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The infrared spectra of some aliphatic monocarboxylic acids in the 700-350 cm−1 region
Spectrochlmlca Acts,1964,
Vol. 20, pp. 686 to 695. Pergamon Pram Ltd.
The kfrared
spectra of some
Printed in Northern Ireland
aliphatic
monocarboxylic
acidsin the 700-350 cm-l region* F. F. BENTLEY and M. T. RYAN Aeronautical Systems Division, United St&es Air Force, Wright-Patterson Air Force Base, Ohio
J. E. &TON Monsanto Research Corporation, Dayton 7, Ohio (Received 2 August 1963) Abstr8&--The infrared spectra of 28 aliphatic monocarboxylic acids of varying st~cture are recorded in the 700-350 cm-l region. It is shown that the structure1conflgmution 8dj8cent to the carbonyl group affects the spectra in a specific manner. It is therefore possible to differentiate a-alkyl branched acids from straight-chain acids. Striking differences in the spectr8 of liquid and solid forms of the straightch8in acids were observed. The spectr8 of the solids below about 510 cm-’ are quite characteristic of the individual Bcids and m8y be used for differentiationof these compounds.
ALTHOUGH the infrared spectra of aliphatic monocarboxylic acids have been widely studied in the 4000-700 cm-l region, [l] there is little published data on their spectra below 700 cm-l. HADZI and SHEPPARD [2], who published a comprehensive discussion of the infrared spectra of this class between 1600 cm-l and Of the 600 cm-l, were concerned primarily with bsnds occurring above 700 cm-l. individual compounds, formic acid has received the most study [3-7). Acetio acid has been assigned by WILMSHURST [S] and a number of other halo and amino acids have received attention [C&14]. * This work was supported in part by the Aeroneutical Systems Division, United Stetee Air Force, under Contract AF 33(616)-8465. [l] L. J. BELLAMY,The Infra-red Spectra of CumpZex Mo.?ecde8 (2nd Ed.), John Wiley, New York (1958). [2] D. HADZI 8nd N. SHEPPARD, Proc. Roy. Sot. A216, 247 (1963): [3] R. C. MILLIKANand K. S. PITZER,J. Chena. Phys. 87, 1306 (1967). [4] R. C. MILLIKAN8nd K. S. PITZER,J. Am. Chem. See. 80.3616 (1968). [6] T. MIYAZAWA8nd K. S. Prrzxn, J. C%em. Phye. N&l076 (1969). [6] J. K. WILBISIIIJ’XST, J. Chem. Phye. 26, 478 (1966). [7] V. LORENZELLI 8nd K. D. Morzxn, Cmpt. rend. 249,620 (1969). [8] J. K. WI~MSEURST,J. Chem. Phy8. a6, 1171 (1966). [9] N. WRIQHT,J. Biol. Chem. 120, 641 (1937). [lo] H. C. CEEN~ and J. LECOMTE,Coopt. red. aO1, 199 (1936). [ll] R. E. KAoARISE,J. Chem. Phy8. m, 619 (1967). [12] M. TSUBOI,T. ONISIII, I. NAKAGAWA. T. SIIIMANOU~III 8nd 8. Bf~zun, Bw. Acta l.2, 263 (1968). [13] J. R. BARCELOand C. OTERO,Spectrochim. Aeta 18, 1231 (1962). [la] E. STEQER,A. TTJRCU and V. MACOVEI,S~ectroclr&m. Actu 19,293 (1963).
