JUNE, 1949
A SIMPLE KETENE GENERATOR1 S. C. WANG and F. W. SCHUELER The State University of Iowa, Iowa City, Iowa
To
THE average chemistry student thermal decomposition seems to imply a highly specialized type of reaction which employs only complicated apparatus or an elaborate commercial process, not suitable for laboratory operation, and which often results in tarry or gaseous products of unknown composition. We have found, however, that the pyrolysis of acetone to yield ketene and methane can be effected in a simple apparatus readily assembled in the laboratory. The amount of ketene so produced can he quantitatively measured and the reaction may be used for the preparation of pure organic compounds. A highly efficient ketene generator has been designed
by Williams and Hurd (1). We have constructed an apparatus by following in principle all of the essential features described by them and yet simplifying the assembly through the use of ordinary glass equipment. Of particular interest is the pyrolysis tube which can be made by anyone with no more than a few hours' training in glassblowing and a t an expense of about 30 cents for materials. The exceedingly low cost and the ease with which the apparatus may be constructed suggests its possible use for thermal decomposition reactions in elementary organic chemistry.
This work was aided by a grant from the U . S. Public Health Service.
It consists essentially of a flask b in which acetone is
APPARATUS
The complete apparatus is illustrated in Figure 1. vaporized, the pyrolysis tube c where acetone vapor is decomposed by use of a metallic filament, two condensers to separate the excess acetone from the ketene, a trap d for the removal of condensed liquid, and one or two reaction tubes e where ketene is allowed to react with aniline or some other liquid or solution. The pyrolysis tube is constructed out of a pyrex glass test tube about 25 mm. inside diameter and 20 cm. long. The pyrex test tube is narrowed a t the open end and attached to a segment of pyrex tubing about 10 cm. long with an inside diameter of 10 to 12 mm. For the heating element, No. 24 chrome1 A wire is used. The coil is made by winding 175 cm. of the wire tightly around a 6-mm. glass rod and is suspended by hooking to two leads made of double strands of the same wire twisted together. The upper lead is sealed into a smallbore glass tube which is then sealed into the pyrex test tube about 3 cm. from the closed end of the latter. The lower lead is similarly sealed a t a position about 12 cm. below the upper lead. Finally, a T-tube is sealed
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JOURNAL OF CHEMICAL EDUCATION
At the end of the reaction, the water bath is first removed, the current through the filament is turned off, and finally the stopcock is opened. The acetanilide separates as a crystalline solid upon cooling the reaction mixture. For purification it is treated with dilute hydrochloric acid to remove any excess aniline, filtered and washed with more of the dilute acid. It is next dried and weighed. The pure acetanilide may he obtained by recrystallization from boiling water. The results for several representative runs are as follows: when ketene was passed into 10 g. of aniline for 30 minutes the average amount of acetanilide isolated was 10.2 g. This represents a 70 per cent conversion of aniline to acetanilide. When 15 or 20 g. of aniline was used and ketene run in for the same length of time, the amounts of acetanilide produced remained practically the same, 10.5 g. and 11.0 g., respectively. The yield of ketene based on the meight of the isolated EXPERIMENTAL acetanilide 7vas 0.154 mole per hour and the net consumption of acetone 31.2 ml. per hour, corresponding to Acetone is placed in the flask b, containing some glass a 34.4 per cent net conversion of acetone into ketene. Cold beads or porous chips to prevent humping. water is passed through the condenser and the flask is OTHER REACTIONS heated by a water bath on a hot plate a, while the stopInstead of using aniline as the reactant, ketene may cock on the trap d is open. In order to avoid