Alkanoic Acids

The ¹³C-NMR spectra of long-chain fatty saturated acids show six easily recognised signals at about 180.6, 34.2, 24.8, 32.1, 22.8 and 14.1 ppm for the C1 to 3 and ω1 to 3 carbon atoms, respectively. The remaining signals, usually close together in the region 29.3-29.8 ppm, are described as the 'methylene envelope'. Some of these can be identified in single compounds as can be seen in Table 2 below, but generally these are not useful signals.

In an interesting paper concerned with medium- and long-chain saturated acids, Bengsch et al. (1986) placed the chemical shifts for each chain length in decreasing order. For the C₁₄-C₁₈ acids, they gave the sequence shown below. This is often useful in making chemical shift assignments in more complex acids.

18:0	2 ω-3 12 ω-6 11 10 9 ω-5 8 7 6 ω-4 5 4 3 ω-2 ω-1
16:0	2 ω-3 - - - - - --11 10 9 ω-5 8 7 6 ω-4 5 4 3 ω-2 ω-1
14:0	2 ω-3 - - - - - - - - - - - 9 ω-5 8 7 6 ω-4 5 4 3 ω-2 ω-1

Acids and methyl esters give slightly different shifts for carbon atoms close to the acyl group, especially for C1 and C3 with smaller differences for C2 and C4-6. Glycerol esters give two signals for each of the first 6 or 7 carbon atoms, but only those for C1 and C2 are observed in most spectra. These have relative intensities of 2:1 and correspond to the α and β chains, respectively. Carbon atoms in the α-chain have the higher shift for C1 and C4, and the lower shift for other carbon atoms. Although the chemical shifts of the atoms in the α and β chains vary a little from spectrum to spectrum, their differences are fairly constant at values around 0.41, 0.17 and 0.04 ppm for the C1-3 atoms. In saturated chains, the ω1-3 signals do not change between acids and esters or between the α and β chains of glycerol esters. They may be different in unsaturated acids and in short- and medium-chain saturated acids. The characteristic shifts for short- and medium-chains are significant in the spectra of butter fat and of lauric oils (see Table 3 below)

Several authors have given values for the effect that end groups have on the chemical shifts of nearby carbon atoms (Table 1):

 

Table 1
		α	β	γ	δ	ε
Gunstone et al. (1976)	COOH	+4.43	-5.00	-0.58	-0.42	-0.24
	COOCH3	+4.43	-4.73	-0.48	-0.39	-0.20
	CH3	-7.01	+2.26	-0.30	-	-
Bus et al.	COOH	-	-	-	-	-
(1976)	COOCH3	+4.40	-4.75	-0.50	-0.45	-0.20
	CH3	-7.00	+2.15	-0.35	-0.10	-
Johns et al.	COOH	+4.4	-5.0	-0.6	-0.45	-0.25
(1977)	CH3	-7.0	+2.2	-0.35	0	+0.1
(These figures are changes from the chemical shift of mid-chain CH2 groups in saturated acids: Gunstone et al., 29.80 acids, 29.84 esters; Bus et al., 29.75 for methyl esters; Johns et al., 29.8 for acids).

Knothe and Nelson (1998) have developed an equation for predicting ¹³C-NMR shifts for the saturated carbon atoms C₁₈ acids, which also contain other functional groups such as cis and trans double bonds or oxo, hydroxy, acetoxy, and epoxy functions.

Howarth et al. (1995<) have also discussed the shifts of saturated carbon atoms on unsaturated acids and esters.

 

Table 2. Chemical shifts (ppm) for 14:0 and 18:0 acids, methyl esters, and glycerol esters (Gunstone et al. (1976), acids and methyl esters. Bus et al. (1976), glycerol esters)
	Acids		Methyl esters		Glycerol esters
	14:0	18:0	14:0	18:0	14:0 α	14:0 β	18:0 α	18:0 β
1	180.68	180.58	174.04	174.06	173.26	172.84	173.27	172.88
2	34.25	34.24	34.15	34.18	34.07	34.24	34.07	34.24
3	24.80	24.81	25.12	25.13	24.90	24.94	24.91	24.94
4	29.22	29.23	29.34	29.37	29.16	29.12	29.16	29.12
5	29.39	29.38	29.44	29.47	29.31	29.35	29.32	29.34
6	29.56	29.56	29.64	29.66	29.52	29.54	29.52	29.54
ω5	-	-	-	-	29.70		29.72
ω4	29.49	29.51	29.51	29.56	29.41		29.42
ω3	32.06	32.07	32.11	32.12	31.97		31.98
ω2	22.80	22.79	22.84	22.84	22.73		22.73
ω1	14.12	14.12	14.13	14.14	14.13		14.11

 

 

Table 3. Chemical shifts of glycerol esters of butyric (4:0), caproic (6:0), and caprylic (8:0) acids (Bus et al., 1976).
	Tributyrin (C4)		Tricaproin (C6)		Tricaprylin (C8)
1	173.14	172.74	173.31	182.89	173.29	172.89
2	35.94	36.09	34.03	34.19	34.08	34.24
3	-	-	24.56	24.59	24.88	24.92
5	-	-	-	-	28.94	28.96
ω-3	-	-	31.26	31.22	31.68	31.70
ω-2	18.37	18.40	22.31		22.63
ω-1	13.63	13.57	13.90		14.07

References

• Bengsch, E., Perly, B., Deleuze, C. and Valero, A. A general rule for the assignment of the carbon-13 NMR peaks in fatty acid chains. J. Mag. Res., 68, 1-13 (1986).
• Bus, J., Sies, I. and Lie Ken Jie, M.S.F. ¹³C-NMR of methyl, methylene, and carbonyl carbon atoms of methyl alkenoates and alkynoates. Chem. Phys. Lipids, 17, 501-518 (1976).
• Gunstone, F.D., Pollard, M.R., Scrimgeour, C.M., Gilman, N.W. and Holland, B.C. ¹³C Nuclear magnetic resonance studies of acetylenic acids. Chem. Phys. Lipids, 17, 1-13 (1976).
• Howarth, O.W., Samuel, C.J. and Vlahov, G. The σ-inductive effect of C=C and C≡C bonds: predicting NMR shifts of sp² carbon in non-conjugated polyenoic acids, esters, and glycerides. J. Chem. Soc. Perkin Trans. 2, 2307-2310 (1995).
• Johns, S.R., Leslie, D.R., Willing, R.J. and Bishop, D.G. Relaxation times of carboxylic acids. Austral. J. Chem., 30, 813-822 (1977).
• Knothe, G. and Nelson, T.C. Evaluation of the ¹³C-NMR signals of saturated carbon atoms in some long-chain compounds. J. Chem. Soc. Perkin Trans. 2, 2019-2026 (1998).