8
686
686
F. F. BENTLEY, M. T. RYAN and J. E. KATON
&WAN0 [15] has recently studied the spectra of a number of acids in the CsBr region. No definitive spectra-structure correlation of aliphatic monocarboxylic acids below 700 cm-l has been published, however. In addition, the assignment of fundamental frequencies in this region does not appear to be completely satisfactory. This paper reports the results of a spectral study in the range 700-350 cm-l of 28 aliphatic monocarboxylic acids of various structure, using both the liquid and solid states. EXPERIMENTAL Apparatus
A special spectrophotometer equipped with cesium bromide optics [I63 was used. It is essentially a conventional Perkin-Elmer Model 21 modified to include a double-pass system. The spectrophotometer was calibrated by using atmospheric carbon dioxide and water vapor bands. The wave length accuracy is probably better than 0.05 ,u (I.5 cm-l at 18.25 ,u). The instrument is equipped with an automatic absorbance integrator. The low-temperature cell used to obtain the spectra of the normally liquid acids in the crystalline state was designed and has been described by HARRAH [17]. It is essentially a demountable cell surrounded by a coil through which dry nitrogen, which has been previously cooled, is circulated. Materials Most of the straight-chain acids used were obtained from the Applied Science Laboratories, State College, Pennsylvania and were better than ‘99.5 per cept pure. Those not available eleswhere were synthesized by carbonation of the alkylmagnesium bromide and purified by vacuum distillation. The branched-chain acids were obtained from commercial sources and are thought to be at least 95 per cent pure; those not rated at this purity were purified by gas-liquid chromatography prior to spectra determination. Procedure The spectra of the solid compounds were obt’ained as Nujol r&lls or as benzene solutions in cesium bromide cells of 0.5 mm pathlength. The spectra of the liquid compounds were obtained using pathlengths of 0.1-0.3 mm. The low-temperature measurements were made with a nominal cell thickness of 0.05 mm. The latter cell utilized potassium bromide windows and so spectra Since the acids are known to exist in several were obtained only to 360 cm-l. polymorphic forms, care was taken to cool each one slowly to a temperature well below its freezing point in order to obtain the most stable form. [15] M.
KAWANO, Nippon Kagaku Zasshi 81,1652 (1960); Chem. Aba. 55, 704Og (1961). F. F. BENTLEY, E. F. WOLFARTH, S. E. SRP and W. R. POWEI.L, N’ilDC TR 57-359, Analytical Applications of Far Infrared Spectra I. Historical Review, Apparatus and Techniques; also S;oectTochim. Acta 13, 1 (1958). El71 L. A. HARRAH and D. W. MAYO, Tenth Annual Symposium of American Association of Spectrographers, Chicago (1959).
[is]
Infrared spectra of some aliphatic monocarboxylic acids
687
The quantitative measurements were made by dissolving weighed amounts of the acid in benzene and diluting in volumetric flasks. The absorbance of each acid was obtained by recording the spectra of each solution in the same mounted
Fig.
I’ilrtiibl i~lf’r;Iwd spcwtnl of ucrtic, cbncl wlcric acids. Solid line --liquid; dotted line-solid.
I.
propionic, htyric
cell. B-50
Pig. 2. Partiai infrared spectra of caproic, heptamoic, caprylic and pelargonic acids. Solid lin+liquid; dotted line--solid.
The absorbance integrator was turned on at 1840 p and turned ,u. A benzene blank value was subtracted from each reading.
off at
RESULTS The absorption spectra of a number of the acids are reproduced in Figs. l-5 and a correlation chart is given in Fig. 6. The major absorption bands for the acids whose spectra are not reproduced in the figures, and their estimated relative intensities, are listed in Table 1.
688
F. F. BENTLEY, M. T. RYAN and J. E. KATON
i E
Fig. 3. Partial infrared spectra of capric, undeoylic, lauric and tridecylic acids. Solid line-liquid; dotted line-solid.
Fig. 4. Partial infrared spectra of myristic, palmitic, mergaric and stearic acids. Solid line-liquid; dotted line-solid.
Table 1. Major absorption frequencies of some liquid aliphatic monocarboxylic acids in the 700-350 cm-1 region (in cm-l, v=very, s=strong, m=medium, b=broed, sh=shoulder) Pentadeeylic acid 2-Methylpentsnoic acid 2,2-Dimethylpentanoia acid 3-Methylbutanoic acid 2,2-Diethylbutanoic aaid 2,2-Dimethylpropanoio aoid 2-Ethylbexanoic acid 3,3-Dimethylbutanoia acid 4-Methylpentanoic acid
667 (a), 625 (8). 699 (a), 476 (8~ 650 (a), 635 (vs), 622 (a), 551 (vB), 543 (8, ah), 362-375 (8, b) 641 (III), 592 (vB), 552 (~8). 639 (8, sh), 393 (~8, b) 676 (a), 619 (a), 602 (m. sh), 533 (a), 516 (8, b), 488 (8, b), 357-375 (8, b) 697 (s), 628 (8, b), 400 (m), 378 (m) 690 (8). 642 (8), 623 (a), 367-380 (8, b) 656 (s), 638 (s), 621 (a, sh), 651 (s), 640 (a, ah) 700 (s), 668 (m), 613 (a), 488 (~8). 471 (6, sh), 408 (B), 352-386 (e. b) 661 (8). 664 (a), 602 (m), 612 (m), 491 (a), 472 (8, ah), 433 (m), 363 (m), 361 (In)
Infrared spectra of some aliphatic monoctwboxylicacids “Aw*uY.EW I” CM-’
Fig. 5. Partial infrared spectra of isobutyric, 2-methyl-butanoic, 2-ethylbutanoic and 2,2dimethylbutanoic a&is.
