Chemistry and Physics of LipMs 15 (1975) 48-52 © North-Holland Publishing Company
RAMAN STUDIES OF SOME DIUNSATURATED OLEFINIC AND ACETYLENIC FATTY ACIDS AND THEIR DERIVATIVES
J.E.D. DAVIES and P. HODGE Department of Chemistry, University of Lancaster, Lancaster, LA 1 4 YA, England F.D. GUNSTONE and M.S.F. LIE KEN JIE Department of Chemistry, The University, St. Andrews, Scotland Received April 5, 1975,
accepted June 23, 1975
The Raman spectra of CC14 solutions of the 6,12; 7,12; 8,12; 9,12; and 10,12 isomers of octadecadiynoic acid and of the octadecadienoic acid methyl esters of both the cis,cis and trans, trans series are reported. Provided that there are two or more methylene groups between the unsaturated groups, the double and triple bond vibrational wavenumber values are close to those found in monounsaturated derivatives. An attempt has been made to obtain a correlation between the relative intensities of the v(C=C) and 6(CH2) bands and the ratio of tile number of double bonds to methylene groups in the molecule.
I. Introduction A previous publication [1] reported infrared and Raman spectra of some monounsaturated olefinic and acetylenic fatty acids and their derivatives. In this publication we report similar studies on diunsaturated derivatives carried out in order to observe the effect on the double bond vibrational wavenumber value of bringing the two unsaturated groups progressively closer together.
II. Olefinic esters The Raman spectra in the u(C=C) region of CC14 solutions of the various octadecadienoic acid methyl esters are given in table 1. These bands were not observed in the infrared spectra since the olefinic groups are in symmetric environments towards the centre of the molecule. This is in agreement with the study [1] of monounsaturated fatty acids where the v(C=C) band was observed in the infrared spectrum only when the olefinic group was in the terminal position or when conjugated with the carbonyl group. Provided that there are two or more methylene groups between the unsaturated
J.E.D. Davies et al., Raman spectra o f some diunsaturated f a t t y acids
49
Table 1 The v(C=C) Raman band of the cis, cis and trans, trans octadecadienoic acid methyl esters. Isomer
cis, cis
trans, trans
6,11 6,12 7,12 8,12 9,12 10,12 Monounsaturated compound
1656 1657 1656 1655 1661 1646 1656 ± 1
1670 1670 1669 1670 1660 1670 + 1
groups the values are very close to those found for the monounsaturated compounds [ 1]. The cis-9,cis-12 isomer gives a value 5 c m - 1 higher than the monounsaturated value, but there seems to be no variation in the value for the corresponding trans, trans isomer. Conjugation of the two double bonds, i.e. in the 10,12 isomers, lowers the values by 10 cm -1 giving values which are very close to those observed when the olefinic group was conjugated with the carbonyl group in the monounsaturated compounds.
IlI. Acetylenic fatty acids Table 2 lists the values observed for the v(C=C) mode in a series of octadecaliynoic acids. In agreement with the study [1] of monounsaturated acetylenic acids this mode did not give rise to an infrared band. The vibration of the triple bond gives rise to two Raman bands due to Fermi resonance between the triple bond stretching mode and the first overtone of the Table 2 The v(C=-C) Raman band of the octadecadiynoic acids. Isomer
v(C=C)
6,11 6,10 6,8 6,12 7,12 8,12 9,12 10,12 Monounsaturated compound
2231/2290 2231/2296 2258 a 2230/2291 2230/2290 2231/2296 2216/2263 2257 a 2232 ± is, 2291 -+2m
a Methyl ester.
50
J.E.D. Davies et aL, Raman spectra o f some diunsaturated fatty acids
skeletal C - C stretching mode. Provided that there are two or more methylene groups between the unsaturated groups the values for the two bands are very close to those found for the monounsaturated compounds. The 9,12 isomer gives two bands at 2216 and 2263 cm -1 and the 10,12 and 6,8 isomers give single Raman bands at 2257 and 2258 c m - 1 respectively. A triple bond conjugated with a carbonyl group gives [1] a single band at 2245 cm-1.
IV. Application Comparison of the Raman spectra of methyl cis-octadec-8-enoate and methyl cis-8,cis-12- octadecadienoate (fig. 1) shows a variation in the relative intensities of the bands at 1656 cm -1 and 1440 cm -1. Jones and Ripley [2] have made a detailed analysis o f the Raman spectrum of methyl dodecanoate (methyl laurate) CH3(CH2)9CH2CO2CH3, and have shown by deuteriation studies that the band at 1440 c m - 1 arises from the bending mode of the -(CH2)9-grou p. We therefore thought it worthwhile to try and obtain a correlation between the relative intensities o f the 1440 and 1656 cm 1 bands and the ratio o f the number o f methylene to unsaturated groups in the molecule. The results are summarized in fig. 2, where R is the ratio of the number of methylene groups (ignoring the group adjacent to the carbonyl) to the number o f
(a)
(b)
j!i 1800
I 1600-
1400
i.I
i
1200 1800
Z 1600
M
I -1400
; 1200
Fig. 1. The Raman spectra (1200-1800 cm 1) of (a) methyl eis-octadec-8-enoate and (b) methyl cis, cis-octadec-8,12-dienoate examined as neat liquids.
