William A. Bonner and Joseph I. DeGraw, Jr. Stanford University Stanford. California
Infrared Spectra of Rlver Salts to Characterize Carboxylic Acids
Customary procedures for the characterization of carboxylic acids generally involve conversion of the pure acid to a suitable crystalline derivative such as a substituted amide, substituted phenacyl ester, substituted ammonium salt or S-l-arylmethylthiuronium salt, followed by identification of the derivat,ive through its physical properties (1-3). Such procedures have the disadvantage that they require reasonably large samples of the pure acid and, further, are applicable to water-soluble carboxylic acids in aqueous solution only after rather tedious isolation and purificat,ion procedures. A recent need to identify a small quantity of the acid fragment (in aqueous solution) resulting on hydrolysis of the Beckmann rearrangement product of the oxime of a naturally occurring ketone (4) has led us to seek a more suitable method for carboxylic acid characterization. Matthews and co-workers (5) have conveniently prepared the insoluble silver salts of a number of carboxylic acids in aqueous solution and have been able to characterize each salt by its unique X-ray diffraction patt,ern. This
report has prompted us to explore a more practical and convenient technique, namely, measurement of t,he infrared spectra of such silver salts. The infrared spectra of free carboxylic acids have been well studied ( 6 ) , both in potassium bromide planchets (7) and in Nujol mulls (8,O ) , and found to he quite suitable for the identification of the individual acids. For characterizing small quantities of such acids in aqueous solution, however, such techniques are again hampered by the above mentioned disadvantageous need to isolate and purify the free acid. The silver salts of eleven representative aliphatic carboxylic acids have been prepared in dilute aqueous solution, and their infrared spectra have been measured in potassium bromide planchets. As seen in Fig. I , each salt proved to be readily distinguishable from all others when the sample concentration in the potassium bromide disc was adjusted so that the "fingerprint" spectral region was well developed. Strong bands, common to all samples, in the 2.9-3.0 p region (residual moisture) and in the 6.4 p region (carboxylate ion)
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authentic silver salt samples. Still another advantage resides in the possibility of converting the silver salts by means of appropriate alkyl halides into esters suitably selected for confirmatory characterization by classical methods. Procedure
The silver salts in the figure were prepared by addmg 0.005 mole of the carboxylic acid to 12 ml. of water containing a few drops of phenolphthalein solution, then titrating t o the pink end-point with 10% sodium hydroxide solution. Silver nitrate solution (0.005 mole in 1 ml. hot water) was then added dropwise with stirring and the precipitated silver salt was filtered and rinsed with water, methanol and ether, then dried over PZOa under vacuum. The infrared spectra in the figure were measured on potassium bromide planchets of the silver salts, employing both the Perkin-Elmer "IR-21" as well as "Infracord" spectrophotometers. The sample concentration in the planchet was adjusted so as strongly to develop the "fingerprint" region of the spectrum as well as the C-methyl region around 7.25 p. The "Infracord" spectra were found to be entirely suitable for comparative purposes, and are the type reproduced in the figure. Acknowledgment
The authors are indebted to the National Institutes of Health for a research grant (RG-6232) which supported this study and the research which suggested it. Literature Cited
A,, A N D CURTIN,D. Y., "Sy8tematic Identification of Organic Compounds," 4th Ed., John Wiley and Sons, New York, 1956, p. 196. (2) M C E L ~ A I N S , M., "The Characterization of Organic Campounds," 2nd Edition, The Macmillan Company, New kork, 1955, p. 189. (3) CHERONIS, N. D. AND ENTRIKIN, J. B., Y3emimicro Qualitative Organic Analysis," Thos. Y. Crowell Comprtny, New York, 1 2 7 , p. 205: (4) BONNER,W. A. AND DEGRAW,JR., 3. I., Tetmhed~on(In Press). F. W., WARREN,G. G., AND MICHEL, J. H., (5) MAT~HEWS, Anal. Chem., 22,514(1950). (6) LECOMTE, J., Oleagineuz, 6.72 (1951). (7) MEIKLEJOHN,R. A,, METER, R. J., ARONOYIC,S. M., H. A , , AND MELOCR,V. W., Anal. Chem., 29, SCHEUT~E, 329 (1957). (8) JONES,R. N., MCKAY,A. F., AND SINCLAIR, R. G., J. Am. Chern. Soc., 74,2575 (1952). (9) S I ~ c ~ a r R. n , G., MCKAY,A. F., A N D JONES,R. N., J . Am. Chem. Soe., 74,2570 (1952). (1) S ~ R I N E R R., L., F u s o ~ R. ,
Infrared spedra of some silver corboxyloter; (I) acetate, (11) propionate, (Ill) butyrofe. IIV) isobulyrote, (V) valerate, (VI) ihvderate, (VII) a-methylbufyrofe, (ViiI) coproate, OX) ibcoproate, (X) caprylote, 1x1)