689
690
F. F. BENTLEY, M. T. RYAN and J. E.
Straight-chain
KATON
acCds
The liquid straight-chain acids show a complex of strong bands in the region 690-675 cm-i. In the series C4-C& this complex appears as three bands located in the regions 590-610 cm- l, 620-640 cm-l and 655-675 cm-l. Acetic and propionic acids have only two partly resolved bends in the 690-675 cm-l region, and the higher acids (e.g., myristic and palmitic) show strong, unresolved absorption through this range and spreading to higher frequencies. For straight-chain acids in the crystrtlline form, this band complex becomes one strong band at 625-680 cm-l with occasionally an unresolved shoulder. FREOUENCY
STRAIGHT-
CHAIN
ACIDS
-BRANCHED
AND ACIDS
e-l
4
ACIDS
MONOd-BRANCHED (TENTATIVE) > C4 6-
5
I
400
500
+il+t+
C4-C13 a
km-‘)
600
700
+-i ACIDS
Y-BRANCHED
STRAIGHT-CHAIN (SOLIDS) > c4
+li+w 1 3 bo”br.
ACIDS
+-I m-S
i
P-l
t---L--l
Fig. 6. Correletion
chart for monocarboxylio
acids.
The liquid acids also show a strong band in the region 465-495 cm-l, which is usually broad and asymmetric (occasionally resolved into two or three components). The solid acids exhibit a number of bands in this region whose positions appear to be specific for each particular compound. In addition, in their crystalline state, all acids containing more than four carbon atoms exhibit a strong band at 525-550 cm-l. a-branched
acids
These acids were investigated in the liquid state only. Tentatively, mono-abranched acids and d&a-branched acids may be distinguished by their absorption in the 600-700 cm-l range. The number of individual compounds studied in the two classes is smell, however, and further data are necessary for confirmation. With the exception of 2-methylpropanoic acid, all mono-a-branched acids studied possess a complex of three bands in the region 610-665 cm-l, which occur at 610-625 cm-l, 630-640 cm-l and 645-665 cm-l. On the other hand, the di-abranched compounds possess at most one band in this region, the location depending on the specific compound. Both mono- and di-a-branched acids show a medium to strong, asymmetric or ill-resolved band in the region 520-555 cm-l.
Infraredspectraof Bornealiphatiomonocarboxylic acids
691
p- and y-branched acids
The three acids available in this class were investigated in the liquid state. All possess three bands in the 60&700 cm-l region, but these do not oorrelate well with the three bands of straight-chain aoids. These acids do, however, possess the broad absorption at 465-495 cm-i that is also oharacteristic of straight-chain acids. DISCUSSION Assignments
From previous assignments of formic and acetic acids [3-81, three fundamental vibrations associated with the carboxyl group of monomeric acids would be predicted in the 400-700 cm-l range. Since the conditions used in this study would result in association of the molecules, the spectra would be expected to be somewhat more romplex. For acetic acid monomer, WILNSHURST[8] has described these vibrations as: (1) the O=C-0 bending motion, r,i-654 cm-‘; (2) the CO, in-plane rocking motion, Y,,-536 cm-l; and (3) the CO, out-of-plane rocking motion, ~~~-582 cm-l. The assignment of the latter two is based on the acetate ion assignment of JONESand MCLABEN[18]. It would seem that this is not necessarily an adequate representation of the motions of vi1 and vi,, for the associated species, however, since hydrogen bonding would tend to hold the carboxyl group more rigidly in position. Comparison of the acid spectra with those previously obtained with ketones [19] and aldehydes [20] suggests that the strong band near 500 cm-l could be better described as a C-C=0 in-plane bending motion. In the generalized structure
where X = C (ketones), H (aldehydes) or 0 (acids), a strong band is always noted in the vicinity of 500 cm-l which shifts to higher frequency upon a-branching. The assignment of the band as a C-C=0 in-plane bending motion in ketones has been previously discussed [19]. Using the notation of WILMSHURST, the motions could then be diagrammed schematically as follows: O+ a d\ - c/ Vll
\t
o-
-i
/
CO
O-
!
