MASSSPECTROMETRIC STUDIES OF UNSATURATED METHYLESTERS
1711
Mass Spectrometric Studies of Unsaturated Methyl Esters1
by W.K. Rohwedder, A. F. Mabrouk, and E. Selke N o r t h Repio.aal Research I~borOtOry,~ P&,
Illdm0 (Received December 23, 1964)
Mass spectra were determined of methyl and ethyl sorbates, four methyl hexenoate isomers, ethyl 2- and ethyl 3-hexenoate1 methyl Z-octenoate, and mme of their deuterated derivatives. Ions of mms 59, 74, 87, and M - 31 were found in accordance with the expected fragmentation of saturated methyl esters. Both methyl 2-hexenoate and methyl sorbate had characteristic peaks a t M - 15. The spectra of deuterium-labeled 2-hexenoate and sorbate and of longer chain esters indicate that.the fragment forms by cleavage at the 5-6 carbon-to-carbon bond. Formation of the mass 87 fragment of methyl 2-hexenoate appears to be due to cleavage of the 3-4 carbon-to-carbon bond and rearrangement of two hydrogen atoms rather than double bond migration followed by cleavage.
Introduction The mass spectra of methyl sorbate and the isomers of methyl hexenoate were measured along with some deuterated derivatives as part of a study on the mechanism of catalytic hydrogenation of methyl sorbate in the presence of pentacyanocobaltate(I1) i ~ n . ~ ? ~ The mass spectra of many esters have been published by Sharkey, Shultz, and Friedells along with a discussion of the fragmentation patterns. Ryhage and Stenhagen have published many papers on esters and two excellent review articles.6-10
Experimental The trans-2-, trans-&, and trans-4-hexenoic acids were prepared by the Knoevenagel condensation reactionll-13; the trans-bhexenoic acid, by the reaction of l-bromo-&butene with diethyl malonate in the presence of sodium ethoxide.13 Their melting points agreed well with those given in the literature as did the melting points of their p-toluidide derivatives, The trans2-octenoate was synthesized by condensing n-hexanal with malonic acid according to the Doebner reaction.13 Sorbic acid was purified by crystallization from aqueous solution. The methyl esters were prepared according to the method of Stoffel, Chu, and Ahrends14; the ethyl esters were prepared according to V0gel.1~ All of the esters were purified and separated by preparative gas chromatography. This means of purification led to the separation of the cis-4hexenoate present in the trans-4hexenoate preparation.
The deuterated sorbic acid was prepared by equilibrating the acid twice with deuterium oxide in the presence of KOH. Sorbic acid (5.2 g.) was dissolved in DzO (99.7% isotopic purity) containing 3.1 g. of KOH, and the mixture was refluxed for 5 hr. on a steam bath. The acid was extracted and refluxed for another 6 hr. (1) Presented at the 12th Annual Conference on Mass Spectrometry and Allied Topics, sponsored by ASTM E 1 4 Committee, at MorrtreBl, Canada, June 7-12, 1964. (2) This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. (3) A. F.Mabrouk, H. J. Dutton, and J. C. Cowan, J . Am. Oil Chemists’ SOC.,41, 153 (1964). (4) A. F. Mabrouk, E. Selke, W. K. Rohwedder, and H. J. Dutton, Abstracts pf pspers, 85, presented at the 37th Fall Meeting, American Oil Chemiets’ Society, MinneapoIis, Minn., Oct. 1963, p. 51. (57 A. G. Sharkey, J. L.Shpltz, and R. A. Friedel, Anal. Chem., 31, 87 (1969). (6) R. Ryhage and E. Stenhagen, Arkiu K m i , 13, 523 (1959). (7) R. Ryhage, 5. Stallberg-Stenhagen, and E. Stenhagen, ibid., 18, 179 (1961). (8) N. Dinh-Nguyen, R. Ryhage, and S. Stallberg-Stenhagen, ibid., 15, 433 (1960). (9) R. Ryhage and E. Stenhagen, ibid., 15, 291 (1960). (10) R.Ryhage and E. Stenhagen in “Mass Spectrometry of Organic Ions,” F. W. McLafferty, Ed., dcademic Press, New York, N. Y., 1963, Chapter 9. (11) A. A. Goldberg and R. P. Linstsad, J. Chem. SOC.,2343 (1928). (12) R. P. Linstead, E. G. Noble, and E. J. Boorman, (bid., 557 (1933). (13) R.P.Linstead and H. N. Rydon, ibid., 1995 (1934). (14) W.Stoffel, F. Chu, and E. H. Ahrends, Jr., Anal. Chem., 31, 307 (1959). (15) A. I. Vogel, “A Textbook of Practical Organic Chemistry,” 3rd Ed., Longmans, Green and Co., Inc., New York, N. Y., 1959, p. 381.
