Determination of the length of polymethylene chains in salts of saturated and unsaturated fatty acids by infrared spectroscopy

Abstract

AbstractThe length of polymethylene chains is determined by counting the number of, or measuring the position of, methylene vibration peaks in the 1070舑710 cm蜢1 and/or the 1380舑1170cm蜢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, ऴ/ॠ=k/(m+1); where ० 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,舰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舑710 cm蜢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舑1170 cm蜢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.