ANALYTICAL CHEMISTRY
334 adding between 0.05 and 0.10 micron to the longest observed wave length and subtracting a similar amount from the shortest. Table I X lists the structures and gives their correlation bands, band intensities, and remarks about the properties of the bands. Table X is the inverse of Table I X but includes some bands not in Table I X which are common to all alkylbenzene spectra. Figure 7 charts the correlations. Figure 8 compares the ranges given here with those of Colthup. Some bands given by Colthup were not found in the alkylbenzene spectra or in the spectra in the Sadtler catalog (12). Recently Young, Du Vall, and Wright have related the structures of the bands between 5 and 6 microns to the type of ring substitution ( 1 6 ) . These results have been found to apply without change to the alkglbenzenes. ACKNOWLEDGMEhT
The authors are indebted to D. C. Smith for providing them with results from his studies which, in many cases, paralleled their own work. LITERATURE CITED
(1) American Cyanamid Co. and Perkin-Elmer Corp., correlation charts.
( 2 ) Anderson, J. S . ,Jr., and Seyfried, W.D., ANAL.CHEX, 20, 998 (1948). (3) Barnes, R. B., Gore, R. C., Liddel, U., and Williams, V. Z., "Infrared Spectroscopy," New York, Reinhold Publishing Corp., 1944. (4) Barnes, R. B., Gore, R. C., Stafford, R. W., and Williams, V. Z., ANAL. CHEY., 20,402 (1948). (5) Colthup, N. B., J . Optical SOC.Am., 40,397 (1950). (6) Dinsmore. H. L., and Smith, D. C., ANAL.CHEM.,20, 11 (1948). (7) Gourlay, J. S., Research, 3, 252 (1960). (8) Hampton, R. R., A N ~ LCHEM., . 21, 889 (1949). (9) Hart, E. J.,and hleycr, A. W., J . Am. Chem. SOC.,71, 1980 (1949). (10) Hibbard, R. R., and Cleaves, -4. P., A x ~ LCHEM., . 21,486 (1949). (11) Rasmussen, R. S., and Brattain, R. R., J . Chem. Phys., 15, 120, 136 (1947). (12) Sadtler and Sons, Philadelphia, "Catalog of Infrared Spectra andcorrelation Chart," 1950. (13) Sheppard, N., ond Sutherland, G. B. B. M., Proc. Roy. SOC., A196, 195 (1949). (14) Smith, D. C., private communication. (15) Thompson, H. TT., and Torkington, P., Proc. Roy. SOC.,A184, 21 (1945). (16) Young, C. IT., DuVall, R. B., and Wright, Sorman, A S ~ L . CHEM.,23, 709 (1951). RECEIVEDApril 23, 1961. Presented before the Symposium on Analytical Research in the Petroleum Industry, .Imerican Petroleum Institute, Tulsa, Okla., April 30, 1951.
Infrared Absorption Band Due to Nitrile Stretching Vibration ROBERT E. KITSON AND NORMAN E. GRIFFITH' Polychemicals Department, Research Division, E. 1. du Pont de ivernours & Co., Inc., Wilmington, Del. In the course of measurement of the infrared absorption spectra of some unsaturated dinitriles, it was observed that the strong band at ca. 2250 cm.-' due to the stretching of the-C=N bond was split, in some cases, into a doublet. The nitrile stretching frequency of about 70 nitriles was studied in an effort to explain this behavior. The nitrile stretching band was found to occur at 2250 & 10 cm.-l in saturated nitriles or in olefinic nitriles wherein no conjugation exists between the nitrile and olefinic groups. In
I
N T H E course of measurement of the infrared adsorption
spectra of some unsaturated dinitriles, it wasobserved that the strong band a t approximately 2250 cm.-' due to the stretching of the C=N bond was split, in some cases, into a doublet which just could be resolved with the sodium chloride optics being used. The splitting always occurred when the two nitrile groups in the molecule differed in their relation to the olefinic bond--i.e., one group was conjugated with the double bond while the other was not. If both groups were conjugated or unconjugated, the usual single sharp band appeared. Reitz and Sabathy (1, 2 ) and Reitz and Skrabel (3) have studied the nitrile stretching frequency of a Raman shift of a number of mononitriles. The former authors ( 2 ) observed that conjugation of an olefinic group xith the nitrile group caused a shift in the C=N frequency. They report the Raman shift occurs a t 2245 cm.-I in aliphatic nitriles, a t 2229 cm.-l in aromatic compounds, and a t 2220 cm. in conjugated compounds. No similar work has been reported on the infrared absorption of the nitrile group. Consequently, the nitrile stretching frequency of a large number of nitrile compounds was measured on an inPresent address, E. I. du Pont de Nemours & Co., Inc , Victoria, Tex.
