished from the internally unsaturated and/or the saturated types. Correlations of absorption bands w-ith molecular structure are given for all spectra. The spectra shoitld be useful in the application of the infrared method to studies involving fats and other longchain systems.
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A preliminwy step in the application of infrared spectroscopy to a variety of problem being conducted in the authors'-laboratories, and in particular to studies of the reaction of fatty materials with oxygen, i t was necessary to olitain refere w e spectra from 2 to 15 microns on a wide variety of pure longchain compounds. With few exceptions (1, 2f) publkhed spectra on long-chain aliphatic compounds cover a limited range (10, 18, 20) or the purity of the starting materials is unknown ( 3 ) . In this paper, infrared absorption spectra from 2 to 15 microns are reported for some pure long-chain monounsaturated and saturated fatty acids, esters (methyl esters and triglycerides), arid alcohols. Interpretations of these spectra have been made on the basis of correlations evident from the absorption curves in conjunction with both published and unpublkhed infrared data. EXPERIMESTAL
Spectrophotometer. All spectra were autornatic:illy recorded with a Beckman IR-2 infrared spectrophotometer housed in an air-conditioned room and maintained at 25' =t0.1 ' C. by the constant temperature circulating bat.h supplied with the inst)rument, The slits were automatieally adjusted during operation x i t h the aid of a mechanical slit drive ( 2 5 ) . Reproducibility of slit width3 was equal to t,hat attainable by manud settings. The instrument was calibrated by means of known absorption bands of cut~l~oii dioxide and ammonia. Liquid samples were run directly in a 0.03-mm. standard Beckman liquid cell consisting of two rock salt windows sep:ir:tterl by a n amalgamated copper spacer; solids were run as dilute solutions in carbon bisulfide in a 1-mm. standard Heckman cell. Esaet cell thicknesses, determined interferomrtricnlly ( U ) ,were 0.0321 and 1.0538 mm. Per ccnt transmittance curves for the solid samples were obtained by plotting rat,ios of recorder deflections for solution plus cell to the corresponding deflect,ions for the cell filled with pure carbon disulfide. A similar procedure was emplo>wl for the liquid samples, except that a rock salt plate equal in thickness to the combined thicknesses of the two cell windows was used as reference blank. Materials Used. The reference compounds employed, somr of their characteristics, and methods of preparation are shown in 'l'ablc. I. RESULTS
Figures 1 to 4 s h o spectra ~ for the various compounds a7. plots of per cent transmittance against wave length on a uniform wavclength scale. Sample form is indicated on the right side of each figure and wavelength positions of absorption maxima are tabulated on each curve. Correlation of Absorption Bands with Molecular Structure. Most of the absorption bands appearing in these spectra can be
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Figure 1. Infrared Absorption Spectra of Pure LongChain .\Ionounsaturated and Saturated Fatty Acids
i,olatcd to specific structuriil fcatwes of the molecules. The assigriments made in the following discwsion are based on a comparative study of these and a number oi the authors' unpublished infrared spectra, together with generalizations drawn from puhlish(3d frequency correlation charts ( 4 , S I ) and other data from the litewturr cited. Although mast of these assignments are believed to be reliable, some must lx considered tentative, particularly those relirting to carbon-oxygen absorption in the µn region and hydroxyl bending vibrations. Spectral Features Common to All Compounds. The following absorption bands occur a t nearly constant wave length in the spectra of all compounds studied, except 10-hendecenoic acid, regardless of cluss :
V O L U M E 2 2 , NO. 1 2 , D E C E M B E R 1 9 5 0 1. A stron band near 3.3 microns. In the methyl esters, triglycerides, and alcohol., this band is due entirely to C-H stretching vibrations; in the acids, however, as explained below, absorption due to stretching vibrations of the bonded hydrosyl group (0-H. , , .O) comhmes with that due to C-H to produce the total absorption obwrved in this region.
