Org. Synth. 1940, 20, 26
DOI: 10.15227/orgsyn.020.0026
DEHYDROACETIC ACID
Submitted by F. Arndt
Checked by W. W. Hartman and A. Weissberger.
1. Procedure
A
250-ml. round-bottomed flask is fitted with a
thermometer reaching nearly to the bottom
(Note 1) and a
three- or four-bulb fractionating column without a side arm, at the upper end of which is attached a
partial condenser (Fig. 11) (Note 2). The side arm (
A) of the partial condenser carries a 110°
thermometer and is connected to a
condenser set for downward distillation. The inside container (
B) of the partial condenser is filled halfway with
toluene, a chip of
porous plate is added, and the top is attached to a
reflux condenser.
Fig. 11.
In the 250-ml. flask 100 g. (0.78 mole) of freshly vacuum-distilled ethyl acetoacetate and 0.05 g. of sodium bicarbonate (Note 3) are placed and heated so that the toluene is kept just boiling until the liquid in the flask has reached 200–210° (Note 4). The time for heating is usually 7–8 hours, during which period 27 g. of distillate boiling at 72° (mostly ethanol) is collected, and the color of the reaction mixture becomes dark brown. The dehydroacetic acid, while still hot (Note 5), is transferred to a 200-ml. distilling flask (Note 6) and distilled under reduced pressure. After a fore-run boiling up to 128° at 12 mm., consisting of ethyl acetoacetate, has been collected, the receiver is changed and dehydroacetic acid is collected up to 140°/12 mm. The yield of dehydroacetic acid melting at 104–110° is 34 g. (53%) (Note 7). A purer product, m.p. 108°, may be secured in 80% yield by recrystallization from ethanol, using 2 ml. per gram of material (Note 8).
2. Notes
1.
The bulb of the thermometer must be immersed in the liquid.
2.
This partial condenser, which is a modification of one described by Hahn,
1 is very effective in reducing the time required for fractional distillation of many mixtures. It is best constructed of Pyrex. The dimensions given are approximate and may be varied to suit individual needs. The inside
container is a 30 by 140 mm.
Pyrex test tube sealed at the top to standard Pyrex tubing. It is very effective in the purification of
cyclohexene [Org. Syntheses Coll. Vol. 1, 183 (1941)].
2 The crude hydrocarbon mixture is first put into the flask with
ethanol in the partial condenser, and the whole heated as long as a distillate is obtained. The
ethanol is then replaced by
ethylene chloride and the
cyclohexene is collected.
3.
It is essential to use
sodium bicarbonate to secure consistent results.
4.
Above this temperature extensive decomposition sets in. The time required varies with the size of the run, being less with smaller quantities.
5.
The residue solidifies on cooling. The
dehydroacetic acid may be filtered and washed at this stage, but the yield is lower, owing to its solubility in the reaction mixture.
6.
An ordinary 200-ml. distilling flask with a large-diameter side arm placed well down on the neck gave the best results. There was no foaming, frothing, or spattering.
7.
The yield falls off with larger amounts; e.g.,
500 g. of ester gave only
35% of the calculated quantity of acid.
8.
The acid may be isolated through the sodium salt, but the quality is poorer and the yield less.
3. Discussion
Dehydroacetic acid has been prepared by the action of
acetic anhydride on
acetonedicarboxylic acid,
3 as a by-product in the pyrolysis of
acetone to give
ketene,
4 by treatment of ketene dimer with
pyridine5 or
sodium phenoxide,
6 and by removal of
ethanol from
acetoacetic ester.
7
This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
acetoacetic ester
ethanol (64-17-5)
acetic anhydride (108-24-7)
Acetonedicarboxylic acid
sodium bicarbonate (144-55-8)
Cyclohexene (110-83-8)
ethylene chloride (107-06-2)
acetone (67-64-1)
pyridine (110-86-1)
toluene (108-88-3)
Ethyl acetoacetate (141-97-9)
sodium phenoxide
Ketene (463-51-4)
Dehydroacetic acid (520-45-6)
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