Volume 69,Number 6 Mau 1966
W. K. ROHWEDDER, A. F. MABROUK, AND E. SELKE
1712
Table I : Normalized Mass Spectra
Methyl Mass number
15 26 27 28 29 30 31 31.5 32 32.5 33 33.5 34 37 38 39 40 41 42 43 44 45 46 46.5 47 47.5 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
2-
hexenoate
28.2 7.3 53.8 13.5 35.9 2.5 3 .9 0.16 0.64
2.0 7.7 65.7 21 .I 89.1 32.1 14.1 I .3 4.8 0.16
Methyl Methyl Methyl 4Methyl 34hexenoate 5(cis) hexenoate hexenoate hexenoate
27.0 3.5 28.6 9.3 14.7 1.8 3.6 0.17 0.08 0.60 0.16 0.95 4.1 38.9 7.1 100.0 12.9 14.3 1.2 2.2
2 1
7 7 30 8 1 3
28.0 5.0 43.8 16.7 34.9 2.8 6.3
30.1 5.0 33.2 17.3 18.0 2.4 7.8
0.26 0.10 1.2 0.10 0.41 1.8 6.5 69.1 14.2 100.0 23.0 39.2 3.1 5.8 0.14
0.04 0.89 0.04 0.19 1.3 5.7 62.0 12.2 96.9 22.5 44.5 3.5 5.4
0.29 0.03 0.69 0.03 0.22 1.1 4.4 53.9 10.3 76.2 16.4 62.8 6.0 5.2 0.26
0.06
0.32
0.32 2.9 5.6 2.6 18.9 5.9 100.0 6.3 3.0 1.4 24.4 0.64 0.48 0.80 1.4 0.42 2.9 2 0 10.6 49 7 30 4 2 6 11 2
37.6 5.9 49.4 17.3 35.5 3.3 7.5
0.19 2.4 4.7 2.6 15.1 8.6 17.8 1.9 1.1 0.54 27.0 0.86 0.48 0.46 1.1
0.29 2.9 1.3 10.4 54.0 48.9 3.6 12.1 1.1 0.67 41.3 1.8 0.32 0.08
The Journal of Physical Chemistry
0.18 0.24 3.6 7.1 3.9 23.8 15.7 79.3 5.3 2.7 0.89 27.3 1.1 0.85 0.98 1.8 0.41 3.7 2.4 23.4 96.8 60.2 4.5 15.7 1.6 0.85 79.7 3.0 0.49 0.24
2.9 5.8 3.1 23.3 15.1 67.1 5.1 2.6 0.78 27.1 1.2 0.78 0.78 1.5 0.27 3.2 2.1 23.0 100.0 64.0 4.8 15.3 1.6 0.97 83.3 3.3 0.39 0.19
0.06 0.11 1.6 3.2 1.7 10.3 15.1 31.4 2.7 1.6 1.3 21.4 0.78 0.45 0.45 0.89 0.16 1.9 1.1 9.3 47.7 25.2 2.6 3.6 0.76 0.70 100.0 4.1 0.71 0.13
Ethyl
Methyl aorbate
Ethyl 23hexenoate hexenoate
26.3 5.1 20.6 8.3 15.5 3.2 3.8 0.40 0.72 0.67 0.54 0.27
5.8 8.0 67.7 15.8 81.8 2.8 3.0
4.0 14.2 88.5 19.0 83.1 4.9 5.4 0.54 2.0 0.27
Methyl Ethyl eorbate
4.1 4.9 39.3 11.1 100.0 3.3 2.4
6.6 8.5 44.6 15.1 47.3 3.6 1.9 0.10 0.35 0.21 0.21
0.93 4.3 52.9 14.7 76.3 21.7 21.2 2.9 7.0 0.18
0.08 0.10 0.08 0.08 0.49 2.2 29.4 5.8 92.3 11.7 8.6 0.95 3.4 0.12
0.27
0.18
0.11
1.1 5.9 11.4 5.4 7.7 1.9 12.8 0.91 1.1 0.27 8.7 0.46 2.0 4.0 8.2 2.3 24.1 35.4 100.0
0.14 2.5 5.9 2.9 16.1 5.4 100.0 10.7 11.8 1.6 3.0 3.6 0.91 0.68 1.4 0.32 3.1 2.0 10.9 24.8 21.8 5.5 6.2 1.7 25.0 1.4 0.3
10.0 2.4 0.32 0.80 3.7 0.27
0.80
2-
octenoate
Methyl 2hexenoated14,
dab
Methyl aorbate-