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2 . A single or doublet absorption nrar 6.9 or 6.9 and i microns related to C-H bending (.Gj 31 ). This absorption does not appeal’ in the case of solids, run as dilute solutions in the thick cell, as complete absorption of the solvent rendered the instrument inactive in this region. 3. A band (or inflection) ranging in wave length from about 7 . 2 t>o7.4 mirrons caused by symmetrical deformation vibrations of the methyl group ( 5 2 ) ,wit,h the exception of 10-hendecenoic acid, which does not contain this group. 4. -4band near 13.9 microns. A band near this wave length has been observed in spectra of a large number of hydrocarbons containing a continuous chain of four or more carbon atoms ( 3 2 ) and reccntly h:ts been ascribed to a CH? wagging mode ( 2 3 ) . Tn the cis nionoiinsaturated compounds, out-of-plane bending vibrations of the hydrogens attached to the double bond carbons probably contribute to the total absorption observed in this region. Spectral Features Common to Members of Each Class. In addition to the common bands discussed above, the individuals within each of thr four classes of compounds studied show a nuniher of common spectral features which scwe for class distinction. LOXG-(:H.IISF.~TTT ACIDS (Figure 1). The band near 3.7 microns occurring on the long wave-length side of the strong C-II stret,ching ithsorption has bren observed in the spectra of a nuintler of organic acids and has been attributed to the presence of thp carliosyl group in such compounds ( 2 ) . The presence of this band, as ell as thr. a h e n c r of any resolved 0-H stretching baritl near 3 microns, is explained hy the fact that cttrbosylic acids associate through the formation of hydrogen bonds, even at fairly high dilution in nonpolar solvents (12, 1 3 ) . 111 the lower fatty acids and :it lesst one higher acid (lauric) (10, 18, 20), infraWAVELENGTH, M I C R O N S red ahsorption measurements in the %micron region have been employed by a number of investigators (9, 11-15) in the study of Figure 2. Infrared ribsorption Spectra of Pure Longthis association, In general, these and similar studies show that, Chain Monounsaturated and Saturated llethyl Esters although the normal free 0 --I1 stretching band in the nionomeric acid molecule occurs Table 1. Characteristics of Reference Compounds near 2.8 microns, in the dimer (associated) moleculc the ahMethod of PreparaSerit. Sapon. tion and/or Litera31. P.. Iodine Reference sorption inasimum due to n $ o ture Reference E ‘1 ii i v . Eqiiiv. S o . c. Compound 0-€I ...O“fuses”intotheC--lr ... 1 ,4364 90.0... Oleic acid 4 . . 208 90.3 dxorption near 3.3 microns S O , .3 ... 44 ... Elaidic acid ... and estends to somewhat .. 256.:1 6 1 . 1 . . . Palmitic acid ... 62.3 longer wave length, Thus, \vc 282.4 .. 69.4.. ... ... (88) Stearic acid G9.6 conclude t,hat the 3.7-micron From celery seed oil . . 88.8 ... ... 3 0 . .5... Petraselinic arid band in the long-chain fatty by methanolysis. 31.0 (cis-6-ocrafractional distildecenoic acid acid spectra represents :I lation, and recrystallization of C-18 branch of the 0-H ...0 “assoacids froni acetone ciation” band and that thr From petroselinic 52 8... 89.8 ... I’etroselaidic arid ... acid b y isomeriza33 2 (trans-6-octamain branch of this band tion with selenium decenoic acid! (6,7 ) c o m b i n e s w i t h t h e C-H IS3 ... 10-Hendecenoie 1 ao 26.3 24.3, . . 138 (16) stretching band to produce the 24 5 acid . . Esterification of pure 4 1.4484 . . ... 3Iethyl oleate 180 total absorption seen near 3.3 acid Esteritication of pure . . 4 .. 1.4492 .. ... Methyl elaidate 180 microns. acid Esterifiration of pure . . In addition to the 3.7-micron 4 3 6 . sorpt’ionin the two solid alcohols, The 0-H elaidyl and stearyl (run in dilute solution),appears as a weak band near 2.7 microns, indicating :i normal “free” alcoholir 0-H vibration (IO). In the spertruiii of olcyl alcohol (run as the pure liquid), this absorption OCCUI’Y :it about 3 microns and is much more intense relative to the adjacent C-H absorption, indicating strong hydrogen bonding in the condensed phase, as expected (IO). On dilution, the ”association” hand decreases in intensity (relative to C-H) and shift.; t o the normal 2.7-micron position.