conjugated nitriles, the band is shifted to 2225 i 8 cm.-' In dinitriles containing both types of groups, both bands are found. Aromatic nitriles in which the nitrile is attached to the ring have an intermediate nitrile stretching frequency, the exact value depending on the nature of other substituents on the ring. Thus the study of the nitrile band in a compound of unknown structure permits some conclusions to be drawn as to the relations between the nitrile group and other parts of the molecule.
frared spectrometer to determine whether or not the shifts in Raman spectra, due to conjugation of the nitrile group with an olefinic double bond, could be substantiated by infrared data. The authors have found that the nitrile stretching frequency in saturated nitriles, or in unsaturated nitriles in which no conIn unsaturated jugation exists, occurs a t 2250 & 10 cm.-' nitriles in which conjugation does exist between the nitrile group and an olefinic tlouble bond, the band is found a t 2225 i 7 cm.-' Dinitriles having both types of group have both bands. Aromatic nitriles in which the nitrile is attached to the ring have an intermediate nitrile stretching frequency, the exact value depending on the nature of other substituents on the ring. Thus, by a study of the exact position of the nitrile band in an unknown compound, the relation of the nitrile group to any unsaturated groups can be established. In the course of this work, it was noted that the intensity of the nitrile absorption varied considerably. In nitriles rontaining only carbon and hydrogen in addition to the nitrile group(s), the band is usually intense. However, as electronegative groups, particularly those containing oxygen, are introduced into the molecule, the band becomes neaker. When the carbon atom bonded to the nitrile group is also bonded to an electronegative group, the nitrile stretching frequency is very weak or absent.
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2
335 DISCUSSION OF RESULTS
Table 1. Nitrile Stretching Frequency of Some Aliphatic Mononitriles
The saturated mono- and dinitriles listed in Tables I and I1 (excluding malononitrile) have an average frequency of Compound Formulaa Sourceb 2250 f 10 ern.-' The 17 compounds Acetonitrile c-cs E.K. No. 488C 2254 listed in Table I11 having a nitrile group Propionitrile C-c-CN GeneseeC 2249 8-Chloropropionitrile CI-c-c-CN M. No. 6227 2260 unconjugated with the olefinic double 8-Bromopropionitrile Br-C-C-CN Sapon 2268 c bond show the band a t 2252 f 11 cm.+ \ ( I t is doubtful that the difference between 8-Isopropoxypropionitrile C-0-C-C-CN .4m. Cy. Co. 2254 / the two averages is significant.) C 8-Hydroxypropionitrile HO-C-C-CN hl. No. 6367 2266 The 12 compounds in Table I11 having CI one or more nitriles conjugated with the 2,3-Dichloro-2-niethylpropionitrile CI-c-c-cri D u Pont 2244 (R)d double bond (excluding fumaronitrile) chave a strong band at 2225 f 7 cm.-' n-Butyronitrile C-c-c-CN E.K. No. 741C 2253 Only 4 compounds having both types of 4-Chlorobut yronitrile CI-C-C-C-CN Sapon 2253 4-Bromobutyronitrile Br-C-C-C-CN Sapon 2252 nitrile group were available, but, as exn-Valeronitrile C-c-c-c-CN Sapon 2254 5-Bromovaleronitrile Br-C-C-C-C-CN Sapon 2249 pected, they all show both bands. Isocapronitrile Examination of the data for the various E.K. No. 65 2249 substituted propionitriles listed in Table 6-Aminocapronitrile -c-CN Du P o n t 2252 n-Heptanonitrile Sapon 2250 I suggests that the nitrile frequency is Nonanonitrile (n-octyl cyanide) Sapon 2250 shifted slightly to lower wave lengths by Undecanonitrile (n-decyl cyanide) Sapon 2250 Pentadecanonitrile Sapon 2250 substitution of electronegative groups on Heptadecanonitrile (cetyl cyanide) Sapon 2249 the beta-carbon. This shift is not nearly so pronounced as that due to conjugaAcetone cyanohydrin c-c' M. No. 6655 2246 tion, but it may be of some significance. c' 'cri Study of more compounds of this type a For simplicity, only carbon skeletal formula is given. will be necessary to establish this point. b E . K . = Eastman Kodak Co., Rochester, N . Y . >I = hlatheson