!+
4 ‘lo-
Vl!?
i/
‘\-
oVlS
The band or band complex between 600 cm-l and 700 cm-l is then due to vi1 and the strong band in the vicinity of 500 cm-l is due to viz. In view of WITAISHURST’S assignment for yiB for acetic acid, it is likely that this is the usually ill-resolved [18] L. H. JONESand E. MCLAREN, J. Chem. Phys. $32,1790 (1954). [lQ] J. E. KATON and F. F. BENTLEY, &xctmchim. Acta 19,639 (1963). [20] J. V. PUSTINGER,Jr., J. E. KATON and F. F. BENTLEY, Appt. Spedroscopy, in pm.
692
P.
F. BENTLEY, M. T. RYAN and J. E. KATON
band occurring with yi2 at about 500 cm-l. Further evidence in favor of this assignment is provided by Fig. 7. Here the integrated absorbance of this broad, ill-resolved bend has been plotted against the number of carbon atoms for several straight-chain acids at both a, constant molarity and a constant weight percent of acid. Nearly s, constant integrated absorbance is observed for solutions of the same mole&y, indicating that the absorption is due to the carboxyl group. If only one of these two bands were due to the carboxyl group a definite trend away from a straight line should be noted. Solid state spectra Rather striking differences are noted in the spectra of the solid and liquid forms of straight-chain acids. The spectra of the solids are simpler than those of the liquids between 600 om-l and 700 cm- l, but are considerably more complex below about 600 cm-l. The reduction of complexity in the 600-700 cm-l region is probably associated In the stable, crystalline state these with the hydrogen bonding phenomenon. acids exist as dimers, whereas in the liquid state they exist as a complex mixture of dimers, trimers, etc. An analogous situation was discussed by BELLAMY and Pacl;: [21] as it occurs in the two crystalline forms of oxalic acid below. \ O---H-O / \
O-H-
- -0
/
O---H-O \
c-
/
/?-oxalic acid
c=o
/ \
O-H--
a-oxalic
-0
acid
It is noteworthy that a-oxslic acid possesses a complex of bands between 600 cm-land 700 om-l similar to that which we observed with the liquid acids, while &oxalic acid possesses a single band at 720 cm-l [21]. Since the p-form is the more stable in monooarboxylic acids, this would support the hypothesis that the complexity of the absorption in this region in the liquid straight-chain acids is due to a mixture of and the p- and y-branched acids hydrogen-bonded species. The mono-a-branched have similar spectra in this region for the same reason. The di-a-branched acids do not show analogous behavior in this region, however. The corresponding sbsorption is generally lower and consists of at most two bands. It is likely that steric hindrance is the major cause of this behavior, probably resulting in the exclusion of some of the rtssociated species which would otherwise occur. [Zl] L. J. BEIZAMY and R. J. PACE, Speotrochim.
Acta 19, 436 (1963).
Infrared epeatra of some aliphatic monocarboxylic acid8
7d
4
6 8 10 NUMBER OF CAR2CN
I2 I4 ATOMS
693
I
Fig. 7. Plot of integrated absorbance of the 466-496cm-1 band vs. number of carbon atom8. CurveA aorresponda to a oon&ant 4 x 10-r mole{cc of 8cid. Curve B corresponds to a oon&ant 0.1 g/w?of acid.
Although all the solid acids containing more than four carbon atoms possess a strong band at 626450 om-l, at lower frequencies their spwtra vary greatly. A number of relatively strong bands are noted below 610 om-l which are characteristic of the individual acids. Their appearance is not unusual in that solidification often causes band sharpening and splitting. These bands are particmhrly useful since they allow ready determination of chain length. Although ahain length of aoids can often be determined in the higher frequency region, the procedure is not entirely satisfactory [l]. emen&-The authors 8cknowledg8 with gmtitude ~evBlrale&ghtening dkuka~8 A&?KWltX& with Profewor E. R. LIPPINCO~ 8nd hi8w&l auggeefionarelev8nt to thk work. It ia a w BIB0to thenkMr. C. TABSBORSKIfor the aynthe& of seversl of the eaide UBBd, Mr. LEE SXI!~ESONfor his help in obtaining spectra end Mr. W. D. Rosa and c.o-wor3~es~~ for purS88tion of 8ever8.l~amplee by ges-liquid chromatography.
