The FRACTIONATING COLUMN in PREPARATION of ACETONE G. ROSS ROBERTSON University of California a t Los An&s
Isoprofiyl alcohol, now available at low cost, is readily converted into acetone in a n experiment suited to beginners i n organic chemistry. The yield i s high if the temperature of oxidation be kept low. The resulting aqueous acetone solution i s then completely distilled huice; first by the ordinary method and then through a fractionating column. The difference in boiling point between acetone and water i s just suficient to give a clear demonstration of the superiority even of a commonplace column.
+ + + + + +
A
SPECIFIC demonstration of the value of a fractionating column in organic synthesis is generally not provided in laboratory manuals. To be sure, one may find the common but arbitrary scheme of mixing two unrelated solvents. The student has the task of undoing this unnecessary operation, and may
not have great intellectual respect for the assignment. Much better is a genuine organic synthesis requiring the use of a column for purification. Unfortunately, the few examples of such a separation in standard manuals, such as alcohol from water after a fermentation and benzoyl chloride from phosphorus oxychloride, have drawbacks. The preparation of acetone by oxidation of isopropyl alcohol is so much more satisfactory than by the conventional acetate-dry distillation method that its inclusion very early in an elementary course is practicable. The aqueous solution obtained by the oxidation process is readily fractionated. The 44' interval between the boiling points of acetone and water is just sufficientto make the limited virtues of a simple column stand out. The accompanying figure shows the behavior of the same aqueous acetone when distilled with and without the column.
The oxidation of hot isopropyl alcohol by the dropwise addition of dichromate, with prompt expulsion of the volatile ketone, similar to the procedure in preparing acetaldehyde, is largely a failure on account of the continuation of oxidation to the acetic acidcarbon dioxide stage. A reduction of temperature to 40' or less almost entirely stops the second oxidation, and a large yield of acetone is a t once available. The following directions, obviously intended for beginners, give good results in the hands of careful inexperienced students: To a 700- or 1000-cc. flask containing 25 cc. water add carefully 50 cc. concentrated sulfuric acid, and mix by gentle shaking of the vessel. Cool the flask under the tap; then add 100 cc. water and one-half mol isopropyl alcohol (33 g. of the commercial aqueous alcohol contains approximately this amount). Provide an ice-and-water bath of size large enough to permit the complete immersion of the body of the flask. This bath should contain a t least 500 g. finely crushed ice, together with enough water to facilitate frequent introduction and removal of the reaction flask. Dissolve 52 g. sodium dichromate in about 25 cc. hot water; this quantity is slightly in excess of the theoretical requirement. A thermometer is now mounted in the flask in such a manner that it is held rigidly, with its bulb in the reaction mixture, but with some arrangement for adding a liquid reagent from time to time. A common cork, bored to receive the thermometer, and with one side cut off, is satisfactory. Immerse the flask in the ice bath, and add the dichromate solution in from ten to fifteen portions. After each addition shake the vessel momentarily in the open with a gentle rotary motion, and immediately plunge i t into the ice bath. If the temperature threatens to rise above 40°, remove the flask from the bath, shake again momentarily and return promptly to the ice bath. As soon as the temperature falls to 25', add a new portion of dichromate. Do not try to lower the temperature below 25", lest unreacted material accumulate. At the end of this operation the color should be green, possibly with a slight olive tint, indicating excess of dichromate. When the temperature of the reaction mixture shows no further tendency to rise spontaneously, transfer the material to a 500-cc. distilling flask containing boiling stones. Distil rapidly, using a 100- or 125-cc. distilling flask as receiver. A receiver with low sidestem is preferable, so that the difference in the two redistillations will be accentuated. When the temperature of distillation finally reaches the boiling point of water, allow 10 cc. of distillate to collect a t this temperature, and then discard the residue in the boiler. Attach the distilling flask, with its accumulation of crude aqueous acetone, and some fresh boiling stones, to the condenser, and provide a bent adapter to convey the next distillate to a 50- or 100-cc. graduated cylinder, which is used as a receiver. In warm weather stand this cylmder in ice water.
Now distil the entire contents of the flask as rapidly as the following program of observations will permit. (Note that a slow distillation, permitting excessive reflux, will obscure the point of the experiment as far as manipulation is concerned.) At each 2- or 3-cc. interval record in the notebook the boiling fioint and yield so far collected. A few drops of relatively less volatile liquid will always be found as residue in the distilling flask. Investigate this material; is it pure water? Transfer the distillate from the graduated cylinder to a 200-cc. round-bottomed flask to which is fitted a small Hempel or other fractionating column. From this flask and column redistil the aqueous acetone slowly, (about 2 drops per second) making a record of temperatures and yields just as in the previous distillation. There must be no interruption of the distillation while these data are being taken. Watch out particularly for drafts in the laboratory which cause irregular cooling of the column. Any fall in temperature is definite evidence that the apparatus is not being manipulated properly. As the rate of distillation falls, gradually heat the flask more strongly. When the temperature a t the top of the column reaches 65', substitute a test-tube for the graduate and set aside the distillate as your preparation of acetone. As soon as possible place the emptied graduate back in position, add to i t whatever material has accumulated in the test-tube, and continue the distillation until you can get no more liquid to go over into the receiver. The product from this experiment may be treated with about 5 g. of coarsely crushed quicklime, and allowed to stand overnight. When finally distilled from the lime, the resulting liquid should be highgrade acetone of narrow boiling range. Draw on a single piece of aoss-section paper a graph representing the two distillations, with and without
the column. In this graph plot the boiling points as ordinates, and yields in cubic centimeters as abscissas.
