Org. Synth. 1965, 45, 50
Submitted by S. Andreades and H. D. Carlson1
Checked by R. David Clark, James J. Fuerholzer, and Hentry E. Baumgarten.
Caution! Ketene (b.p. −41°) is a poisonous gas of the same order of toxicity as phosgene. All operations with ketene should be carried out in an efficient hood.
The pyrolysis apparatus consists of a vertical, electrically-heated Vycor® tube (25 mm. I.D.) packed with 6-mm. lengths of Pyrex® tubing (10 mm. O.D.) and mounted in an electric furnace about 45 cm. long (Note 1)
and (Note 2)
. Attached to the top is a 100-ml. dropping funnel
with a pressure-equalizing side arm2
that has an inlet for nitrogen (Note 3)
. A thermocouple
well inside the tube holds a movable thermocouple and extends to the bottom of the heated section (Note 4)
. The bottom of the reactor is fitted to a 500-ml. side-arm flask packed in ice
. The side arm leads to two traps in series cooled in ice
and to a final trap cooled in a bath of dry ice and acetone (Note 5)
The hottest part of the tube, which is near the middle of the heated section, is maintained at 550° ± 10° while dry oxygen-free nitrogen is passed successively through a flowmeter and the tube at about 150 ml. per hr. for at least 30 minutes (Note 6). The dropping funnel is charged with 56 g. (0.67 mole) of diketene (Note 7) and (Note 8), which is then introduced into the hot tube at a rate of about 0.5 ml. per min. while the nitrogen flow continues. Essentially pure ketene (Note 9), yield 26–31 g. (46–55%) (Note 10), collects in the dry-ice trap as a colorless or nearly colorless liquid. The ketene is distilled directly from this trap for use in reactions.
If the ketene is not to be used at once, drying tubes should be attached to the trap, which should then be stored at −80°. Ketene can be kept for as long as 2 weeks in this way, although some transformation to high-boiling material occurs (Note 11). However, pure ketene can be readily obtained from a partially decomposed mixture by simple distillation from the trap (Note 11) and (Note 12). Caution! Do not store ketene under pressure, as an explosion may result.
The furnace used by the submitters and checkers was an 1870-watt hinged type manufactured by the Hevi-Duty Electric Company, Milwaukee, Wisconsin; length 18 in.; inside diameter 2 3/8; in.; catalog No. M-2018. With the packing described, the total surface area in the packed tube is about 2000 cm.2
In addition to this setup the checkers used a similar tube (20 mm. inside diameter) packed with 1/8;-in. I.D. single-turn Pyrex®
glass helices and inserted in a 550-watt furnace manufactured by the Hoskins Mfg. Co., Detroit, Michigan; length 12 in.; inside diameter 1 ½ in.; catalog No. FD303A.
To prevent heat loss from the ends of the furnace opening, the ends are packed with Pyrex®
glass wool or Fiberglas®
insulation PF-105. In addition, the checkers capped each end of the furnace with a plate constructed from ¼-in. Transite®
sheet in which a hole just large enough for a loose sliding fit on the tube had been bored. The plate at the bottom of the furnace was held in place with a rubber stopper
also bored to fit the tube, and the plate at the top of the furnace was held in place with an iron ring attached to the ring stand supporting the funnel.
The lower part of the barrel of the dropping funnel
should be bent in such a way as to offset it from the path of the thermocouple to permit adjustment of the latter. The checkers used a 8-mm. glass tube bent for offset
and fitted at the upper end with a nitrogen inlet and a standard-taper ground joint to permit attachment of funnels of various sizes.
Attachment of dropping funnel and thermocouple well to the pyrolysis tube may be made with a rubber stopper suitably bored.
The first two traps may be packed loosely with glass wool to prevent mechanically entrained impurities (or aerosol) from passing through into the final trap. Omission of the glass wool may allow as much as 0.5–1.0 g. of colored material to be collected in the product. However, this colored impurity is easily removed by simple distillation. The checkers cooled the two traps in ice-ethanol.
The checkers passed the nitrogen through a gas absorption bottle filled to a depth of 10 cm. with concentrated sulfuric acid
which had been calibrated roughly for flow rate. The rate of flow used was one bubble per 7 seconds (ca.
145 ml. per hr.) for the larger furnace, and one bubble per 10 seconds (ca.
100 ml. per hr.) for the smaller furnace.