Vol. 20, pp. 686 to 695. Pergamon Pram Ltd.
The kfrared
spectra of some
Printed in Northern Ireland
aliphatic
monocarboxylic
acidsin the 700-350 cm-l region* F. F. BENTLEY and M. T. RYAN Aeronautical Systems Division, United St&es Air Force, Wright-Patterson Air Force Base, Ohio
J. E. &TON Monsanto Research Corporation, Dayton 7, Ohio (Received 2 August 1963) Abstr8&--The infrared spectra of 28 aliphatic monocarboxylic acids of varying st~cture are recorded in the 700-350 cm-l region. It is shown that the structure1conflgmution 8dj8cent to the carbonyl group affects the spectra in a specific manner. It is therefore possible to differentiate a-alkyl branched acids from straight-chain acids. Striking differences in the spectr8 of liquid and solid forms of the straightch8in acids were observed. The spectr8 of the solids below about 510 cm-’ are quite characteristic of the individual Bcids and m8y be used for differentiationof these compounds.
ALTHOUGH the infrared spectra of aliphatic monocarboxylic acids have been widely studied in the 4000-700 cm-l region, [l] there is little published data on their spectra below 700 cm-l. HADZI and SHEPPARD [2], who published a comprehensive discussion of the infrared spectra of this class between 1600 cm-l and Of the 600 cm-l, were concerned primarily with bsnds occurring above 700 cm-l. individual compounds, formic acid has received the most study [3-7). Acetio acid has been assigned by WILMSHURST [S] and a number of other halo and amino acids have received attention [C&14]. * This work was supported in part by the Aeroneutical Systems Division, United Stetee Air Force, under Contract AF 33(616)-8465. [l] L. J. BELLAMY,The Infra-red Spectra of CumpZex Mo.?ecde8 (2nd Ed.), John Wiley, New York (1958). [2] D. HADZI 8nd N. SHEPPARD, Proc. Roy. Sot. A216, 247 (1963): [3] R. C. MILLIKANand K. S. PITZER,J. Chena. Phys. 87, 1306 (1967). [4] R. C. MILLIKAN8nd K. S. PITZER,J. Am. Chem. See. 80.3616 (1968). [6] T. MIYAZAWA8nd K. S. Prrzxn, J. C%em. Phye. N&l076 (1969). [6] J. K. WILBISIIIJ’XST, J. Chem. Phye. 26, 478 (1966). [7] V. LORENZELLI 8nd K. D. Morzxn, Cmpt. rend. 249,620 (1969). [8] J. K. WI~MSEURST,J. Chem. Phy8. a6, 1171 (1966). [9] N. WRIQHT,J. Biol. Chem. 120, 641 (1937). [lo] H. C. CEEN~ and J. LECOMTE,Coopt. red. aO1, 199 (1936). [ll] R. E. KAoARISE,J. Chem. Phy8. m, 619 (1967). [12] M. TSUBOI,T. ONISIII, I. NAKAGAWA. T. SIIIMANOU~III 8nd 8. Bf~zun, Bw. Acta l.2, 263 (1968). [13] J. R. BARCELOand C. OTERO,Spectrochim. Aeta 18, 1231 (1962). [la] E. STEQER,A. TTJRCU and V. MACOVEI,S~ectroclr&m. Actu 19,293 (1963).
