OXIDATION of ISOPROPYL * + e * + ALCOHOL to ACETONE 6
A Student Experiment
E. C. WAGNER University of Pennsylvania, Philadelphia, Pennsylvania
I
N THE description of a student experiment "The Fractionating Column in Prepasation of Acetone" by Robertson,' it is stated that the hot oxidation of alcohol by chromic acid "is largely a failure on account of the continuation of the oxidation to the acetic acid-carbon dioxide stage." Oxidation a t 25' to 40' is accordmgly specified, requiring the mixture to be chilled in ice after each addition of oxidant. In 1926 the writer worked out a student experiment which involves the same reaction, the oxidation being effected by gradual addition of chromic acid solution to the boiling solution of isopropyl alcohol in water. The reaction is carried out in a flask surmounted by a short fractionating column, through which the acetone is distilled during and after the oxidation. The crude acetone (collected below 65'), and several subsequent fractions, are redistilled through the column, permitting the easy isolation of a high yield (70% to 80% or more) of acetone boilmg 57' to 60' and over 99% pure, and also the isolation of any unchanged isopropyl alcohol (76' to 82'). This procedure involves only two major operations and a minimum of manipulation. As i t seems more direct and effective than Robertson's, a description of the experiment is appended. It is appreciated that Robertson's experiment is designed both to illustrate the oxidation of a secondary alcohol to a ketone, and to demonstrate the value of a fractionating column in the isolation of acetone. This incidental study of the usefulness of a column is evidently intended to serve as a substitute for the usual experiment on fractional distillation, whose somewhat artificial character (mixing alcohol and water and "undoing this unnecessary operation") is pointed out. A careful examination of Robertson's directions, however, reveals the fact that in order to accomplish the double purpose of the experiment the isolation procedure is made fully as artificial, involving three distillations from three different flasks, after which the student will appreciate that he should have used a column for the first distillation, and it may occur to him to wonder why a product so much distilled should finally be collected as an 8' fraction (up to 65'). If he had previously performed an experiment involving a study of fractional distillation with aid of a small c o l ~ m n it , ~ would not be necessary for him to toil
through an unnecessary distillation in the isolation of acetone in order to appreciate that a column is certain to be advantageous for such separations. DIRECTIONS FOR THE HOT OXIDATION OF ISOPROPYL
.
-
ALCOHOL TO ACETONE
Apwralus: Support a 500sc. ring-neck flask an a stand so as t o permit heating with a burner. Adjust t o the flask an a d d tion-tube. Through the side arm of the addition-tube pass the bent stem of a 250-c~.separatory funnel, and into the vertical arm fit a 3-ball Snyder column (or a Vigreux or packed column of the same size), provided with a thermometer and attached to a short condenser. Use as receiver a 200-cc. ring-neck flask. All the corks of the apparatus must be tight. Procedure: Introduce into the reaction flask 25 g. of isopropyl alcohol and 100 cc. of water. Transfer to the addition-funnel a cooled solution of 45 g. of sodium dichromate (NaCrZOr2H2O)! 75 cc. of water, and 40 cc. of concentrated sulfuric acid. Heat the liquid in the flask just t o boiling, then withdraw the Bame and add chromic acid solution slowly until boiling is spontaneously resumed. Continue addition of the chromic acid a t such rate that gentle boiling is maintained, but without active distillation until about half of the oxidant has been added. Then apply a very small &me, causing acetone t o distil, the temperature a t the head of the column not exceeding 60'. When the chromic acid solution is all added, distil offthe residual acetone slowly (about 30 drops per minute); most of i t passes over below 60". When this temperature is just exceeded change the receiver (use 50cc. Erlenmeyer flasks for this and subsequent fractions; stopper and set aside the flask containing the acetone) and collect in separate receivers the 60- t o 70- fraction, the 70' t o 85' fraction, and finally about 25 cc. above 85". Test the reaction of this aqueous fraction (?). Note the odor of the steam from the green liquid in the oxidation flask (?). Disconnect the fractionating column and condenser, and dry both of them internally by rinsing successively with alcohol and ether, and then passing a current of dry air. Adjust the column t o the flask which contains the crude acetone, attach the condenser and thermometer, and seat the flask in a perforated board. Place as receiver a dry 50-cc.Erlenmeyer flask,previously weighed. and distil offthe acetone slowly (30 drops per minute). When the temperature just exceeds 60°, introduce the 60' t o 70° fraction and redistil, collecting as acetone anything which passes over below 60'. Similarly redistil the other fractions so as t o collect as much as possible of the acetone in the h t receiver, and so as to separate as a final 76- t o 82" fraction any unchanged isopropyl alcohol. Calculate the yield of acetone on the actual weight of isaprapyl alcohol used and not recoverable, and take into consideration the purity of the acetone obtained. The purity of the isaprapyl alcohol can be derived from its specific gravity.' Assume the isopropyl alcohol recovered in the experiment t o approximate constant-boiling composition (87.9% propanal-2). Determine the specific gravity of the acetone, and calculate its purity from a
LANDOLT-BORNSTEIN, "Physikalisch-Chemisthe Tabellen."
5th ed.. Vol. I, p. 454.
09
the values reported by ICrug."' It should be above 997& The yield thus determined should be 75% to 85% of the theoretical. COMMENTS
Robertson apparently assumes commercial isopropyl alcohol to be about 90% pure. The Eastman Co. product used in this laboratory, however, had the corresponding to 97.6% specific gravity 0.7919 propanol-2. It is hence advisable to determine the specific gravity of the alcohol used. The amount of dichromate specified is a moderate excess, 41.3 g. being required by equation. Even with 48 g. of dichromate some isopropyl alcohol was recovered. As is shown below, cold oxidation as directed by Robertson leaves some isopropyl alcohol unoxidized. It would seem that in this reaction some over-oxidation can hardly be avoided, especially near the end, when the mixture contains little alcohol and much acetone. I n the accompanying table are listed results of four runs, of which three involved hot oxidation and one cold oxidation as directed by Robertson. In all cases the isolation was effected as described above. Yields KRUG,2.anal. C h . ,32, 106 (1893).
r:),
-
* Densities of acetone-water mixtures at 20": 100% acetone 0.79197. 95% 0.80748. 90% 0.82197.
are corrected for the purity of the isopropyl alcohol (97.6%) and for that of the acetone, and are calculated both with and without correction for recovered isopropyl alcohol. Vicrdofaurac Is*
Ncnlrclira Colc.
3"-
alcohol Mahod of
rcodcd
oxidoiiol
....
Hor Oxidolion, s described: 44 g. diehromate Somr:48~.dichromate Cold Oridolion: 25" to 40' (Robertson)
79.4 17.9
99.7
3.1
75.5
84.9
17.3
98.9
1.8
72.5
79.4
In two other trials using hot oxidation the acetone was collected only up to 59'. The yields were then smaller, but the product had the specific gravity of pure acetone. I t therefore seems better to extend the acetone fraction to 60'. as the yield is higher and the purity still above 99%. In the directions given the quantity of dichromate is set at 45 g. somewhat arbitrarily in absence of definite data as to the effect of excess oxidant upon yield. The assistance of Dr. J. K. Simons and of Benjamin Einhorn in the trials reported above is appreciatively acknowledged.
