Organic Syntheses, CV 2, 408
Submitted by Frank C. Whitmore, W. L. Evers, and H. S. Rothrock.
Checked by W. W. Hartman and L. J. Roll.
1. Procedure
In a
1.5-l. round-bottomed Pyrex flask, fitted with a
dropping funnel, an
efficient mechanical stirrer, and a
thermometer, is placed
176 g. (2 moles) of tert.-amyl alcohol (redistilled, b.p. range 0.5°). The flask is surrounded by a
water bath, and the temperature of the alcohol is held at 50–60° while
320 g. (103 cc., 2 moles) of bromine is slowly added with stirring during about two hours (Note
1). The stirring is continued for a few minutes until the
bromine color has disappeared (Note
2).
To the reaction flask containing the crude
trimethylethylene dibromide is added 540 cc. of water, and the flask is fitted with a
long reflux condenser and a
mercury-sealed stirrer (Note
3). The mixture is refluxed with stirring for three to five hours until hydrolysis is practically complete (Note
4). The reflux condenser is then replaced by a condenser for distillation, and the crude
methyl isopropyl ketone is removed from the reaction mixture by direct distillation, with stirring, until the temperature rises (Note
5) and the oil nearly stops coming over (about one and one-half hours), or until the oil coming over is heavier than water. The residue in the flask can be distilled to yield about
380 cc. of constant-boiling hydrobromic acid.
To the distillate, consisting of a yellow oil with a small lower water layer, powdered
sodium carbonate (about 10 g.) is added, and the mixture is shaken until the water layer is alkaline and nearly saturated (Note
6). The layers are separated, and the oil is refluxed for about sixteen hours with
20 g. of powdered potassium carbonate and 5 cc. of water (Note
7). The oily layer is again separated and dried over about
6 g. of anhydrous calcium chloride or potassium carbonate (Note
8). It is then placed in a flask containing
2 g. of dry sodium carbonate and distilled through an
efficient column. The yield of
methyl isopropyl ketone boiling at
92–94° (Note
9) is about
102 g. (
59 per cent of the theoretical amount). The product is slightly yellow.
2. Notes
1. The
bromine is added at such a rate that only a small amount is present at any time, as shown by an orange-red color. The speed of the reaction depends on the temperature and efficiency of stirring. The temperature of the reaction mixture remains a few degrees above that of the water bath.
After about half the
bromine has been added, the mixture becomes cloudy owing to the separation of water, and a somewhat lower temperature (40–45°) suffices to prevent the accumulation of unreacted
bromine.
2. The crude
trimethylethylene dibromide can be purified by washing with
sodium carbonate solution and water, drying, and distilling under reduced pressure; b.p.
49–51°/11 mm.;
60–70 per cent yield. Hydrolysis of the purified dibromide gives a
78 per cent yield of
methyl isopropyl ketone. However, the losses in purifying the dibromide result in a slightly decreased over-all yield of ketone so that the purification of the dibromide is not recommended.
3. The quantity of water used is 50 per cent more than the theoretical amount for hydrolyzing the dibromide and converting all the
bromine to constant-boiling
(47.3 per cent) hydrobromic acid. Less water can be used, but more darkening of the reaction mixture occurs, and the yield of ketone is slightly decreased.
4. Efficient stirring lessens the time required for hydrolysis and avoids loss of material by intermittent sudden boiling. To be sure of complete hydrolysis, refluxing should be continued for at least an hour after the specific gravity of the oily layer becomes less than that of the water layer.
5. The distillate and residue contain a very powerful lachrymator and should be handled under a good
hood.
7. The crude ketone contains small amounts of bromides, and it darkens even after repeated distillation unless first refluxed over
potassium carbonate.
8. It is necessary to dry the ketone thoroughly since water distils with it at about 78°. This fraction can be dried again to recover the ketone.
9. A
column 55 cm. high packed with glass tubes 6 mm. in length and diameter was employed with good results. A few grams of
trimethylethylene, b.p.
34–36°/735 mm., is always obtained if adequate condensers have been used throughout the preparation. The presence of a little
sodium carbonate during the final distillation hinders decomposition.
3. Discussion
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved