892
R. T. O’CONNOR, R. R. MOD,31, D. MURRAY AND E. L. SKAU
Vol. 77
[CONTRIBUTION FROM THE SOUTHERN REGIONAL RESEARCH LABORATORY’]
The X-Ray Diffraction and Infrared Spectra of Molecular Compounds of Acetamide and Long-chain Saturated Fatty Acids BY ROBERT T. O’CONNOR, ROBERT R. MOD,MILDREDD. MURRAY AND EVALD L. SKAU RECEIVED AUGUST2, 1954 Infrared spectra and X-ray diffraction data have been obtained for the four molecular compounds formed between acetamide and lauric, myristic, palmitic and stearic acids. The infrared spectra in the rock salt region resemble those of the constituent molecules and, like the spectra of the saturated fatty acids, are identical for all members of the homologous series. Infrared spectra are not useful for the identification of the individual molecular compounds. From X-ray diffraction patterns “long” and “short spacings” have been measured. The “long spacings” of the molecular compounds have been shown t o correspond in length to two molecules of saturated fatty acid and two molecules of acetamide. Each pattern also exhibits a weak “long spacing” identical t o that of the “C” form of the parent acid. In addition the myristic acid-acetamide compound shows a very weak “long spacing” equal to that reported for an unstable third modification of a Clr fatty acid. These spacings can be accounted for by the sublimation of a small amount of acetamide from the saturated fatty acidacetamide addition compounds during the exposure to the X-ray radiation. “Short spacings,” obtained by the usual “powder” technique and useful for analytical identifications, are also reported.
Recently Magne and Skau2 showed that 1 : l compounds of the general formula RCOOH. NH&OCHj are formed between acetamide and long-chain saturated fatty acids. They reported binary freezing point data and apparent molecular weights in 1,4-dioxane for the molecular compounds formed between acetamide and lauric, myristic, palmitic and stearic acids. Optical properties of these compounds have now been investigated. The infrared spectra in the rock salt region from 2 to 12 p and the “long” and “short” crystal spacings from X-ray diffraction patterns are reported in this paper. Experimental The saturated fatty acids and acetamide were purified2 and the molecular compounds prepared by fusing together equimolecular proportions of the acid and acetamide and allowing the melt t o solidify. The infrared spectra were obtained both in chloroform solution and as hTujo13mulls with a Beckman3 IR-2T infrared spectrophotometer. instrument was housed in a room maintained a t about Thz 23 and 20% relative humidity and its temperature was maintained constant at 25 f 0.1’ by water circulated from a constant temperature bath. The X-ray diffraction patterns were obtained with a General Electric* Diffraction Unit Model XRD, using Cu Kcu radiation with a nickel filter (0.0007 inch thick). The “long spacings” were measured from photographs obtained by pressing a small amount of sample, enough t o produce a layer several tenths of a millimeter thick, on a thin piece of glass. This sample was mounted in the X-ray apparatus with a plate distance of 10 cm. and the mount was oscillated through an angle from 0 to 10”. The “short spacings” were calculated from patterns obtained by photographing the finely powdered samples in sealed capillary tubes at a plate distance of 6 cm. Exposure times for both “long” and “short” spacing patterns were 2.25 hours.
Infrared Spectra.-Infrared spectra were obtained both from chloroform solutions, containing about 25 g. of sample per liter of solution, and as finely ground Nujo13 mulls. The solid and solution spectra were identical throughout the rock salt region, in spite of the fact that the molecular compounds are highly dissociated into fatty acid and acetamide molecules in solution.2 This is not surprising in light of the fact that the addition (1) One of the laboratories of the Southern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture. (2) F . C. Magne and E . L. Skau, THIS JOURNAL, T4, 2628 (1952). (3) Mention of names of firms or trade products does not imply that they are endorsed or recommended by the U. S. Department of Agriculture over other firms or similar products not mentioned.
compound contains no new vibrating groups. The molecular compound formation most probably involves hydrogen bonding with 0-H. . .O and N-H. . .O groups, but these groups are also present in the spectra of fatty acids and acetamide, even in relatively dilute solution. The vibrating groups which give rise to the other characteristic bands in these molecules are likewise not perceptibly altered by the addition compound formation. Hence no change would be expected in going from the spectra of the addition compound in the solid form (Nujol mull) to a dissociated form (chloroform solution). Comparison of the infrared spectra of the molecular compounds with those of the fatty acids4 and of acetamide5 affords further confirmation. The spectra of the four molecular compounds in chloroform solution are shown in Fig. 1. TABLEI THE PRINCIP.4L ABSORPTIOSBANDS IN THE INFRARED FATTY -4CIU MOLECUSPECTRA OF ACETAMIDE-SATURATED LAR COMPOVNDS Wave length of absorption max., H
Probable vibratmg group which gives rise to the obsd. absorptlon band
3.12
S-H stretching (only partially resolved from
sti ong C- H stretching) C-H stretching (any C-H group) n’eak shoulder, probably branch of 0-H. . e 0 (boiided (OH) stretching) 5 . 8 5 C-0 stretching, characteristic of fatty acids 5 . 9 3 C=O stretching, amide; poorly resolved from C=O of fatty acids XHz bending 6 28 6 . 6 7 S-H bending 6 . 8 7 Doubly degenerate CHI methyl deformation arid
