MAY, 1949
THE FORMATION OF ACETONE FROM ACETATES WILLIAM K. NOYCE University of Arkansas, Fayetteville, Arkansas
ONE
of the earliest reports on obtaining acetone from acetates of the alkali metals was from the work of Boyle, ~ h noticed o that the heating of potassium acetate formed a liquid. In 1732 Boerhaave showed that the liquid was not alcohol, and in 1805Trommsdorff pointed out that the liquid obtained by heating potassium or sodium acetate "stands between alcohol and ether." Chenevix, in 1809, found that any of the acetate salts formed the same liquid (I). (The constitution of the compound, which we now know as acetone, was later shown by Williamson.) This decomposition of the acetates was referred to by Perkin (2) in connection with his work on the reaction of sodium acetate and acetic anhydride. Kronig (3) made a study of the comparative yields of acetone from various metallic acetates, and has reported a yield of 37.5% from sodium acetate as compared to 82.5% from calcium acetate and 100% from the lithium salt. Others have made similar studies (4). It would appear that the acetates of the metals of the second group of the periodic table have received the most attention, but in view of the literature on the pyrolysis of the alkali metal acetates, it seems somewhat surprising to find this reaction of these salts practically ignored in most present-day textbooks. Some laboratory manuals even indicate that the reaction does not take place. In the preparation of acetone several manuals for the elementary organic chemistry laboratory call for the use of calcium acetate alone, while others prescribe a mixture of calcium and sodium acetates. According to one procedure (5), "Sodium acetate is mixed with calcium acetate in this preparation in order to facilitate the formation of acetone." Just what the author had in mind in using the word "facilitate" is problematical,
as the amount suggested for the probable yield is from about 115 to 155 per cent of theoretical, if based on the calcium acetate alone. In another manual (6) it is stated that, "The acetone is formed from the calcium acetate alone; sodium acetate is used as a flux." The student is then expected to base his calculation of the theoretical yield on the calcium acetate only. This is unsatisfactory since on this basis many students get yields of well over 100per cent. The following experiments were carried out to determine the comparative yields of acetone, under the usual laboratory conditions, from sodium acetate, calcium acetate, and a mixture of the two acetates. It was also of interest to determine whether the low yields could be attributed to insufficient heating and consequent incomplete decomposition of the acetates. The sodium acetate used was the reagent grade, anhydrous salt. It was given a preliminary heatmg in a casserole to eliminate m y moisture present. The hydrated calcium acetate was Baker's o.p. andyzed monohydrate, and was used directly. The anhydrous calcium acetate was prepared by dissolving 40 g. of calcium metal in one liter of glacial acetic acid (99.5%) to which 35 ml. of acetic anhydride had been added toavoid any water in the product. The reaction took place in a three-necked flask provided with a reflux condenseranda stirrer. - The precipitate of calcium acetate was filtered off and washed three times with 200 ml. portions of ether and then heated in a vacuum oven for several hours a t 100 to 105°C. The procedure for the preparation of acetone was essentially the same as that given by Norris (5) and
Wertheim (6). From 16 to 28 g. of the salt or mixture of salts were thoroughly pulverized and placed in a 25 X 200-mm. pyrex ignition tube. This was clamped in a nearly horizontal position and attached to a watercooled condenser by means of a bent piece of 8-mm. glass tubing, using cork stoppers. The charge was heated with a Fisher burner and the tube was rotated to several positions to make the heating as uniform as possible. This was continued until no more liquid would distill over, and usually took from 12 to 15 minutes. The distillate was collected directly in a 25-ml. distilling flask. For the second distillation the distilling flask was heated in a water bath maintained between 75 and 80°C., and the distillate collected in a weighed vial. The corrected boiling range for each run is given i? the table. For those runs in which anhydrous salts were used as starting materials, the upper limit was determined not by a change of receivers as is usually done in a fractionation, but by the upper limit of the boiling that would take place with the given temperature of the water bath. That is, the rate of distillation would diminish as the maximum temperature was reached. Then it would practically stop and the temperature would begin to drop, leaving the higher-boiling residue in the flask. In each case the per cent yield is based on the total acetate used.
