Organic Syntheses, CV 4, 348
Submitted by C. W. Smith and D. G. Norton
1.
Checked by T. L. Cairns and J. C. Sauer.
1. Procedure
A.
α-Bromoisobutyryl bromide. To a mixture of
250 g. (2.85 moles) of isobutyric acid and
35 g. (0.28 mole) of red phosphorus in a
1-l. three-necked flask, fitted by ground-glass joints to a dropping funnel,
mechanical stirrer, and
reflux condenser, is added, dropwise with stirring,
880 g. (5.5 moles) of bromine. After the addition is complete, the solution is warmed to 100° over a period of 6 hours. The unreacted
bromine and
hydrogen bromide are removed under reduced pressure (30 mm.). The
α-bromoisobutyryl bromide is decanted from the
phosphorous acid and fractionated through a
short helices-packed column. After a considerable fore-cut, the main fraction,
493–540 g. (
75–83%), is collected at
91–98° (100 mm.).
B.
Dimethylketene (Note
1). The apparatus for this preparation consists of a
500-ml. flask equipped with an
inlet tube for nitrogen and a dropping funnel and fitted to a
6-in. modified Claisen still head leading to a tared spiral inlet trap having stop-cocks on the inlet and exit sides and cooled in Dry Ice-acetone (Note
2). This trap is connected to a
vacuum line, and the reaction is carried out at 300 mm. pressure. After
40 g. (0.61 g. atom) of zinc turnings and
300 ml. of ethyl acetate have been placed in the flask and the system has been flushed with
nitrogen (free of
oxygen and moisture) and heated to incipient boiling,
111 g. (0.48 mole) of α-bromoisobutyryl bromide is added dropwise at such a rate that the
ethyl acetate boils gently. A slow stream of
nitrogen is continued throughout the reaction.
Dimethylketene distils along with
ethyl acetate and is obtained in
46–54% yield as a
9–10% solution in
ethyl acetate (
15–18 g. of
dimethylketene in
190–200 ml. of ethyl acetate) (Note
3).
2. Notes
1. This ketene reacts rapidly with
oxygen to form an explosive peroxide. Drops of solution allowed to evaporate in air may detonate. Washing with water is an efficient means of decontamination.
3. Discussion
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