Org. Synth. 1943, 23, 45
DOI: 10.15227/orgsyn.023.0045
KETENE DIETHYLACETAL
[Ketene, diethyl ketal]
Submitted by S. M. McElvain and D. Kundiger.
Checked by R. L. Shriner and C. H. Tilford.
1. Procedure
In a
2-l. round-bottomed flask, preferably fitted with an interchangeable ground-glass joint, are placed
650 g. (820 ml.) of absolute tert-butyl alcohol (Note 1) and
39.1 g. (1 gram atom) of potassium (Note 2). A
reflux condenser is attached to the flask and the mixture is refluxed until all the
potassium is dissolved (about 8 hours). The solution is allowed to cool slightly, and
198 g. (1 mole) of bromoacetal, together with some boiling chips
(Note 3) is quickly added. A cream-colored precipitate of
potassium bromide begins to deposit immediately. The flask is attached at once to a closely indented
46-cm. Vigreux column, equipped with a
glass insulating jacket and a total-reflux partial take-off still head1 (Note 4), and the
tert-butyl alcohol is distilled from an
oil bath (about 120–130°) at the rate of 25 drops per minute with a reflux ratio at the still head of about 6:1. This operation requires 16–18 hours
(Note 5), and at the end of this time the temperature of the oil bath is raised to 160° and maintained there until no more alcohol comes over. The bath is then lowered and allowed to cool while the pressure within the fractionating system is very gradually reduced to 200 mm. and held there by a barostat
(Note 6).
A small amount of the alcohol comes over at 51–52° /200 mm., and then, after the heating bath is replaced, 4–6 ml. of an intermediate fraction distils. This is followed by a fraction which boils at 83–86°/200 mm. and which is collected as pure ketene acetal. A total of 78–87 g. is obtained (67–75%). The major portion is collected while the temperature of the bath is 120–140°, and the remainder is obtained by raising the bath temperature to 170–180°.
Ketene acetal is best stored in a bottle made of alkaline glass which is preferably new and dusted with sodium tert-butoxide (Note 7). The glass stopper should be very well greased. Even with these precautions a small amount of a voluminous precipitate of the white polymer will develop.
2. Notes
1.
The
tert-butyl alcohol is refluxed over quicklime, distilled, and then redistilled over
1 g. of potassium per
100 g. of the alcohol. Improved yields of
ketene acetal are obtained from
tert-butyl alcohol that has been recovered from a previous preparation of the acetal.
2.
The
potassium should be cut into pieces sufficiently small to pass through the neck of the flask.
Sodium in tert-butyl alcohol can be used, but it is necessary to carry out the subsequent reaction at 125° in
sealed tubes. The amount of
tert-butyl alcohol specified is sufficient to provide for complete solution of the
potassium as the
tert-butoxide.
3.
An
ebullator tube through which dry
nitrogen was drawn has been used for the subsequent distillation under reduced pressure, but it is far more advantageous to use about six boiling chips. Because of its rapid reaction with water,
ketene acetal must be protected from moisture of the air.
4.
The still head described by Whitmore and Lux
1 is most satisfactory for controlling this distillation. The tube leading from the take-off of the column is attached to the
receiver through a
fraction cutter protected from moisture by a large tube of calcium chloride. The checkers replaced the Vigreux column by a
50-cm. column filled with glass helices and surrounded by a heating jacket. With this column the removal of the
tert-butyl alcohol was complete in 5–6 hours.
5.
A slower rate of fractionation does not result in an increased yield, but interrupted fractionation results in a decreased yield.
6.
A good barostat is necessary. Control of the reduced pressure by adjusting a "leak" in the system is entirely unsatisfactory, for, as a result of a small increase in pressure, the liquid ceases to boil, the column drains, and the boiling chips are rendered ineffective. The barostat used by the submitters is essentially that described by Ellis
2 in which the relay is replaced by the thermionic relay described by Waddle and Saeman.
37.
The column and apparatus should not be washed with acid cleaning solution because the glass surface is left acidic and it then catalyzes the polymerization of
ketene acetal.
4 A thin coating of the polymer on the walls of the apparatus is not detrimental. If polymer must be removed, it is best done by dissolving it in a
10% solution of hydrochloric acid in acetone; a deep red solution results.
3. Discussion
Ketene acetal may be prepared by the action of
potassium tert-butoxide on
iodoacetal5 or
bromoacetal.
4
This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
Bromoacetal
absolute tert-butyl alcohol
iodoacetal
hydrochloric acid (7647-01-0)
nitrogen (7727-37-9)
acetone (67-64-1)
sodium (13966-32-0)
potassium bromide (7758-02-3)
potassium (7440-09-7)
Ketene diethylacetal (2678-54-8)
Ketene, diethyl ketal
ketene acetal
tert-butyl alcohol (75-65-0)
potassium tert-butoxide (865-47-4)
tert-butoxide
sodium tert-butoxide (865-48-5)
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