J.E.D. Davies et al., Raman spectra o f some diunsaturated fatty actas
5l
e
O
I
[
J
5
10
15
R
Fig. 2. I vs R plots for: (a) methyl cis, cis-octadeca-8,12-dienoate; (b) methyl cis, cis-eicosa-7,13dienoate; (c) cis-tetradec-8-enoic acid; (d) cis-hexadec-10-enoic acid; (e) methyl cis-octadec-9enoate.
unsaturated groups, and I is the intensity ratio of the 1440 cm-1 band to the 1656 cm- 1 band. The scatter of the points probably arises from the fact that several of the samples fluoresced badly and thus had sloping base lines which made measurement of the band intensities difficult. The plot cannot consequently be used to obtain an accurate R value for an unknown fatty acid, but it could be used to determine the number of unsaturated groups, providing the chain length is known approximately. Furthermore the wavenumber value of the band arising from the vibration of the unsaturated group would indicate whether the group is cis or trans disubstituted.
V. Experimental The diunsaturated compounds were prepared using the literature methods [3]. The samples were examined as 20% solutions in carbon tetrachloride. A. Raman spectra
The Raman spectra were excited using the 568.2 nm line of a CRL Model 52K
52
J.E:D. Davies et aL, Raman spectra o f some diunsaturated fatty acids
krypton laser. The spectra were recorded on a Cary 81 spectrometer using 4.5 cm-1 slit widths and an exciting power of 30 mW at the sample. The instrument was calibrated with krypton emission lines. The diunsaturated compounds, like the monounsaturated compounds, gave a high background whose intensity could be reduced by exposure to the laser beam.
References [ 1] J.E.D. Davies, P. Hodge, J.A. Barve, F.D. Gunstone, and 1.A. Ismail, JCS Perkin 11 (1972) 1557 [2] R.N. Jones and R.A. Ripley, Canad. J. Chem. 42 (1964) 305 [31 F.D. Gunstone and M.S.F. Lie Ken Jie, Chem. Phys. Lipids 4 (1970) 1
RAMAN STUDIES OF SOME DIUNSATURATED OLEFINIC AND ACETYLENIC FATTY ACIDS AND THEIR DERIVATIVES
J.E.D. DAVIES and P. HODGE Department of Chemistry, University of Lancaster, Lancaster, LA 1 4 YA, England F.D. GUNSTONE and M.S.F. LIE KEN JIE Department of Chemistry, The University, St. Andrews, Scotland Received April 5, 1975,
accepted June 23, 1975
The Raman spectra of CC14 solutions of the 6,12; 7,12; 8,12; 9,12; and 10,12 isomers of octadecadiynoic acid and of the octadecadienoic acid methyl esters of both the cis,cis and trans, trans series are reported. Provided that there are two or more methylene groups between the unsaturated groups, the double and triple bond vibrational wavenumber values are close to those found in monounsaturated derivatives. An attempt has been made to obtain a correlation between the relative intensities of the v(C=C) and 6(CH2) bands and the ratio of tile number of double bonds to methylene groups in the molecule.
I. Introduction A previous publication [1] reported infrared and Raman spectra of some monounsaturated olefinic and acetylenic fatty acids and their derivatives. In this publication we report similar studies on diunsaturated derivatives carried out in order to observe the effect on the double bond vibrational wavenumber value of bringing the two unsaturated groups progressively closer together.
II. Olefinic esters The Raman spectra in the u(C=C) region of CC14 solutions of the various octadecadienoic acid methyl esters are given in table 1. These bands were not observed in the infrared spectra since the olefinic groups are in symmetric environments towards the centre of the molecule. This is in agreement with the study [1] of monounsaturated fatty acids where the v(C=C) band was observed in the infrared spectrum only when the olefinic group was in the terminal position or when conjugated with the carbonyl group. Provided that there are two or more methylene groups between the unsaturated
J.E.D. Davies et al., Raman spectra o f some diunsaturated f a t t y acids
49
Table 1 The v(C=C) Raman band of the cis, cis and trans, trans octadecadienoic acid methyl esters. Isomer
cis, cis
trans, trans
6,11 6,12 7,12 8,12 9,12 10,12 Monounsaturated compound
1656 1657 1656 1655 1661 1646 1656 ± 1
1670 1670 1669 1670 1660 1670 + 1
groups the values are very close to those found for the monounsaturated compounds [ 1]. The cis-9,cis-12 isomer gives a value 5 c m - 1 higher than the monounsaturated value, but there seems to be no variation in the value for the corresponding trans, trans isomer. Conjugation of the two double bonds, i.e. in the 10,12 isomers, lowers the values by 10 cm -1 giving values which are very close to those observed when the olefinic group was conjugated with the carbonyl group in the monounsaturated compounds.