as
20.2 4.0 49.7 14.3 69.2 2.5 2.9 0.07
2.1
0.02 0.02 1.3 3.0 1.9 11.4 7.7 14.6 4.4 3.4 0.39 0.36 5.4 1.4 0.34 0.61 0.15 1.6 1.0 8.1 31.1 80.1 8.7 9.7 1.1 2.6 0.24
2.2 9.5 78.3 17.9 81.3 6.2 14.0 1.5 3.9 0.27 0.04 0.42 0.02 0.27 4.0 9.5 5.4 5.8 1.5 11.5 2.5 1.0 1.4 0.10 0.17 0.81 2.5 6.2 2.1 20.4 28.8 100.0 14.8 5.4 1.5 0.83 0.06
0.17 0.02 0.54
0.52
0.56 2.9 50.0 17.0 81.5 29.8 19.9 1.3 5.1 0.20
1.1 6.0 41.1
2.4 8.1 25.2
67.4 71.4 77.7
92.4 59.6 70.5
24.0 8.6
42.4 12.4
0.35
2.5
1.4
0.07 0.10 1.4 4.5 2.8 22.2 11.9 100.0 16.1 16.2 2.1 31.5 1.2 0.52 0.17 0.87 0.17 2.7 2.6 13.6 34.3 22.1 12.9 8.7 2.9 1.0 29.4 2.3 0.14 1.4
0.86 1.4 4.6 5.2 11.8 10.4 80.6 100.0 31.4 11.4 32.0 5.1 2.3 0.57 1.7 1.0 1.7 2.1 5.7 14.9 45.7 32.0 28.6 28.3 12.7 34.3 28.3 9.3 2.9
1.0 4.3 10.0 15.2 15.2 10.0 7.6 10.5
8.1 5.7 22.8 4.2 1.4 2.4 3.8 5.7 6.2 4.7 13.6 29.5 46.7 76.2 100.0 57.2 8.5 3.8 6.2 3.3 0.50
MASSSPECTROMETRIC STUDIES OF UNSATURATED METHYL, ESTERS
1713
Table 1 (Continued) Methyl Methyl 4Methyl Methyl 23hexenoate 54hexenoate hexenoate hexenoate (cis) hexenoate Methyl
Mass number
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 109 110 111 112 113 114 115 116 117 124 125 126 127 128 129 130 131 132 133 134 135 138 139 140 141 142 143 144 155 156 157 158
0 42
0.29
0.28
0.31
15 0 1 3 0 96
3.2 0.40 0.37 0.16 2.4 1.6 3.7 1.9
4.7 0.43 0.41 0.26 5.1 1.4 6.0 0.41
4.7 0.39 0.39 0.39 4.1 1.3 6.2 0.47
8 1 57 3 0
2 6 53 4 2 2 0
1 3 4 2 45
6 6 5 8 9 6 32
0.95 3.8 6.7 0.70 1.2 0.48 0.16
2.4 6.8 23.6 2.2 1.2 0.57 0.20
2.2 7.4 25.5 2.1 1.2 0.78 0.39
0.03 0.45 0.03 1.7 0.14 0.17 0.11 2.0 0.56 5.4 0.33
0.67 9.3 24.5 2.4 0.67 0.78 0.17
Ethyl
Methyl sorbate
0.21 2.4 0.40 2.9 1.2 5.6 1.3 1.3 0.03
0.21 2.1 3.5 64.1 6.7 2.9 1.9 0.43
0.59 0 48
0.32
32 1 2 9 0 16
4.22 0.37
0 2 43 5 0
10 2 3 5 80
0.56 22.7 2.2 0.24
0.41 0.16 6.7 0.59
1.0 37.4 3.9 0.47
0.47
0.13
7.2 0.85
2.4 0.33
0.19 1.2 39.5 4.3 0.70
16.0 2.6 0.39
68.9 5.9 0.80
0.43 4.7 41.3 4.5 0.80
Ethyl 23hexenoate hexenoate
0.45 2.5 0.45 6.8 1.6 6.3 2.3 7.5 3.2 1.7 3.9 0.23 0.86 0.07 3.8 0.48 2.7 4.4 74.5 7.2 42.1 4.5 7.6 0.73 0.14 0.05 7.3 0.95 3.0 8.8 2.1 0.39
0.84 14.5 2.8 0.32
0.07 0.59 7.5 1.3 0.18
1.6 0.64 1.0 0.12 1.8 0.34 0.06 11.2 0.51 0.04
0.75 3.5 7.8 0.89 1.5 0.46 0.36
0.07 0.16 2.4 2.4 0.3
