Allenic Fatty Acids
Fatty acids with allenic fatty acid systems are rather uncommon in nature, and we have not encountered them in our work. However, we do have spectra from a series of synthetic allenes, both as the 3-pyridylcarbinol ('picolinyl') and methyl esters. As these spectra are unique, and have relevance to the interpretation of the spectra of acetylenic fatty acids, selected examples are illustrated with a minimal commentary. Further details are available in the published paper, but more spectra can be illustrated here than is possible in a printed publication (Christie, W.W., Brechany, E.Y., Lie Ken Jie, M.S.F. and Wong, C.F. Mass spectrometry of derivatives of isomeric allenic fatty acids.Biol. Mass Spectrom., 21, 267-270 (1992); DOI: 10.1002/bms.1200210508).
Interpretation is not straightforward in all instances, but the main diagnostic fragmentation is beta to the allenic moiety on either side, as illustrated.
3-Pyridylcarbinyl 9,10-octadecadienoate. Note that from this isomer onwards, cleavage alpha to the double bond system on the distal side (at m/z = 244 in this instance) becomes more prominent.
As the double bond system moves further from the carboxyl group, rearrangement ions within the double bond system stand out more, e.g. that at m/z = 258 in this example.
Attempts to prepare the 3-pyridylcarbinol ester of a 3,4-dienoate isomer were not successful.
Allenic systems are more stable and less prone to migrate than conventional double bonds under electron-impact ionization, so that mass spectra of methyl esters of allenic acids are also informative. Three useful ions are formed by cleavage on the far side of the molecule beta to the double bond system, at least for the 3,4- to 10,11-isomers, although the relative intensities of the relevant ions vary with the distance from either end of the molecule. They include all or part of the carboxyl group, as shown in the figure (ions a, b and c).
For example, in the spectrum of methyl 3,4-tetradecadienoate-
- ions a, b and c are at m/z = 66, 94 and 126, respectively, although only the last stands out from the background.
However, with methyl 5,6-octadecadienoate, all three ions are distinctive. This spectrum is remarkable similar to that of methyl octade-6-ynoate (see our web page on mass spectra of acetylenic acids).
In the mass spectrum of methyl 9,10-octadecadienoate, an ion that probably represents a fragment from the terminal part of the molecule (at m/z = 152) now becomes a useful diagnostic aid, while ions a, b and c are now less abundant. However, this spectrum and that of the methyl 8,9-octadecadienoate have only a superficial resemblance to the spectrum of methyl octadec-9-ynoate.
Methyl 13,14-octadecadienoate- now only the ion from the distal portion of the molecule is significant.
We have spectra on file for further allenic fatty acids, both as the methyl and 3-pyridylcarbinol ester derivatives, and these can be found in our Archive pages, but without interpretation - Archive of methyl esters --- Archive of 3-pyridylcarbinol ('picolinyl') esters. In addition, the spectra of ethyl 2,3-tetradecadienoate and 3,4-tetradecadienoate and 3,4-tetradecadienoic acid are online here. The first of these is important as no other derivative is available for this isomer.
Updated March 10, 2014