An Improved Method for the Preparation of Chloroform
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OLIER L. BARIL Holy Cross College, Worcester, Massachusetts
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T IS a fact to be regretted that many of the more recent laboratory manuals in organic chemistry have either omitted or deleted the preparation of chloroform. Preference has been givencto experiments in which the products are obtained in good yields and in which comparatively inexpensive chemicals are used. In perusing the different laboratory manuals which contain the preparation of chloroform, one wonders why so many different methods are given for the preparation of the same compound. If these different methods are tried, some will give almost negative results. The variations in procedure are evidently given for the purpose of increasing the yield of chloroform. The low yield obtained by the student may cause him to doubt the value of the haloform reaction. Very satisfactory yields are obtained in the preparation of iodoform and bromoform and there is no reason why chloroform should not give as large a yield as these, provided the proper precautions are taken to avoid loss of so volatile a liquid. Many laboratory books (1,2, 3), call for the distillation of the product immediately after the addition of the alcohol or acetone, resulting in a mixture of alcohol or acetone and chloroform in the distillate. Adams
and McElvain (4) shake the mixture until no more heat is evolved before distilling over the chloroform. Norris (5) uses steam distillation in tlie removal of chloroform from the reaction mixture. Cumming, Hopper, and Wheeler (6) call for the analysis of the calcium hypochlorite, as they claim that commercial bleaching powder is rather variable; the correct equivalent quantity should be taken from those results, otherwise poor yields are obtained. They add the acetone and distil off the chloroform directly. If alcohol is used instead Of acetone, they suggest the passing of chlorine gas through the alcohol, forming chloral alcoholate. This is then mixed with bleaching powder and lime, the whole mixture heated, and the chloroform distilled over. Muldoon (7) refluxes the mixture thirty minutes before distilling over the final product. Porter, Stewart, and Branch (8) distil the chloroform over into ice water in an effort to prevent any loss of chloroform. Lowy and Baldwin (9) have recourse to H.T.H., a high-test hypochlorite containing approximately 65 per cent available chlorine. As ordinary bleaching powder as obtained from chemical warehouses or used in laundries contains only about 30 per cent available chlorine, they claim that the use of H.T.H. always tends to increase the
yield due to the large amount of chlorine present, and to the ultimate complete chlorination of the reagent used. If alcohol is the reagent used, it must first be oxidized to acetaldehyde by the chlorine. Acetaldehyde is a very volatile liquid, and there is a possibility that some is lost a t this phase of the reaction. The acetaldehyde must then be chlorinated to chloral (trichloroacetaldehyde) before decomposition by the calcium hydroxide can take place. If a ketone is used, it must of necessity contain a methyl group, as does acetone, since the haloform reaction is possible only with compounds containing the acetyl group; the reaction is, in fact, a test for the presence of the acetyl group. Acetone, the ketone commonly used in this reaction, is a volatile liquid. The methyl radical of the acetyl group is 6rst chlorinated, forming trichloroacetone, which is then decomposed by the bleaching powder to chloroformand calcium acetate. Refluxing or distilling the whole reaction mixture a t once seems to give a low yield, due principally to the volatility of the chloroform itself. Unreacted acetone also distils over, thereby lowering the yield of chloroform. There may also be a loss of acetaldehyde, if alcohol is used. The solution to the problem seems to be to add the reagent slowly, and to distil the chloroform over as soon as i t is formed, using as little heat as possible, thereby preventing a too rapid volatilization of chloroform and allowing a complete condensation of vapors by means of slow distillation. The method of preparing chloroform outlined below seems to overcome most of these difficulties, and i t has worked without fail with students in elementary organic chemistry. PREPARATION OF CHLOROFORM