8
686
686
F. F. BENTLEY, M. T. RYAN and J. E. KATON
&WAN0 [15] has recently studied the spectra of a number of acids in the CsBr region. No definitive spectra-structure correlation of aliphatic monocarboxylic acids below 700 cm-l has been published, however. In addition, the assignment of fundamental frequencies in this region does not appear to be completely satisfactory. This paper reports the results of a spectral study in the range 700-350 cm-l of 28 aliphatic monocarboxylic acids of various structure, using both the liquid and solid states. EXPERIMENTAL Apparatus
A special spectrophotometer equipped with cesium bromide optics [I63 was used. It is essentially a conventional Perkin-Elmer Model 21 modified to include a double-pass system. The spectrophotometer was calibrated by using atmospheric carbon dioxide and water vapor bands. The wave length accuracy is probably better than 0.05 ,u (I.5 cm-l at 18.25 ,u). The instrument is equipped with an automatic absorbance integrator. The low-temperature cell used to obtain the spectra of the normally liquid acids in the crystalline state was designed and has been described by HARRAH [17]. It is essentially a demountable cell surrounded by a coil through which dry nitrogen, which has been previously cooled, is circulated. Materials Most of the straight-chain acids used were obtained from the Applied Science Laboratories, State College, Pennsylvania and were better than ‘99.5 per cept pure. Those not available eleswhere were synthesized by carbonation of the alkylmagnesium bromide and purified by vacuum distillation. The branched-chain acids were obtained from commercial sources and are thought to be at least 95 per cent pure; those not rated at this purity were purified by gas-liquid chromatography prior to spectra determination. Procedure The spectra of the solid compounds were obt’ained as Nujol r&lls or as benzene solutions in cesium bromide cells of 0.5 mm pathlength. The spectra of the liquid compounds were obtained using pathlengths of 0.1-0.3 mm. The low-temperature measurements were made with a nominal cell thickness of 0.05 mm. The latter cell utilized potassium bromide windows and so spectra Since the acids are known to exist in several were obtained only to 360 cm-l. polymorphic forms, care was taken to cool each one slowly to a temperature well below its freezing point in order to obtain the most stable form. [15] M.
KAWANO, Nippon Kagaku Zasshi 81,1652 (1960); Chem. Aba. 55, 704Og (1961). F. F. BENTLEY, E. F. WOLFARTH, S. E. SRP and W. R. POWEI.L, N’ilDC TR 57-359, Analytical Applications of Far Infrared Spectra I. Historical Review, Apparatus and Techniques; also S;oectTochim. Acta 13, 1 (1958). El71 L. A. HARRAH and D. W. MAYO, Tenth Annual Symposium of American Association of Spectrographers, Chicago (1959).
[is]
Infrared spectra of some aliphatic monocarboxylic acids
687
The quantitative measurements were made by dissolving weighed amounts of the acid in benzene and diluting in volumetric flasks. The absorbance of each acid was obtained by recording the spectra of each solution in the same mounted
Fig.
I’ilrtiibl i~lf’r;Iwd spcwtnl of ucrtic, cbncl wlcric acids. Solid line --liquid; dotted line-solid.
I.
propionic, htyric
cell. B-50
Pig. 2. Partiai infrared spectra of caproic, heptamoic, caprylic and pelargonic acids. Solid lin+liquid; dotted line--solid.
The absorbance integrator was turned on at 1840 p and turned ,u. A benzene blank value was subtracted from each reading.
off at
RESULTS The absorption spectra of a number of the acids are reproduced in Figs. l-5 and a correlation chart is given in Fig. 6. The major absorption bands for the acids whose spectra are not reproduced in the figures, and their estimated relative intensities, are listed in Table 1.
688
F. F. BENTLEY, M. T. RYAN and J. E. KATON
i E
Fig. 3. Partial infrared spectra of capric, undeoylic, lauric and tridecylic acids. Solid line-liquid; dotted line-solid.
Fig. 4. Partial infrared spectra of myristic, palmitic, mergaric and stearic acids. Solid line-liquid; dotted line-solid.
Table 1. Major absorption frequencies of some liquid aliphatic monocarboxylic acids in the 700-350 cm-1 region (in cm-l, v=very, s=strong, m=medium, b=broed, sh=shoulder) Pentadeeylic acid 2-Methylpentsnoic acid 2,2-Dimethylpentanoia acid 3-Methylbutanoic acid 2,2-Diethylbutanoic aaid 2,2-Dimethylpropanoio aoid 2-Ethylbexanoic acid 3,3-Dimethylbutanoia acid 4-Methylpentanoic acid
667 (a), 625 (8). 699 (a), 476 (8~ 650 (a), 635 (vs), 622 (a), 551 (vB), 543 (8, ah), 362-375 (8, b) 641 (III), 592 (vB), 552 (~8). 639 (8, sh), 393 (~8, b) 676 (a), 619 (a), 602 (m. sh), 533 (a), 516 (8, b), 488 (8, b), 357-375 (8, b) 697 (s), 628 (8, b), 400 (m), 378 (m) 690 (8). 642 (8), 623 (a), 367-380 (8, b) 656 (s), 638 (s), 621 (a, sh), 651 (s), 640 (a, ah) 700 (s), 668 (m), 613 (a), 488 (~8). 471 (6, sh), 408 (B), 352-386 (e. b) 661 (8). 664 (a), 602 (m), 612 (m), 491 (a), 472 (8, ah), 433 (m), 363 (m), 361 (In)
Infrared spectra of some aliphatic monoctwboxylicacids “Aw*uY.EW I” CM-’
Fig. 5. Partial infrared spectra of isobutyric, 2-methyl-butanoic, 2-ethylbutanoic and 2,2dimethylbutanoic a&is.
