Abstract
The length of polymethylene chains is determined by counting the number of, or measuring the position of, methylene vibration peaks in the 1070–710 cm−1 and/or the 1380–1170cm−1 regions of the IR spectrum of salts of fatty acids. Plotting this peak position against the phase relationship of the vibration in adjacent methylenes gives a curve which is independent of the chain length. (Thephase relationship, Φ/π=k/(m+1); where φ is the phase difference in radians between adjacent methylenes in a chain;m is the number of methylenes in the chain;k=1,2,3,…m, withk=1 generally assigned to the in-phase vibration.) Separate curves are obtained for methylene wagging and for two arrays of coupled twisting-rocking vibrations.
Coupled twisting-rocking vibrations give as many as one peak per methylene group in the 1070–710 cm−1 region with silver, sodium, potassium and barium salts of saturated acids. Lead salt peaks split. These peaks show the total length of salts of both saturated andtrans-unsaturated acids, but only the length of the carboxylate segment in salts ofcis-unsaturated acids. (The carboxylate segment comprises the carbons from the carboxylate carbon to the first unsaturated carbon, inclusive.)
Wagging vibrations in the 1380–1170 cm−1 region show the total chain length of saturated salts and the length of the carboxylate segment in unsaturated salts, bothcis andtrans. This region also has peaks for twisting-rocking vibrations, and they are most conspicuous in the spectra of silver and barium salts.
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Kirby, E.M., Evans-Vader, M.J. & Brown, M.A. Determination of the length of polymethylene chains in salts of saturated and unsaturated fatty acids by infrared spectroscopy. J Am Oil Chem Soc 42, 437–446 (1965). https://doi.org/10.1007/BF02635587
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DOI: https://doi.org/10.1007/BF02635587