[CONTRIBUTION FROM
THE
RESEABCH LABORATORY OF ARMOUR AND COMPANY, CHICAGO, ILLINOIS]
ESTERS OF ALIPHATIC THIO ACIDS OF HIGH MOLECULAR WEIGHT A. W. RALSTON, E . W. SEGEBRECHT,
AND
S. T. BAUER
Received A p r i l 3, 1939; revised May 13, 1939
Previous work upon the preparation of thio esters has been confined to the thio esters of aromatic acids or of the aliphatic acids of low molecular weight. Muhler' prepared ethyl thioacetate by the action of acetyl chloride upon ethyl mercaptan. Wallach and Bleibtreu2 reported the formation of several thioacetates by the hydrolysis of thioacetanilides. Obermeyer3prepared methyl thioacetate and thioisobutyrate by the action of the acid chlorides upon lead mercaptides. Later Wheeler4 prepared ethyl thiobenzoate by the action of ethyl bromide upon potassium thiobenzoate. Reid" reported the preparation of ethyl thiobenzoate by the esterification of benzoic acid with ethyl mercaptan. Pratt and Reid,6in a study of the equilibrium reactions between methyl, ethyl, and propyl mercaptans with benzoic acid, stated that the stability of the esters decreased with increase in molecular weight. In a study of the methyl, ethyl, propyl, isobutyl, and isoamyl esters of thioacetic and thiopropionic acids Faber and Reid7 restated this conclusion. The preparation of ethyl thioacetate from acetyl chloride and ethyl mercaptan as reported by Muhler' was later repeated by Baker and Reid8. This earlier work casts some doubt upon the stability of the higher members of the aliphatic series. Because of this question. and the fact that no attempt to prepare these higher members has been reported previously, the author have undertaken the synthesis of some representative esters of this type. After several alternate methods for their preparation were investigated the method chosen was the action of the acid chlorides upon the respective mercaptans. The methyl, ethyl, n-propyl, and n-butyl esters of thiolauric, thiomyristic, thiopalmitic, and thiostearic acids and 1
MUHLER, Ann., 176, 182 (1875).
* WALLACH AND BLEIBTREU, Ber., 12,
* OBERMEYER, ibid., 20, 2918 (1887).
1061 (1879).
WHEELER, Am. Chem. J . , 24, 69 (1900). REID,ibid., 43, 489 (1910). 6 PRATT AND REID,J . Am. Chem. SOC., 37, 1934 (1915). 7 FABER AND REID,ibid., 39, 1930 (1917). 8 BAKER AND REID,ibid., 61, 1567 (1929). 6
502
ESTERS OF ALIPHATIC TRIG ACIDS
503
the n-propyl ester of thiooleic acid have been prepared. These are stable compounds which can be distilled under reduced pressure without decomposition. EXPERIMENTAL
Preparation of the acid chlorides.-Stearic acid (586 g., 2 moles) m.p. 67-70', was placed in a three-necked &ask fitted with a dropping funnel, reflux condenser, mechanical stirrer, and thermometer. Thionyl chloride (285.5 g., 2.4 moles) was then added over a period of two and one-half hours, and the mixture heated at 75" for two hours, The temperature was then increased t o go", and heating was continued for an additional two hours. The excess thionyl chloride was then removed under a vacuum, and the product was fractionally distilled. An 81% yield of stearyl chloride boiling a t 200-215" a t 1 mm. was obtained. The acid chlorides of lauric, myristic, palmitic, and oleic acids were prepared in a similar manner. Approximately 80% yields were obtained with the following boiling ranges: lauryl chloride, 146-150" at 16-17 mm.; myristyl chloride, 175-176" at 16-17 mm.; palmityl chloride, 191-194" a t 14-15 mm.; and oleyl chloride, 184-186" a t 12-13 mm. Preparation of methyl thioZaurate.-Tri-n-butylamine (18.5 g., 0.1 mole) was weighed into a 125-cc. stoppered distilling flask, and the flask and contents were cooled to -30" in a n acetone-carbon dioxide bath. Chilled methyl mercaptan (5.8 g., 0.12 mole) was then added, and the flask was shaken to insure a uniform solution. Lauryl chloride (21.9 g., 0.1 mole) was then added dropwise over a period of thirty minutes. The acid chloride must be added in such a manner that i t does not come in contact with the walls of the reaction vessel and solidify. The reaction mixture was then kept at a temperature of -15" for twelve hours and was then held in an ice bath at 4" for twenty-four hours. It was then removed from the ice bath and held at room temperature for an additional twenty-four hours. The reaction mixture was then dissolved in 50 cc. of ether, and the ether solution was washed with water until the washings were neutral to litmus. The ether solution was then dried with anhydrous sodium sulfate, a t e r e d , and the ether was removed under a vacuum. The crude ester was then purified by fractional distillatJion and 20.5 g. retained. The methyl esters of thiomyristic, thiopalmitic, and thiostearic acids were prepared in a similar manner. The methyl thiostearate was purified by crystallization bo a constant melting point from a 1 : l acetone-alcohol mixture. Preparation of n-butyl thio2aurate.-n-Butyl mercaptan (10 g., 1.1 mole) and lauryl chloride (21.9 g., 0.1 mole) were weighed into a 50-cc. flask. The flask was then fitted with a reflux condenser; the contents were cooled t o 20", and then mixed by shaking the flask. When the evolution of hydrogen chloride was first observed the mixture was cooled immediately t o 0" and maintained a t this temperature until the generation of hydrogen chloride had subsided. The mixture was then heated slowly t o 50" and held at this temperature for one hour, after which i t was heated to 80"for one-half hour. Dry nitrogen was then passed through the mixture for two hours a t 60". The product was then treated in a manner similar to that described for the methyl thiolaurate; 27.5 g. of n-butyl thiolaurabe was obtained. T h e ethyl, n-propyl, and n-butyl esters of thiolauric, thiomyristic, thiopalmitic, and thiostearic acids, and the n-propyl ester of thiooleic acid, were prepared by a similar procedure.
504
A.
W. RALSTON, E. W. SEGEBRECHT AND 8. T. BAUER
ii
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ESTERS OF ALIPHATIC THIO ACIDS
505
The following table shows the melting or boiling points, sulfur analyses, refractive indices, densities, and molecular refractions of these thio esters. I t will be noted that the densities a t 60" of the methyl esters are higher than those of the ethyl, n-propyl, and n-butyl esters of the corresponding acids. The observed molecular refractions are, without exception, somewhat higher than the calculated values.
Acknowledgment.-The authors wish to acknowledge their indebtedness to M. R. McCorkle who checked several of the syntheses, and to E. J. Hoffman who determined the refractive indices and densities of the esters. SUMMARY
The methyl, ethyl, n-propyl, and n-butyl esters of thiolauric, thiomyristic, thiopalmitic, and thiostearic acids, and n-propyl thiooleate have been prepared, and their refractive indices, densities, and molecular refractions have been determined.