689
690
F. F. BENTLEY, M. T. RYAN and J. E.
Straight-chain
KATON
acCds
The liquid straight-chain acids show a complex of strong bands in the region 690-675 cm-i. In the series C4-C& this complex appears as three bands located in the regions 590-610 cm- l, 620-640 cm-l and 655-675 cm-l. Acetic and propionic acids have only two partly resolved bends in the 690-675 cm-l region, and the higher acids (e.g., myristic and palmitic) show strong, unresolved absorption through this range and spreading to higher frequencies. For straight-chain acids in the crystrtlline form, this band complex becomes one strong band at 625-680 cm-l with occasionally an unresolved shoulder. FREOUENCY
STRAIGHT-
CHAIN
ACIDS
-BRANCHED
AND ACIDS
e-l
4
ACIDS
MONOd-BRANCHED (TENTATIVE) > C4 6-
5
I
400
500
+il+t+
C4-C13 a
km-‘)
600
700
+-i ACIDS
Y-BRANCHED
STRAIGHT-CHAIN (SOLIDS) > c4
+li+w 1 3 bo”br.
ACIDS
+-I m-S
i
P-l
t---L--l
Fig. 6. Correletion
chart for monocarboxylio
acids.
The liquid acids also show a strong band in the region 465-495 cm-l, which is usually broad and asymmetric (occasionally resolved into two or three components). The solid acids exhibit a number of bands in this region whose positions appear to be specific for each particular compound. In addition, in their crystalline state, all acids containing more than four carbon atoms exhibit a strong band at 525-550 cm-l. a-branched
acids
These acids were investigated in the liquid state only. Tentatively, mono-abranched acids and d&a-branched acids may be distinguished by their absorption in the 600-700 cm-l range. The number of individual compounds studied in the two classes is smell, however, and further data are necessary for confirmation. With the exception of 2-methylpropanoic acid, all mono-a-branched acids studied possess a complex of three bands in the region 610-665 cm-l, which occur at 610-625 cm-l, 630-640 cm-l and 645-665 cm-l. On the other hand, the di-abranched compounds possess at most one band in this region, the location depending on the specific compound. Both mono- and di-a-branched acids show a medium to strong, asymmetric or ill-resolved band in the region 520-555 cm-l.
Infraredspectraof Bornealiphatiomonocarboxylic acids
691
p- and y-branched acids
The three acids available in this class were investigated in the liquid state. All possess three bands in the 60&700 cm-l region, but these do not oorrelate well with the three bands of straight-chain aoids. These acids do, however, possess the broad absorption at 465-495 cm-i that is also oharacteristic of straight-chain acids. DISCUSSION Assignments
From previous assignments of formic and acetic acids [3-81, three fundamental vibrations associated with the carboxyl group of monomeric acids would be predicted in the 400-700 cm-l range. Since the conditions used in this study would result in association of the molecules, the spectra would be expected to be somewhat more romplex. For acetic acid monomer, WILNSHURST[8] has described these vibrations as: (1) the O=C-0 bending motion, r,i-654 cm-‘; (2) the CO, in-plane rocking motion, Y,,-536 cm-l; and (3) the CO, out-of-plane rocking motion, ~~~-582 cm-l. The assignment of the latter two is based on the acetate ion assignment of JONESand MCLABEN[18]. It would seem that this is not necessarily an adequate representation of the motions of vi1 and vi,, for the associated species, however, since hydrogen bonding would tend to hold the carboxyl group more rigidly in position. Comparison of the acid spectra with those previously obtained with ketones [19] and aldehydes [20] suggests that the strong band near 500 cm-l could be better described as a C-C=0 in-plane bending motion. In the generalized structure
where X = C (ketones), H (aldehydes) or 0 (acids), a strong band is always noted in the vicinity of 500 cm-l which shifts to higher frequency upon a-branching. The assignment of the band as a C-C=0 in-plane bending motion in ketones has been previously discussed [19]. Using the notation of WILMSHURST, the motions could then be diagrammed schematically as follows: O+ a d\ - c/ Vll
\t
o-
-i
/
CO
O-
!