0.08 0.34 0.04
0.18 0.49 15.8 2.2 0.36
Ethyl sorbate
0.25 1.7 0.21 3.3 0.87 3.8 5.1 0.37 0.62
0.04 1.7 4.3 89.6 9.5 63.3 4.9 1.1 0.37 0.17
0.02 6.8 8.5 1.0 0.21
20.9 2.5 0.02
0.21 1.4 38.1 4.8 0.62
Methyl 2octenoate
0.45 1.8 1.1 18.2 31.4 12.6 1.3 7.6 0.80 79.0 7.6 0.70 0.03 0.17 0.52 0.17 6.3 22.4 4.5 0.87 2.7 16.0 20.2 1.7 0.14 0.70 0.49 0.35 31.2 10.2 2.1 0.24 27.6 29.4 3.8 4.7 1.1 0.28
Methyl 2hexenoate d1*, dl5
1.8 1.7 0.57 4.6 3.4 12.5 10.6 13.3 5.3 37.3 48.6 12.3 2.3
1.1 3.4 11.4 8.0 21.3 74.9 13.6 4.2 2.9
0.86 0.97 6.6 4.1 11.4 42.9 8.0
6.9 4.4 15.7 57.1 11.0 2.4 1.71 3.43
Methyl sorbatedr
0.50 1.*o 1.9 1.4 2.4 2.9 3.3 4.3 5.7 5.2 3.8 1.0
0.5 1.4 4.8 15.2 47.2 85.7 54.8 7.6 1.9 1.0 0.50 11.4 84.7 84.7 11.4 3.8 1.4
0.50 1.9 12.4 40.5 64.2 40.5 4.8 1.0
1.01 0.07 0.77 7.6 1.8 0.38
Volume 69,Number 6 May 1966
1714
after dissolving it in 40 ml. of 7.7% KOH in DzO. Nuclear magnetic resonance spectra of both the deuterated and nondeuterated methyl esters were measured on a Varian A-60 spectrometer, but the spectra are being published elsewhere. The n.m.r. studies indicated that there was 0.6 proton on the number 2 carbon atom, 1.3 protons on the 3-, 4-, and 5-carbon atoms, and 0.4 proton on the number 6 carbon atom. Deuterated 2hexenoic acid is a product of the reduction of sorbic acid with deuterium in the presence of DzO and pentacyanocobaltate(I1) For the deuterated methyl 2-hexenoate, n.m.r. indicated that there was 1.0 proton on the number 4 carbon atom and 0.3 proton on the number 5 carbon atom with a full compIement of protons on the rest of the molecule. The mass spectra of the samples were measured in a Bendix Model 12 time-of-flight mass spectrometer equipped with a heated inlet at 150”. Backgrounds were taken before each sample and, except for air peaks, the background peaks were not significant. The air background was quite reproducible and was subtracted out of the spectrum almost completely. The deuterated esters were trapped from a gas chromatograph and were subjected to severe air leakage both during trapping and during injection into the heated inlet system. The spectra below mass 37 and the peaks at 40 and 44 have not been included because corrections could not be made for the air peaks.