Co., Inc., East Rutherford, N. J. Table IV shows the data obtained for Sapon = Sapon Laboratories, Brooklyn, N. Y. Du Pont = E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. (used to identify both commersome aromatic nitriles. As expected, cial and laboratory preparations). the behavior is about the same as with Genesee = Genesee Research Corp. Rochester N. Y . Am. Cy. Co. = American Cyanamid Co., New k o r k , N. Y aliphatic nitriles. Compounds having C Redistilled before use. Raman shift. Band too weak to measure in infrared. the nitrile group attached to the aromatic nucleus show the low frequency, while insertion of one or two methylene groups between the ring and the nitrile EXPERIMENTAL group causes the frequency to shift to the higher value. Infrared absorption measurements were made with a PerkinSince the unsaturation in the aromatic ring is not equivalent Elmer Model 12-c spectrometer. A lithium fluoride rism was to olefinic unsaturation, it is not surprising to find that most of used to obtain the necessary resolution for this work. 8 h e wavethe compounds examined showed bands a t slightly higher frelength calibration was made by comparison with the rotational fine structure of the carbon monoxide and carbon dioxide bands quencies than the conjugated olefinic compounds. In substia t 2046 to 2225 ern.-' and 2292 to 2347 cm.-l Liquid samples were run directly in 0.025-mm. sodium chloride cells or as Table 11. Nitrile Stretching Frequency of Some Aliphatic Dinitriles solutions in suitable solvents in 0.050C=N mm. cells. Solids were run as solutions Frequency i? suitable solvents, as Xujol suspenCompound Foriiiulaa Sourcea Cm.-1 sions, or as films melted onto salt hlalononitrile NC-C-Ch' M. KO.6660 2278 plates. Succinonitrile NC-c-c-CN Sapon 2261 C=N Frequency, Cm.-1
~
The nitrile samples were assembled from a variety of sources, as shown in Tables I to IV. Some of the c o m m e r d samples were subjected to further purification before measurement. Many of the unusual unsaturated nitriles were prepared in the laboratory. In all such cases, the structure of the sample was proved by suitable independent techniques before their inclusion in this work.
Giutaronitrile Adiponitrile
NC-C-C-C-CN NC-C-C-C-C-CN C
8-hle thylglii taroni trile
NC-c-A-c-CY
Ethylsuccinonitrile Pimelonitrile
Sapon D u Pont
2254 2248
Du P o n t
2247
NC-C-d-CN NC-(C)a-CN C
D u Pont Sapon
2247 2248
@-Methyladiponitrile Suberonitrile
h-C-c-C-c-c-cN EC-(C) 6-cN
Du Pont Sapon
2249 2249
Tetramethslsuccinononitrile
xc-c-c-Cri
Du Pont
2243
Azeleonitrile Sebaconitrile
NC-(C),-CN NC-(C)s-CN C y QH
Sapon Sapon
2249 2249
DL-Biacetylcyanohydrin
c-c-b-c I 1
Du P o n t
2259
D u Pont
2259
C'
EXPERIMENTAL RESULTS
The results obtained are summarized in Tables I to V. Except for acetonitrile and fumaronitrile, none of the compounds showed any absorption bands in the region scanned (2281 to 2146 cm.-1) other than those listed.
C
I
F:?
cI cI
HO CN meso-Biacetylcyanohydr in See Table I.
CN CN
336
ANALYTICAL CHEMISTRY
337
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2 tuted benzonitriles in which the substituents tend to reduce the electron density in the benzene ring and, consequently, the bond force constants so that the aromatic nucleus appears less unsaturated (as in the nitrobenzonitriles), the nitrile frequency is shifted toward the saturated value. When substituents are present which increase the electron density (as in aminobenzonitrile) the opposite effect is noted. Additional work on substituted benzonitriles should be most interesting.
ing band should be done with some caut,ion. If the nitriles in the sample are all of the same type, the determination can be carried out without difficulty. If, however, both conjugated and unconjugated nitrile groups are present in the sample, as in a mixture of propio- and acrylonitrile, care must be used to ensure that the analytical conditions used either resolve the two nitrile bands or quantitatively sum them. Similarly, care should be used that the intensity'of the nitrile band is constant for the compounds being determined. SUMMARY
Table V.