IlI. Acetylenic fatty acids Table 2 lists the values observed for the v(C=C) mode in a series of octadecaliynoic acids. In agreement with the study [1] of monounsaturated acetylenic acids this mode did not give rise to an infrared band. The vibration of the triple bond gives rise to two Raman bands due to Fermi resonance between the triple bond stretching mode and the first overtone of the Table 2 The v(C=-C) Raman band of the octadecadiynoic acids. Isomer
v(C=C)
6,11 6,10 6,8 6,12 7,12 8,12 9,12 10,12 Monounsaturated compound
2231/2290 2231/2296 2258 a 2230/2291 2230/2290 2231/2296 2216/2263 2257 a 2232 ± is, 2291 -+2m
a Methyl ester.
50
J.E.D. Davies et aL, Raman spectra o f some diunsaturated fatty acids
skeletal C - C stretching mode. Provided that there are two or more methylene groups between the unsaturated groups the values for the two bands are very close to those found for the monounsaturated compounds. The 9,12 isomer gives two bands at 2216 and 2263 cm -1 and the 10,12 and 6,8 isomers give single Raman bands at 2257 and 2258 c m - 1 respectively. A triple bond conjugated with a carbonyl group gives [1] a single band at 2245 cm-1.
IV. Application Comparison of the Raman spectra of methyl cis-octadec-8-enoate and methyl cis-8,cis-12- octadecadienoate (fig. 1) shows a variation in the relative intensities of the bands at 1656 cm -1 and 1440 cm -1. Jones and Ripley [2] have made a detailed analysis o f the Raman spectrum of methyl dodecanoate (methyl laurate) CH3(CH2)9CH2CO2CH3, and have shown by deuteriation studies that the band at 1440 c m - 1 arises from the bending mode of the -(CH2)9-grou p. We therefore thought it worthwhile to try and obtain a correlation between the relative intensities o f the 1440 and 1656 cm 1 bands and the ratio o f the number o f methylene to unsaturated groups in the molecule. The results are summarized in fig. 2, where R is the ratio of the number of methylene groups (ignoring the group adjacent to the carbonyl) to the number o f
(a)
(b)
j!i 1800
I 1600-
1400
i.I
i
1200 1800
Z 1600
M
I -1400
; 1200
Fig. 1. The Raman spectra (1200-1800 cm 1) of (a) methyl eis-octadec-8-enoate and (b) methyl cis, cis-octadec-8,12-dienoate examined as neat liquids.
J.E.D. Davies et al., Raman spectra o f some diunsaturated fatty actas
5l
e
O
I
[
J
5
10
15
R
Fig. 2. I vs R plots for: (a) methyl cis, cis-octadeca-8,12-dienoate; (b) methyl cis, cis-eicosa-7,13dienoate; (c) cis-tetradec-8-enoic acid; (d) cis-hexadec-10-enoic acid; (e) methyl cis-octadec-9enoate.
unsaturated groups, and I is the intensity ratio of the 1440 cm-1 band to the 1656 cm- 1 band. The scatter of the points probably arises from the fact that several of the samples fluoresced badly and thus had sloping base lines which made measurement of the band intensities difficult. The plot cannot consequently be used to obtain an accurate R value for an unknown fatty acid, but it could be used to determine the number of unsaturated groups, providing the chain length is known approximately. Furthermore the wavenumber value of the band arising from the vibration of the unsaturated group would indicate whether the group is cis or trans disubstituted.
V. Experimental The diunsaturated compounds were prepared using the literature methods [3]. The samples were examined as 20% solutions in carbon tetrachloride. A. Raman spectra
The Raman spectra were excited using the 568.2 nm line of a CRL Model 52K
52
J.E:D. Davies et aL, Raman spectra o f some diunsaturated fatty acids
krypton laser. The spectra were recorded on a Cary 81 spectrometer using 4.5 cm-1 slit widths and an exciting power of 30 mW at the sample. The instrument was calibrated with krypton emission lines. The diunsaturated compounds, like the monounsaturated compounds, gave a high background whose intensity could be reduced by exposure to the laser beam.
References [ 1] J.E.D. Davies, P. Hodge, J.A. Barve, F.D. Gunstone, and 1.A. Ismail, JCS Perkin 11 (1972) 1557 [2] R.N. Jones and R.A. Ripley, Canad. J. Chem. 42 (1964) 305 [31 F.D. Gunstone and M.S.F. Lie Ken Jie, Chem. Phys. Lipids 4 (1970) 1