Results and Discussion The mass spectra are tabulated in Table I. The molecular ion peaks are somewhat more intense for all the unsaturated methyl esters than for methyl hexanoate.5 The mass spectra of methyl 3-, 4-, and 5hexenoates exhibit the same general fragmentation pattern as saturated methyl esters. They have a small peak at mass 113 due to loss of a CH3 group, not found in the spectrum of methyl hexanoate. They also have a 97 peak due t o the loss of a methoxy group and a 59 peak due to the methoxy carbonyl group. The 74 peak, due to cleavage of the 2-3 carbon-to-carbon bond and rearrangement of a hydrogen atom, is the largest in the methyl 5-hexenoate spectrum. This peak is considerably smaller for the methyl 3- and 4-hexenoates. The 87 peak due to cleavage of the 3-4 carbon-to-carbon bond is quite small in the methyl 3-, 4-, and 5-hexenoate. The hydrocarbon fragments are more prominent for the methyl 3- and 4-hexenoates than they are for the other isomers. The spectrum of the methyl cis-4-hexenoateis almost identical with the methyl trans-Chexenoate as would be expected.8 Both methyl 2-hexenoate and methyl sorbate have A4 - 15 peaks at respective mass numbers 113 and 111. The Journal of Physical Chemistry
W. K. ROHWEDDER, A. F. MABROUK, AND E. SELKE
Ryhage and Stenhagenlo have observed a mass 113 peak in the spectrum of methyl 2-octadecenoate which they attribute to the cleavage of the 5-6 carbon-tocarbon bond. The 5-6 carbon-to-carbon bond cleavage is also indicated by the presence of a major peak at 113 in methyl 2-octenoate due to the loss of a C3H, group, which is equivalent to the loss of CH3 for the methyl 2-hexenoate. Holmanl‘j has observed that the mass spectrum of methyl 2,4-decadienoate has an important peak at mass 111 indicating a cleavage at the 5-6 carbon-to-carbon bond like that found for methyl sorbate. Proof that the carbon lost is the number 6 is seen in the spectrum of the deuterated methyl sorbate. The intensities of the parent peaks at masses 129, 130, and 131 are 40.5, 64.2, 40.5; accordingly, the respective molecules contain 3, 4, and 5 atoms of deuterium. The peaks at 112 and 113 are equivalent to M - 17 and M - 18 or the loss of CHDz and CDs. The only carbon atom in methyl sorbate which could contain that much deuterium is the sixth. Since the mass spectrum of ethyl sorbate has an M - 15 peak identical with the M - 15 peak of methyl sorbate, the CH3 lost cannot be from the methoxy group. The 113 peak in methyl 2-hexenoate is due to the cleavage of the 5-6 carbon-to-carbon bond and probably forms the ring structure proposed by McLaff ertyl’ to explain characteristic peaks in the spectra of methyl esters of a-unsaturated long-chain fatty acids. CH3
C F
‘0 CH=CH \ \ / CH2 C
+o \\
0, ,CH=CH
\CH2
C
--t
4-
CH3
>o-c/Hz
\+Iz CH3
The large methyl sorbate peak at 111 appears to be formed by the same mechanism as the 113 peak in the methyl 2-hexenoate spectrum. C p
0 CH=CH \ I
$0
\
CH
qi-I
-
C F
0 CHy,CH \ I