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Figure 3.
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MICRONS
Infrared rlbsorption Spectra of Pure LongChain Triglycerides
The strong triplet absorption a t about 8.0, 8.3, and 8.5 microns is a characteristic feature of the spectra of methyl esters of longchain acids. One or more of these bands must be related in this case to vibrations involving the C-0 linkage in the ester group 0 0 0
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C-0-CHJ; the replacement of C-OH with C-O-CHP has resulted in considerable alteration in the position and general pattern of the absorption involving C-0 links. TRIGLYCERIDES OF LONG-CHAINFATTY ACIDS (Figure 3). The triglycerides are readily distinguished from the acids or methyl esters by their characteristic absorption. The position of the C=O stretching band corresponds closely to that observed in the methyl esters, but the nature of the absorption in the 8micron region, some or all of which must be attributed in this case to the presence of C 4 linkages in the triester structure, is considerably altered. This general absorption pattern, comprising a strong band near 8.6 microns flanked by weaker bands near 8 and 9 microns, is common to all triglycerides of long-chain fatty acids whose spectra the authors have examined. LONG-CHAINALCOHOLS (Figure 4). Because a large number of hydroxy compounds show strong absorption in the 9- to 10-micron region of the infrared (4, SI),the strong band near 9.5 niicrons in the spectra of Figure 4 may be attributed to the alcoholic hy-
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Infrared 4ibsorption Spectra of Pure LongChain 4lcohols
Spectral DBerences within Each Class. In addition to the common features which distinguish each of the four classes of long-chain compounds, significant spectral differences are apparent mithin each class. These may be summed up and interpreted as follows: DIFFERENCES RELITED TO C-H BESDINGVIBRATIOXS AT DOUBLEBOND. -411 thc trans monounsaturated compounds, regardless of class, shoa a strong absorption band a t 10.36 microns, but this band is absent in the cis monounsaturated and saturated compounds (Figures 1 to 4). Recent studies ( 1 7 , 22) indicate that this band is common to a variety of monounsaturated hydrocarbons and other compounds having the trans configuration a t the double bond. The band undoubtedly arises from out-of-plane vibrations of the two hydrogen a t o m attached to the double bond in a trans structure ( 2 4 ) . The corresponding hydrogen-bending vibrations in a cis monounsaturated compound do not seem to give rise to appreciable absorption in this region. This absorption in hydrocarbon spectra has recently been found to occur in the 14- to 16-micron region (I 7, 22). In the present case, comparison of the spectra of the A$ cis-trans methyl ester pair and the A6 cis-trans acid pair shows the absorption in the 13.8- to 14.3-micron region to be greater for the cis compound in each case. The As cis-trans acid, glyceride, and alcohol pairs cannot be diiectly compared in this respect. The ratio of the absorption intensity in this region to that a t the C = O stretching peak, hon-ever, is greater for the cis acid and glyceride than for the trans acid and glyceride. Thus i t would seem that the absorption near 13.9 micions in cis compounds is due only partly to a methylene group “wagging” mode (see above); the remainder is contributed by the bending vibrations of the two hydrogens attached to the double bond carbons. In the acid and methyl ester series (Figures 1 and 2), the terniinally unsaturated compounds (IO-hendecenoic acid and its methyl ester) show two strong bands near 10 and 11 microns, which are absent in the internally unsaturated acids and esters. The assignment of bands near these wave lengths to bending motions of the hydrogens attached to a terminal double bond is well known (24) and seems fii nily established.
