The infrared spectra of saturated fatty acids with even number of carbon atoms from caproic, C6 (Hexanoic), to stearic, C18 (Octadecanoic), and of their methyl and ethyl esters

Summary Infrared spectra from 1 to 12 microns have been obtained for 7 of the homologous series of monobasic, straight‐chain, saturated fatty acids of even carbon atom content from C6 to C18 and of their methyl and ethyl esters. Infrared data are presented as plots of the percentage transmission against the wavelength in microns on a linear wavelength scale for each compound, and the exact wavelength positions of maxima of the 11 most prominent bands are tabulated. Correlations of each of these bands with molecular structure are given. Methods for distinguishing the acids from the esters and of differentiating an ethyl ester from a methyl ester by observation of infrared spectra are described. An explanation, supported by earlier work with deuterium‐substituted compounds, is given to account for the nonappearance, in the spectra of the fatty acids of any absorption attributable to either the free O—H group or the bonded O—H… O group. Evidence has been accumulated which indicates an association of some sort of the esters and that this association probably involves the carbonyl and the methyl groups. Data are presented to show which of the bands in the infrared spectra do and which do not follow Beer's law.