a possible explosion the filament must not be heated when a con- also be allowed to pass through an excess of standard siderable amount of air is present. Fifteen minutes' NaOH solution and the excess of the alkali back titrated refluxing will be sufficient to expel the air almost com- with standard acid. Other reactions of ketene which pletely from the pyrolysis tube. The stopcock on the may be similarly carried out are tested in Table 1. trap is then closed and the filament heated to a dull red. A very rapid rate of boiling of acetone is essential TABLE I (3, 4). I t is necessary to remove ketene from the hot Reactions of Ketene pyrolysis tube by the undecomposed acetone vapor to Reactant Product Referace prevent further decomposition of ketene into ethylene Acetic acid (6) and carbon monoxide (2) and to avoid excessive car- Water Acetic acid Acetic anhydride (a bonization, presumably through the formation of car- Hydrogen chloride Acetyl chloride Rromine Bramoacetvl bromide bon and ~vater. The recovered acetone in the gradu- -~cetamide. id ated cylinder is acidic and has a pungent odor due to Ammonia n-Butyl alcohol Butyl acetate (7) the presence of some acetic acid and acetic anhydride Hydrogen sulfide Thioaoetic anhydride (8) magnesium bro(5). . . This acetone, however, can be recycled for subse- Ethyl mide Ethyl methyl ketone (6) quent runs. Phenol Phehyl acetate (9) The high reactivity of ketene makes its isolation Ethyl mercaptan Ethyl thioacetate (10) n-Acetyl-6-naphthylamine (6) imnractical. Therefore. it is used directlv in the reac- @-Naphthylamine ---tion tubes. In our ex~erimentsthe ketene-containinggas is passed for a measured period of time into a definite LITERATURE CITED amount of aniline with which it reacts to form ace(1) WILLIAMS, J. W., AND C. D. HURD,J . Org. Chem., 5, 122 tanilide according to the following equation: 11 ,-040) - .*,.
into the chamber about 4 cm. below and on the opposite side from the top lead. After all glass manipulation is completed the filament is hooked on to the two leads with the aid of a long stiff piece of wire bent at the end. When ready for operation the leads are connected through a 100-ohm variable slide-wire resistance to a source of 110-volt alternating current. Approximately 12 to 16 ohms are required in order to heat the coil to the desired temperature. Care should be taken to make sure that the apparatus is free from leaks. Rubber stoppers are used instead of corks, or else corks should be well softened and painted with water glass one day prior to the experiment (2). The ends of glass tube inside rubber tubing must touch each other in all connections. Since the ketene is invariably contaminated with carbon monoxide, it is necessary to deliver the effluent gas directly into the opening of a hood.
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A
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H CHs=C=O
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0 H -CHz-A-
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In order to obtain complete absorption of ketene two absorption tubes are arranged in series, followed by an empty tube to receive any aniline driven over by the gas, the evolution of which occasionally becomes rather vigorous. Only a small quantity of ketene is absorbed in the second reaction tubes. Furthemore a longer absorption train retards the rate a t which the ketene passes through the pyrolysis tube, thereby reducing the yield of ketene.
(2) HURD,C. D., "OrganicSyntheses,"Coll.,Vol.I, 1 9 4 7 , ~330. . (3) HUED,C. D., AND M. F.DULL,J . Am. Chem. Sac., 54,3428 (1932). (4) Hum, C. D., AND W. H. TALLYN, J . Am. Chem. Soc., 47, 1427 (1925); W. H. TALLYN,M.S. Thesis, Univ. Ill. (1924). (5) HANPORD, W. E., AND J. C. SAUER,"Organic Reactions," 1947, Vol. 111, p. 133. (6) STAUDINGER, H., "DieKetene," Stuttgmt, 1912. (7) MOREY, G. H., Ind. Enq. Chem., 31,1129 (1939). (8) CH~CK, F.,AND N. T. M. WILSMORE, PTOC.hem. SOC.,24. 77 (1909). (9) RICE, F, 0,, J, GREENBERG, C, E, WATERS,AND R, E. VOLLRATB, J . Am. Chem. Soc., 56,1760 (1934). (10) S~EMIDLIN, J., AND J. M. BERGMANN, Bw., 43,2821 (1910).