V O L U M E 22, NO. 12, D E C E M B E R 1 9 5 0 DIFFERESCES RELATEDTO C=C STRETCHIXC T r ~ ~ The two terminally unsaturated cornpounds mentioned above (Ilgures 1 and 2) show a well resolved C'=C stretching band near 6 microns, but no resolved band appears at this wave length in the spectra of the various internally unsaturated compounds slioivri in Figures 1 to 4. Close examination of the spectra of the cis monounsaturated compounds of Figures 1 to 4 reveals that a slight broadening or inflection on the long wave-length side of the strong C=O band is t,he only evidence of C=C stretching absorption under the resolution employed. The prediction from theoretical considerations, that C=C stretching should be infrared active near 6 microns in a cis but not in a trans compound, was confirmed in the case of the ethyl esters of oleic and elaidic acids by McCutcheon, Crawford, and Welsh (20), but to bring out this difference they found it necessary to plot the ratio of the trarismittance of each of the esters to that of ethyl stearat,? in the 6micron region. G E N E R A L C O M M E N T S AND C O S C L U S I O N S
The foregoing discussion shows that the four classes of lonychain compounds studied are readily differentiated by their characteristic infrared absorption. Within each class, the trans monounsaturated compounds are readily distinguished from the cis monounsaturated and/or saturated compounds, and internal arid external unsaturation can easily be differentiated. Becausc, of the close similarity of their spectra, however, distinction bet w e n cis and saturated, or between various individual cis, trans, or saturated compounds within a ~I:LSS requires careful examination of the curves. In general, the spectra presented should prove useful in kwessing the potentialities of the infrared method as applied to studies of fat systems; they are primarily intended to serve as a guidc in the development of spectroscopic or combined chemicalspwti,oscopic methods for the analysis of mixtures of fatty matcri:tls. For example, as a result of this study, the authors have devrloped an infrared spect,rophotometric method for determination of trans components in mixtures of long-chain compounds, utilizing the strong 10.36-micron band (26). .4lthough the structural correlations given are not all firmly established, when applied with caution they should be useful in the detection and possibly the estimation of functional groups in fats and other long-chain svstenis. ACKSOW LEDGhI ENT
The authors are grateful to Waldo C. Ault for the samples of petroselinic and petroselaidic acids, and to R. E. Iioos for assistance in the preparation of some of the reference materials.
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L I T E~R A T. U R E C I T E D ~ ~ ~ ~ ~ d m . Petroleum Inst., A.P.I. Project 44. ,Spec,tra. Sational Bureau of Standards. CHEM., ~. 20, 998 Anderson, J. A , , and Seyfried, IV. I)., A i x . ~ (1948). Barnes. R. B., Gore, R. C., Liddel, C.. and l\-illianis. l-. Z., "Infrared Spectroscopy," Ken- T o r k , Reinhold Publishing Corp., 1944. Barnes, R. B., Gore, R. C., Stafford. R. IV.,niid it-illiams, T. Z.. AMAL.CHEM.,20, 402 (1948). Bauer, S. T., Oil & Soap, 23, 1 (1946). Bertram, S. H., Chem. Weekblad, 33, 3 (1936). Bertram, S. H., Oil Cololcr Trades J . . 