Organic Syntheses, Coll. Vol. 1, p.10 (1941); Vol. 5, p.5 (1925).
In a 5-l. round-bottomed flask (Note 1)
, fitted with a mechanical stirrer
, is placed 3 kg. (1555 cc.) of fuming sulfuric acid
(20 per cent of free sulfur trioxide
). Then the flask is cooled very efficiently with a thick pack of ice and salt, until the temperature of the acid reaches −5° (Note 2)
. The stirring is started, and 700 g. (3.64 moles) of finely powdered u. s. p. citric acid
is added gradually. The speed of the addition is regulated according to the temperature of the reaction mixture. The temperature should not rise above 0° until half of the citric acid
has been added, after which the temperature should not be allowed to exceed 10° until the reaction is complete. The addition requires three to four hours, provided efficient cooling is used. The citric acid
should be in solution at the end of this time; if not, the stirring should be continued until it has dissolved completely.
The temperature of the reaction mixture is allowed to rise gradually until a vigorous evolution of gas commences; at this point the flask is cooled with ice water to stop the excessive frothing, but cooling is not carried far enough to stop the evolution of gas entirely (Note 3)
. After the more vigorous foaming has ceased, the reaction mixture is raised to about 30° and kept there until no more foaming occurs. A convenient way of determining this point is to stop the stirring for a moment and allow the mixture to remain quiet. After a minute or so, a clear brown liquid giving off very few gas bubbles should result. This general procedure requires two to three hours.
The reaction mixture is cooled down again with ice and salt until the temperature reaches 0°, then 2400 g. of finely cracked ice is added in small portions at such a rate that the temperature does not rise above 10° until one-third of the ice has been added. Then the temperature may be allowed to rise to 25–30°. The addition of the ice requires about two hours; after this, the mixture is cooled again to 0° (Note 4)
and then filtered as rapidly as possible through a funnel
fitted with a filtros plate (Note 5)
. The crystals are thoroughly pressed and sucked as dry as possible. The acetonedicarboxylic acid
is light gray to white in color. After the suction and pressing have removed practically all of the sulfuric acid
, the crystals are transferred to a beaker
and stirred with sufficient ethyl acetate (about 200–250 cc.)
to make a thick paste. The crystals are filtered with suction. If acetonedicarboxylic acid
entirely free from sulfuric acid
is desired, the washing with ethyl acetate
should be repeated. The yield of practically dry acetonedicarboxylic acid
varies from 450–475 g.
per cent of the theoretical amount) (Note 6)
. This may be used directly for the preparation of the ester (p. 237)
. The acid itself is not stable and after a few hours gradually decomposes (Note 7)
The reaction must be carried out in a good hood
, since a large amount of carbon monoxide
The use of a very efficient ice and salt mixture around the reaction flask is necessary if the reaction is to be carried out within the time indicated. It is very necessary to regulate the temperature as directed, since a considerably lower yield is obtained if the temperature rises.
Some cooling is necessary, or the rapid evolution of gas will cause the reaction mixture to foam over with consequent loss of material.
Vigorous cooling before final filtration of the acetonedicarboxylic acid
is essential to good yields, since the acid is fairly soluble in the reaction mixture.
The filtros plate for filtration can be very conveniently sealed into the Büchner funnel
with a paste of water-glass and amorphous silica which is then hardened with concentrated sulfuric acid
If the acid is to be esterified at once, careful drying from ethyl acetate
is not necessary.
However, it has been observed that if the acid is purified by recrystallization from ethyl acetate
and thoroughly dried it undergoes no decomposition, at least over a seven-month period, when kept at room temperature in a desiccator
can be prepared from citric acid
by the action of concentrated2
or fuming sulfuric acid
The procedure described is a slight modification of that by Willstätter and Pfannenstiehl.3
While it is more complex than the details given by Ingold and Nickolls,3
it gives somewhat higher yields. The method of Ingold and Nickolls has been checked and found to have the advantage of requiring much less time.
This preparation is referenced from:
Copyright © 1921-, Organic Syntheses, Inc. All Rights Reserved