Intensity of CEN Stretching Frequeiicg
Compound Acetonitrile -4crylonitrile 8-Chloropropionitrile Heptanonitrile Heptadecanonitrile ddiponitrile Acetone cyanohydrin lntensitS = absorbancy (0.25 mm. cell) mg. C N / m l . CCl,
IntensityR 0.25
0.30 0.26 0.33
0.24
0.31 0.09
.
The i1iipui,tuiictsui' thebe observations Beenis obvious: By measurement of the exact frequency of t'he nitrile stretching band, the analyst can quickly establish the relation of the nitrile group to the rest of the molecule. If the band falls a t ca. 2250 cm.-', the nitrile group is probably in a saturated compound or in a position in an olefinic compound unconjugated to the double bond. The presence of the band at 2225 em.-' indicates conjugation with an olefinic double bond. If the band falls outside one of these two ranges, some conclusions can be d r a m as to the nature of the compound. If the band is between these ranges, it is almost certainly associated Kith an aromatic grouping or n-ith some form of conjugation which is less effective than straight conjugation. Similarly, if the band is below 2220 cm.-', the nitrile may be part of a conjugation system which is more effective than a single double bond. The intensity of the nitrile absorption band varies somewhat. In nitriles containing only carbon, hydrogen, and nitrogen the band is usually intense, and its strength is a function of the proportion of the nitrile group to the total molecule. A number of compountls were examined, however, in which the nitrile band was weak or completely absent. These compounds usually contained oxygenated groups. The "quenching" effect was greatest when the oxygenated group was attached to the same carbon 3.5 the nitrile group. In only one case were compounds differing only in their oxygenated groups available. The nitrile band in DL- arid in nieso-biacetylcyanohydrinwas weak, though detectable. In the corresponding acetoxy compounds, however, the band could not be found although the method of synthesis, the chemical analysis, and the chemical reactions for the samples indicated them to contain nitrile groups. Quantitative measurements m:ide on some typical nitriles are included in Table V to illustrate this behavior, Quantitative functional group analysis using the nitrile stretrh-
The study of the nitrile stretching frequency of a series of saturated nitriles has shown the band to occur a t 2250 == ! 10 cm.-l The band occurs a t essentially this same frequency in unsaturated nitriles in which no conjugation exists between the nitrile and olefinic groups. In conjugated nitriles, however, the band is shifted to 2225 =k 8 cni.-l In dinitriles containing both types of groups, both bi~ndsare found. The band is found a t an intermediate position in aromatic compounds having the nitrile group attached to the nucleus. Its exact position in these compounds depends on the nature of other substituents on the ring. -1study of the exact position of the nitrile band, therefore, will permit the analyst to establish, a t least in part, the relation betm-een the nitrile group and an unsahrated group or groups in the molecule. The nitrile band is usually very intense, except in compounds containing osygenated groups. Compounds that have oxygenated groups attached to the same carbon atom as the nitrile group dhow a marked decrease in int,ensity, or even complete absence of the nitrile band. In viev of these observations, soine care must be used in making quantitative functional group analysis based on the nitrile band, a$ both its position and intensit>- can vary from one compound to another. ACKiVOWLEDGMENT
The authors wish to thank W.H. Calkins, B. F. Day, It. H. Hallinell, R. C. Schreyer, I. D. Webb, and the other Du Pont chemists who made samples of many of the nitrile compounds available for this work. Thanks are due also to John Mitchell, Jr., and Donald Milton Smith for their assistance in the work and in the preparation of the manuscript, and to Richard C. Lord, Massachusetts Institute of Technology, and C. F. Hammer of this lahoratory for their many helpful suggestions. LITERATURE CITED
(1) Reitz, A. I\ ., and Sabathy, R., Monolsh., 71, 100 (1938). (2) Ibid., p. 131. (3) Reitz, A . W., and Skrabel, R., lbid., 70, 398 (1937). K a c h r v E D .\larch 15, 1961. Presented a t the Delaware Chemical Sy~uposiurn, Newark, Del., January 13, 1961, and a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, PittRburgh, Pa., March 6.
1961.