94, 1227 (Oct. 8, 1938). Brown, J. B., and Shinowara, G., J . d n r . Cliem. Soc., 59, 6 (1937). Buswell, A. M., Rodebush, IT. H., and Roy, RI. F., Ibid., 60, 2239 (1938). Ilavies, h4. M.,.I. Chpm. Phys., 8, 577 (1940). Davies, AI. M., and Sutherland, G. B., Ibid.,6, i 5 5 (1938). Gillette, R.H., J . Am. Chem. Soc., 58, 1143 (1936). Gillette, R.H., and Daniels, F., IOki., 58, 1139 (1936). Herman, R. C., and Hofstadter. R., J . Chem. P h y s . , 6, 534 (1938). Ibid., 7, 460 (1939). Jordan, E. F., and Swern, D., J . A m . Citetn. Soc., 71, 23i7 (1949). Kitson, R.E., Experimental Station, E. I. du Pont de Nemours & Co., Kilmington, Del., private communications. Klota, I. M., and Grueri. D. M . , J . PhUs. & Colloid Chern., 52, 961 (1948). Knight, H. B.. and Swern, D.. -1.A m . Oil C'herrLists' Soc., 26, 366 (1949). JIcCutcheon, J. IT., Crawford, AI. F., and IVelsh, H. L.. Oil & Soap, 18, 9 (1941). Rao, P. C.. and Dauhwt, B. F., J . -4m.C h e m Soc., 70, 1102 (1948). Rasmussen, 11. S.,Brattain, R . R..and Zucco, P. S.,J . Chenr. Phys.. 15, 135 (1947). Sheppard, N., and Sutherland, G . B., .Vatcue, 159, 739 (194i). Sheppard, T., and Sutherland. G. R., P r o c . Roy. Soc., A196, 195 (1949). Yhrere. 0. D.. and Heether, 11.R.. .&S.AL. CHtor., 22, 836 (1950). Shreve, 0. D., Heethet,, b l . K., Knight, H. 13., and Swern, D., Ibid., in press. Smith, D. C., and lliller, E. C.. ,J, Optical Soc. A m . , 34, 130 (1944). Swern, D., and Jordan, E. F., J . Am. Chetn. Soc., 70, 2334 (1948). Swern, D., Jordan, E. F., and Knight. H. B., Ibid., 68, 1673 (1946). Swern. D.. Knight. - H. 13., and Findley. T. IV., Oil & ,Yoax), 21, 133 (1944). Thompson. H. IT.,J . Chern. Soc., 1948, 328. Thompson, H. W., and Torkington. P.. Trnna. Furctday S'oc., 41, 246 (1945).
RECEIVED March 4 , 1930. Presented in part before the Division of Organio Chemistry at the 116th Meeting of the A M E R I C A NCHEMICALSOCIETY, Atlantic City, S . J. Paper V in series "Reactions of Fatty Materials with Oxygen." Paper I V is ( 1 9 ) .
Polarographic Determination of Platinum F. L. ENGLISH Chumbers Works, E . I . d u Pont d e Nemours C Company, Znc., Deepwater, N. J .
T
HE literature of the polarography of platinum is meage],
consisting of but one article by Willis ( b ) ,who states that this metal is not reduced polarographically. He worked apparently only with divalent platinum, but preliminary experiments showed that in the tetravalent form a definite and reproducible wave is produced in neutral solution. This wave rises instantly from zero applied potential, and attains a well defined plateau in the neighborhood of -0.6 volt, from which the curve drops rather sharply a t about -0.8 volt forming a secondary plateau, approximately two thirds the height of the first one, a t - 1.2 volts. The polarogram then rises again a t the discharge potential of the supporting electrolyte. The Iieight of the first plateau above
the starting line is linearly proportional to the concentration of platinum and serves as the basis of a quantitative determination. No starting plateau is formed, even if the electrolysis is started at a positive voltage of the dropping mercury electrode. APPARATUS
A Model SS visible recording polarograph, manufactured by E. H. Sargent & Company, Chicago, Ill., was used. The polarographic cells were of the H-shaped type recommended by Lingane and Laitinen (Z), one arm serving for the sample solution and the other for a saturated calomel cell anode. These cells were mounted in an air-agitated, vibration-insulated ( 1 ) water bath, 0 05' C. The capillary used thermostaticallv regulated to 25"
