A BENCHSCALE PREPARATION OF THIOACETAMIDE WILLIAM P. O'CONNOR, GEORGE W. COGSWELL, and EMIL J. MORICONI Fordham University, New Yo&, N.Y.
DESPITEthe numerous non-sulfide methods of qualitative cation separation and analyses that have been suggested "to increase pedagogical efficiency while eliminating the unpleasantness and dangers of sulfide separations" (I), the classical hydrogen sulfide scheme, modified only as to the source of sulfide, still remains the most popular and most widely used in undergraduate instruction. Of the various organic and inorganic sources for sulfide proposed and tested (I), the most acceptable has been thioacetamide. The fundamental work of Barber, Grzeskowiak, and Taylor (t,S), Flaschka (4), Swift and Butler ( 5 ) , and more recently, Lehrman and Schneider (6),have placed the use of a 5%-13% by weight aqueous solution of thioacetamide as a source of hydrogen sulfide in qualitative and quantitative analysis schemes (3, 4 , 7 , 8) on a firm experimental basis. For the past four years, the bench-scale preparation of thioacetamide according to the following reaction 0
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has been assigned as a senior undergraduate problem in our Organic Process Laboratory. In addition to the practical advantages of such training which includes use of larger-than-laboratory scale equipment, a study of process variables, and the necessity for solvent recovery, this process exercise has been a source of pride in accomplishment t o the students since the thioacetamide thus obtained a t considerably less than commercial cost,' has been successfully used in all our undergraduate qualitative analysis laboratories. The method of procedure and isolation of thioacetamide reported in this paper is a modification of that originally proposed by Hantzsch (Q), and improved upon by Kindler and Findorff (lo), and Persina (11). These recorded procedures, however, all require repeated and time-consuming extractions of the reaction mass with large volumes of solvent henzene or xylene for complete thioacetamide recovery, and only Persina (11) reports a yield (35%40%). I n our hands, the maximum yield obtainable using Persina's procedure on a laboratory scale has been 32%. Schwarz (19) has also prepared thioacetamide in situ by this reaction in the synthesis of 2,4dimethylthiazole. Process variables which have been studied include the effect of reaction time, temperature, mode of addition of reactants, and solvent concentration, on the yield and purity of product. The optimum conditions used herein are a henzene :acetamide: PZSSmole ratio of 80: 5.08: 1, reaction temperature 70°-75", and reaction time, onehalf hour. Thioacetamide may he purchased from Distillation Products Industries. Armahhoe Chemicals. Inc.. Matheson Coleman and T:. Baker, and A. ~ a i g g i and r Co. (Sulfi-down) a t Bell, ~ n e . , ' ~ prices ranging from $25 per lh. t o $15 per 100 g.
VOLUME 35, NO. 8, AUGUST, 1958
PROCEDURE
I t i s essential that the reaction be conducted only where there is excellent ventilation because oj the toxic effects of benzene and hydrogen sulfide gas. Three and one-half liters of benzene are heated to 70°C. in a three-neck five-liter round bottom flask, fitted with a good stirrer, reflnx condenser, thermometer, and heating mantle with Variac control. One hundred and fifty grams of technical grade acetamide is then added to the hot ;olvent and stirring vonti~~nnl ior ?INJUI fivt. nu nut^^ until IIIP :~wr:irnidei$ i n ~ o l ~ n i o n01.e . 111111drcd and ten grams of technical grade diphosphorus pentasulfide2is slowly added (5-10 min.) while vigorous stirring is maintained. The solution is heated for an additional 30 minutes at which time the original green solution becomes dark yellow. The stirring is stopped and the bright yellow reaction solution decanted into a fourliter beaker immersed in cold water, taking care that none of the dark brown oily material which always forms in the reaction mass is carried over into the beaker. Crystallization of the thioacetamide needles begins almost immediately from the externally cooled solution. After about two hours the product is filtered on a Biichner funnel and air dried. The henzene filtrate is placed in a distillation flask for concentration and recovery. This distillation flask is a twelve-liter round bottom flask fitted with a one inch by thirty inch fractionating column packed with glass heads. The henzene mother liquors from several runs are combined and fractionally distilled. We have noted, however, that extensive solvent contact time reduces the purity and yield of the thioacetamide recovered from the mother liquors. When these combined filtrates have been concentrated to about onethird their original volume, they are cooled, and the thioacetamide filtered to yield an additional 5%-15% of product. When about 90% of the henzene solvent has been recovered, the stillpot residue is filtered and recrystallized from toluene-petroleum ether and finally from 95% ethanol which can also be recovered. The tarry material originally left in the reaction flask after the decantation step is easily removed with hot water. The recovered benzene (90%) is used for the next sequence of runs. Yield of thioacetamide, 35%40%; m.p. 109°-1110. This thioacetamide is used without further purification in qualitative analysis. LITERATURE CITED (1) See WEST,P. W., AND M. M. VICK,J. CHEM.EDUC.,34, 393 (1957), for an excellent summary of both sulfide and non-sulfide schemes. Anal. Chem., 21, (2) BARBER,H. H., AND E. GRZESKOWIAK, 192 (1949). (3) BARBER, H. H., AND T. I. TAYLOR, "Semimicro Qualitative Analysis," rev. ed., Harper & Brothers, New York, 1953.
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Generously supplied by the American Agricultural Chemical Co., New York, N. Y.
(4) FLASCHKA, H., Chemisl Analyst, 44, 1 (1955). (5) SWIFT,E. H., AND E. A. BUTLER, Anal. C h m . , 28,146,170 11010\ \&""",. (6) L E H R ~ NL.., AND P. SCHNEIDDR. J. CHEM.EDUC.. . 32.. 474 (1955).
(7) HOGNESS, T. R., A N D W. C. JOHNSON, ' r Q ~ d i t a t i Analysis ~e and Chemical Equilibrium," 4th ed., Henry Holt and Co., Ine., New York, 1954; SORUM,C. H., "Introduction to Semimicro Qualitative Analysis," 2nd ed., Prenticc Hall, Inc., New York, 1953; Esstmsn Organic Chemic.48 Abstract, "The Precipitations of Insoluble Sulfides with
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Thioacetamide"; Technical Data Sheets from Fisher Scientific Co. and A. Daigger and Co. GUNNING, H. E., J. CHEM.EDUC.,32,258 (1955). HANTZSCH, A,, Ann., 250,264 (1888). KINDLER,K., AND F. FINNDORFF, Ber., 54, 1079 (1921); Ann., 431, 209 (1923); D.R.P. 385, 376; C h . Zatr., 1924 I.-, -2633. ~ -PERSINA, A. G., J . Gem Chem. U.S. S. R., 9,804 (19391; C. A,, 34,426 (1940). SCHWARZ, G., "Organic Syntheses," Vol. 25, p. 38, John W h y & Sons, Inc., 1945. ~~
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