Suitable diketene can be obtained from the Aldrich Chemical Company, Milwaukee, Wisconsin
. If at all colored, this material should be distilled or sublimed before pyrolysis, for use of colored material may lead to a colored product. Distillation is easily carried out through use of the apparatus illustrated in Fig. 1
. The impure diketene
is placed in flask A
and is cooled in ice and stirred with a magnetic stirrer. The Dewar trap is filled with dry ice and ethanol
. Evacuation of the system with a vacuum pump
is begun very carefully with appropriate upward adjustment of the pressure if necessary to prevent bumping and splashing. After the initial degassing and removal of low-boiling materials, the diketene
distils smoothly (at 0.1–1.0 mm. pressure), collecting as a white solid on the cold surface of the Dewar trap. After the distillation is completed, the apparatus is partially disassembled and the dry ice and ethanol are poured out. As the diketene
melts, it flows into flask B
. The diketene
should be stored under nitrogen in tightly stoppered brown bottles in a refrigerator or, better, below its freezing point of −6.5°, as otherwise it may slowly decompose.
Fig. 1. Low-temperature distillation or evaporation apparatus. When solids are being collected, the Dewar flask bottom should be as indicated by the broken line.
Equally good results have been obtained with charges one-fourth to twice that of the present procedure.
obtained by simple distillation from the final trap is essentially pure. The purity can be determined by passing a weighed sample into 1N
sodium hydroxide solution and titrating for excess base. The checkers found it very difficult to avoid entirely the collection of minute traces of high-boiling colored impurities with the ketene
. However, these impurities were easily removed by simple distillation of the ketene
, just before use, from the original trap into a clean trap by warming the former in ice water and cooling the latter in dry ice and ethanol.
The checkers' yields were 54–66%
for the larger furnace and 58–78%
for the smaller furnace. The yield was found to be somewhat dependent on the rate of addition, with the rate specified in the procedure giving a good yield in a reasonable length of time. The checkers used an addition rate of 0.25–0.30 ml. per min. for the smaller furnace to obtain the yields cited.
Major impurities that collect on standing are all high-boiling, e.g.
, b.p. 127°
, and dehydroacetic acid
, b.p. 270°
. If the ketene
has been stored long enough to allow a considerable portion of higher-boiling materials to accumulate, it is desirable to insert a trap cooled in an ice bath
between the ketene-containing trap and the reaction vessel in order to minimize mechanical entrainment of the impurities. Repeated warming of the container to remove portions of the ketene
naturally hastens transformation of the ketene
to high-boiling materials.
The carbonaceous material that is deposited inside the pyrolysis tube is easily removed by passing a stream of oxygen through at about 550° after a thorough flushing with nitrogen.
The submitters no longer consider this method to be the most convenient one for preparing laboratory quantities of ketene
. It has been supplanted at the Du Pont Experimental Station by pyrolysis of acetic anhydride according to the procedure of Fisher, MacLean, and Schnizer.3
It is recommended that the wires of the preheater be glass-covered, since polymerization of the ketene
at −78° seemed to occur frequently when bare wires were used, possibly because of catalysis by trace amounts of metal. Using this procedure 55 g. (1.31 moles) of ketene
per hour could be produced (private communication, J. B. Sieja and H. D. Carson).
can be generated conveniently by pyrolysis of acetone
in a hot tube4
or over a hot wire in a "ketene lamp,"5
or by pyrolysis of diketene
in a hot tube,6,7
or by pyrolysis of acetic anhydride
Other methods of preparation have been summarized.4
It has been shown that diketene
cracks quite cleanly to ketene
although some allene
and carbon dioxide
are formed at the same time.8
4. Merits of the Preparation
The most convenient procedures for the preparation of ketene
are the present one and the pyrolyses of acetone5
or acetic ahnydride
The acetone procedure gives ketene
at a relatively fast rate (0.45 mole per hour), but it takes considerable adjustment to get optimum conditions, and trouble is sometimes caused by the wire getting coated with carbon. Furthermore, because the efficiency of a given wire coil varies with time, passing through a maximum, frequent calibration of the apparatus is necessary. The acetic anhydride method is even faster (1.31 moles per hour) and uses a readily available chemical. It appears to be the method of choice at this time. The diketene
procedure described here is relatively simple and reliable; however, it is relatively slow (0.2 mole per hour) and requires a somewhat less readily available starting material.
This preparation is referenced from:
Chemical Abstracts Nomenclature (Collective Index Number);
acetic anhydride (108-24-7)
carbon dioxide (124-38-9)
Dehydroacetic acid (520-45-6)
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