data given in the table indicate that in spite of normal precautions in trying to make conditions uniform for each run, there is considerable variation in the yields. More elaborate apparatus would be advisable if more consistent results are to be obtained. As already indicated, the variations in the yieldf cannot be accounted for by incomplete decomposition of the acetates due to insufficient heat. In cases where the yields were especially low a further check was made on this point as follows. After the usual heating period the tube was allowed to cool and the residue was removed and pulverized. The well mixed material was then returned to the ignition tuhe for further heating. In none of these cases was there a significant loss in weight. That the decomposition of the acetate was complete is further confirmed by the work of Ardagh (7) and coworkers, who state that the range from 450 to 490°C. is sufficient for satisfactory pyrolysis of calcium acetate. The sodium acetate, used alone, not only gave the lowest yields but the most inconsistent ones. The salt melted readily, and then as decomposition took place it foamed a great deal, filling the tube with a porous solid. Sometimes during the heating the flame had to he removed to prevent the foam from extending into the outlet tuhe and clogging it. These conditions made i t more diicult to heat the charge uniformly. When anhydrous calcium acetate was heated alone the heTARLF: - - -- -- 1havior was very different. The solid mass stayed in its Yields of Acetone from Acetates original form throughout the heating and became someBoilina what caked. Some of the vapors escaped from within Amount. Trial range; Per the mass only after sufficient pressure had built up to Compounds used g. No. 'C. cent blow little geyser-like holes through the surface. The Calcium acetate, monohydrate 9.0 1 56.5-61.0 61 hydrated calcium acetate acted somewhat the same, 2 56.5-60.5 61 Sodium acetate, anhydrous 9.0 and in both cases a longer period of heating was neces3 57.0-60.5 58 sary than for the mixtures. In the case of the mixtures, Calcium acetate, anhydrous 9.0 4 56.0-60.5 52 particularly when the hydrated calcium salt was used, Sodium acetate, anhydrous 9.0 5 57.0-61.5 49 considerable fusion took place, hut without any appreCalcium acetate, monohydrate 16.0 6 57.0-61.0 53 ciable foaming. The vapors appeared to escape more 7 57.0-61.0 59 readily. It has been shown (7) that yields may be Calciumaeetate. anhydrous 18.0 8 56.5-61.0 52 lowered due to the pyrolysis of acetone itself, if the 9 55.5-61.0 55 vapors are not quickly removed from the high temperaSodium acetate, anhydrous 16.0 10 56.0-59.0 30 ture. zone. This may in part account for the better 11 56.0-60.0 43 results obtained from the mixtures. It will be noted that the calcium acetate monohydrate mixed with the acetate gave the most consistent yields (trials It was thought that the lower yields in some cases sodium 1,2, and 3). might be due to inadequate heating of the acetate. In order to check this, the ignition tube was weighed be- LITERATURE CITED fore and after heating to determine how close the actual H. E., AND C. SCHORLEMMER, "A Treatise an Chemloss in weight approached the theoretical loss, based on a (1)ROSCOE, istry," D. Applcton and Company, New York, 1882, Val. residue of corresponding carbonates. In no case was 111, p. 568. the discrepancy great enough to account for more than (2) PERKIN, W. H., J . Chem. Soe., 49,324 (1886). a 1 to 2 per cent difference in the percentage yield of (3) KRBNIG,W., Z. angew. Chem., 37, 667 (1924). acetone. The maximum temperatures in the center of (4) Hum, C. D., "The Pyrolysis of Carbon Compounds," A.C:S. Monoera~hNo. 50. Chemical Catalogue - Company, New the ignition tube ranged from 575 to 625'C., as deter~ o r k , i 9 2 9p. , 481.' mined by a chromel-alumel thermocouple. (5)NORRIS,J. F., "Experimental Organic Chemistry," 3rd DISCUSSION
Simple equipment was used and no attempt was made
to refine the apparatus beyond that which would normally he used by students in the organic laboratory. The
ed., McGraw-Hill Book Co., New York, 1933,p. 96. (6) WERTHEIM, E.,"Orgenic Chemistry Laboratory Guide," 2nd ed.. The Blak'iton Co., Philadelphia, 1940, p. 135. (7) ARD& E. G. R.,A. D: BARBOW,G. E. M&LEI.UN, and Ind.Eng. Chem., 16, 1133 (1924). E. W. MCBRIDE,