Introduce into a one-liter round-bottomed flask a paste made by mixing 200 grams of bleaching powder (available chlorine-30 per cent) with 300 cc. of water. Add a small piece of paraffin wax, the size of a pea, to keep down excessive frothing. A " b p p i n g tube" is then placed into the flask. This is made by sealing the end of a 5-mm. piece of glass tubing, and cutting to such a length ~" that when the oDen end, which has been ~ e ~olished,is resting.on the bottom of the flask, the closed end is leaning against the neck of the flask about one inch below the stopper. The flask is then fitted with a two-hole stopper^ A dropping funnel is inserted in one hole.. the end of the funnel extending. well below the &face of the mixture in the flask. ~ h ; o u the ~ h second hole is placed a piece of glass tubing bent a t an angle. This is then connected to a water condenser set up for distillation. The end of the condenser is attached to a receiving flask by means of an adapter. The receiver is placed in a freezing mixture, such as ice and salt, or ice and hydrochloric acid. Close the stopcock of the dropping funnel and introduce into it a mixture of 32 cc. of acetone and 32 cc. of water. Allow about 5 cc. of the acetone-water solution to enter the flask. Heat the flask gently, protecting the bottom of the flask with an asbestos or wire ~
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gauze. When frothy bubbles, which do not break immediately, rise to the surface, the reaction has begun. Withdraw the flame. The acetone-water solution can now be added slowly to the reaction mixture. If this is done carefully, the heat of the reaction will make the use of external heat unnecessary and there will be a steady condensation of chloroform. Frothing will continue as long as chloroform is being produced. When all the acetone-water solution has been added and the frothing has ceased, slowly bring the mixture to boiling and boil until the condensate is clear. PURIFICATION
Separate the chloroform from the water. Add an equal volume of a 10 per cent solution of sodium hydroxide, shake, and allow to stand. Separate the two layers, run the chloroform into a small dry Erlenmeyer flask, and add anhydrous calcium chloride. When clear, pour the chloroform into a small dry distilling flask, and distil over a steam or hot-water bath into a dry container. Collect the portion which boils between 6O0-62". YIELD
The yield of chloroform obtained by this method was so satisfactory that the preparation was given to students of organic chemistry in two different colleges. A total of ninety-one students prepared chloroform in this way with the following results: The average yield for the ninety-one students was 28.7 grams, or 54.87 per cent of the theoretical yield, which is 52.39 grams. The lowest yield obtained by any one student was 16.0 grams or 30 per cent of the,tbeoretical. The highest was 40.86 grams or 78 per cent of the theoretical. The same preparation was given to advanced students in organic chemistry, and yields of 80-85 per cent of the theoretical were obtained. The yields obtained by students in elementary organic chemistry seem high enough30 warrant inclusion of this method of preparation of chloroform in any general course in elementary organic chemistry. -
LITERATURE CITED
(1) COHEN,"Practical organic chemistry," 3rd ed., The Macmillan Co., New York City, 1928, p. 80. (2) P E R ~ l N AND K1ppINO, 'eOrganic chemistry;' Ist ed., B. Liooincott Co.. Philadelohia, Pa.. 1923. P. 181. (3) SIIDB~RO~GH m n JAMES,"practical orgaiic chemistry," 1st ed., D. Van Nostrand Co., Inc., New York City, 1926, p.
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(4) ADAMS AND MCELVAIN, "Practice of organic chemistry;' 2nd ed., McGraw-Hill Book Ca., Inc., New York City, 1933, p. 2n
(5) NORMS,"Experimental organic chemistry," 2nd ed., McGraw-Hill Book Co., Inc., New York City, 1924, p. 105. ( 6 ) CUMMIND, HOPPER, ~ r n WHEELER,"Systematic organic chemistry," 2nd ed., D. Van Nostrand Ca.. Inc., New York City. 1931, p. 434. (7) MaoooN, "A laboratory manual of organic chemistry," 1st ed.. P. Blakiston's Sons and Co...Inc...Philadelohia. . . Pa.. . 1935, p. 36. (8) PORTER, STEWART, AND BRANca, "The methods of organic chemistry," Gion and Co., Boston. Mass., 1933, p. 67. (9) LowY AND BALDWIN, "A laboratory book of elementary organic chemistry," 2nd ed., John Wiky and Sons, Inc..New York City. 1934, p. 45.