!+
4 ‘lo-
Vl!?
i/
‘\-
oVlS
The band or band complex between 600 cm-l and 700 cm-l is then due to vi1 and the strong band in the vicinity of 500 cm-l is due to viz. In view of WITAISHURST’S assignment for yiB for acetic acid, it is likely that this is the usually ill-resolved [18] L. H. JONESand E. MCLAREN, J. Chem. Phys. $32,1790 (1954). [lQ] J. E. KATON and F. F. BENTLEY, &xctmchim. Acta 19,639 (1963). [20] J. V. PUSTINGER,Jr., J. E. KATON and F. F. BENTLEY, Appt. Spedroscopy, in pm.
692
P.
F. BENTLEY, M. T. RYAN and J. E. KATON
band occurring with yi2 at about 500 cm-l. Further evidence in favor of this assignment is provided by Fig. 7. Here the integrated absorbance of this broad, ill-resolved bend has been plotted against the number of carbon atoms for several straight-chain acids at both a, constant molarity and a constant weight percent of acid. Nearly s, constant integrated absorbance is observed for solutions of the same mole&y, indicating that the absorption is due to the carboxyl group. If only one of these two bands were due to the carboxyl group a definite trend away from a straight line should be noted. Solid state spectra Rather striking differences are noted in the spectra of the solid and liquid forms of straight-chain acids. The spectra of the solids are simpler than those of the liquids between 600 om-l and 700 cm- l, but are considerably more complex below about 600 cm-l. The reduction of complexity in the 600-700 cm-l region is probably associated In the stable, crystalline state these with the hydrogen bonding phenomenon. acids exist as dimers, whereas in the liquid state they exist as a complex mixture of dimers, trimers, etc. An analogous situation was discussed by BELLAMY and Pacl;: [21] as it occurs in the two crystalline forms of oxalic acid below. \ O---H-O / \
O-H-
- -0
/
O---H-O \
c-
/
/?-oxalic acid
c=o
/ \
O-H--
a-oxalic
-0
acid
It is noteworthy that a-oxslic acid possesses a complex of bands between 600 cm-land 700 om-l similar to that which we observed with the liquid acids, while &oxalic acid possesses a single band at 720 cm-l [21]. Since the p-form is the more stable in monooarboxylic acids, this would support the hypothesis that the complexity of the absorption in this region in the liquid straight-chain acids is due to a mixture of and the p- and y-branched acids hydrogen-bonded species. The mono-a-branched have similar spectra in this region for the same reason. The di-a-branched acids do not show analogous behavior in this region, however. The corresponding sbsorption is generally lower and consists of at most two bands. It is likely that steric hindrance is the major cause of this behavior, probably resulting in the exclusion of some of the rtssociated species which would otherwise occur. [Zl] L. J. BEIZAMY and R. J. PACE, Speotrochim.
Acta 19, 436 (1963).
Infrared epeatra of some aliphatic monocarboxylic acid8
7d
4
6 8 10 NUMBER OF CAR2CN
I2 I4 ATOMS
693
I
Fig. 7. Plot of integrated absorbance of the 466-496cm-1 band vs. number of carbon atom8. CurveA aorresponda to a oon&ant 4 x 10-r mole{cc of 8cid. Curve B corresponds to a oon&ant 0.1 g/w?of acid.
Although all the solid acids containing more than four carbon atoms possess a strong band at 626450 om-l, at lower frequencies their spwtra vary greatly. A number of relatively strong bands are noted below 610 om-l which are characteristic of the individual acids. Their appearance is not unusual in that solidification often causes band sharpening and splitting. These bands are particmhrly useful since they allow ready determination of chain length. Although ahain length of aoids can often be determined in the higher frequency region, the procedure is not entirely satisfactory [l]. emen&-The authors 8cknowledg8 with gmtitude ~evBlrale&ghtening dkuka~8 A&?KWltX& with Profewor E. R. LIPPINCO~ 8nd hi8w&l auggeefionarelev8nt to thk work. It ia a w BIB0to thenkMr. C. TABSBORSKIfor the aynthe& of seversl of the eaide UBBd, Mr. LEE SXI!~ESONfor his help in obtaining spectra end Mr. W. D. Rosa and c.o-wor3~es~~ for purS88tion of 8ever8.l~amplee by ges-liquid chromatography.