Org. Synth. 1985, 63, 154
DOI: 10.15227/orgsyn.063.0154
HEXAFLUOROACETONE
[2-Propanone, 1,1,1,3,3,3-hexafluoro-]
Submitted by Michael Van Der Puy and Louis G. Anello
1.
Checked by Evan D. Laganis and Bruce E. Smart.
1. Procedure
Caution! Hexafluoroacetone and its precursor are toxic. Both procedures should be conducted in an efficient hood.
A. 2,2,4,4-Tetrakis(trifluoromethyl)-1,3-dithietane. A 500-mL, three-necked flask is fitted with a good magnetic stirring bar, thermometer, water-cooled condenser, and a fritted gas inlet tube (Note 1). The outlet of the condenser is attached to a tared −78°C cold trap and the inlet tube is connected via flexible tubing to a graduated −78°C cold trap into which 60 mL (96 g, 0.64 mol) of hexafluoropropene has been condensed under nitrogen. The flask is charged with 3 g of potassium fluoride and is flamed gently under vacuum. The apparatus is cooled while purging with nitrogen. Sulfur (23 g, 0.72 mol) and 200 mL of dry dimethylformamide are then added (Note 2). The reaction mixture is heated to 40–45°C with stirring. The heat source is removed, the stopcock on the trap containing the hexafluoropropene is opened, and the trap is gently thawed. The rate of hexafluoropropene bubbling into the reaction mixture is adjusted to about 0.6 mL (1 g) / min by cooling or warming the trap containing the hexafluoropropene (Note 3),(Note 4),(Note 5). When all the hexafluoropropene has been added, the reaction mixture is cooled to −20°C to −30°C and quickly filtered under suction (Note 6). The filtercake is transferred to an Erlenmeyer flask and is allowed to melt. Water (50 mL) is added, and the mixture is filtered. The lower liquid phase is separated, washed with 50 mL of water, and distilled through a 20-cm Vigreux column at atmospheric pressure to give 93.0–99.4 g (80–85%) of product, bp 106–108°C (Note 7).
B.
Hexafluoroacetone. A
1-L, three-necked flask is fitted with a
sealed mechanical stirrer, thermometer, and condenser. A −78°C glass trap is attached to the condenser via flexible tubing. While the system is purged with
nitrogen,
3 g of potassium fluoride is added and the flask and
potassium fluoride are flame-dried
(Note 8). After the flask has cooled,
300 mL of dry dimethylformamide,
80 g (0.374 mol) of powdered potassium iodate, and
60 g (0.165 mol) of 2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithietane are added
(Note 9). The stirrer and water condenser are started and the reaction mixture is heated over a 45-min period to 149°C and is kept at 149°C for an additional 15 min. The heat source is then removed, and a slow stream of
nitrogen is used to flush any remaining product gas into the cold trap
(Note 8). The condensate is transferred under vacuum to a tared,
evacuated-gas cylinder (Note 10). The cylinder contains
37.0–39.9 g (
68–73%) of material
(Note 11). This material is distilled to give
35.0–37.6 g (
64–69%) of pure product, bp
−28°C [lit.
2 bp −27°C]
(Note 12).
2. Notes
1.
The checkers dried the glassware overnight at 150°C in an oven and assembled it hot under a
nitrogen purge.
2.
The checkers obtained
potassium fluoride, potassium iodate, dimethylformamide (reagent grades), and sulfur (sublimed) from Fisher Scientific Co. The submitters purchased
hexafluoropropene from PCR Research Chemicals, Inc.; the checkers used
hexafluoropropene from E. I. du Pont de Nemours & Company, Inc. The
potassium fluoride was predried overnight in a
vacuum oven at 110°C. The
sulfur was dried in a
vacuum desiccator and the
dimethylformamide was distilled from
P2O5 prior to use.
3.
The mixture of
dimethylformamide,
sulfur, and
potassium fluoride turns brown prior to the addition of
hexafluoropropene, which quickly brings the color back to bright yellow. The submitters report that the reaction mixture will turn blue or green prior to the addition of
hexafluoropropene, if the
dimethylformamide is dry (less than ca. 0.05% water).
4.
The reaction is moderately exothermic. The temperature rises to about 55°C and remains there as the reaction proceeds.
5.
With good stirring, the reaction proceeds as fast as the
hexafluoropropene is added. The
dry ice trap attached to the condenser should be checked periodically, however. When the required amount of
hexafluoropropene is added, little or no undissolved
sulfur remains.
6.
2,2,4,4-Tetrakis(trifluoromethyl)-1,3-dithietane melts at
24°C. Thus, this operation must be done quickly to minimize product loss.
7.
The product is more than 99% pure by GLPC (6 ft × 1/8 in. 20% FS-1265 on 60/80 Gaschrome R, 50–200°C) and by
19F NMR (CDCl
3) φ: −73.3 (s). The submitters report that they obtained
78–90 g of 98% pure product, bp
110°C.
8.
The
nitrogen initially should come from the cold trap, itself cooled under a
nitrogen flush. At the end of the reaction, the flow of
nitrogen should be reversed. This can be done by replacing the thermometer with a gas inlet tube.
9.
The submitters report that a ratio of
KIO3 to
2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithietane of 2.26 is near the optimum since a ratio of 2.5 did not increase the yield, whereas a ratio of 2.0 gave
5–10% lower yields.
10.
This transfer is best done on a vacuum manifold system equipped with a manometer. The trap and a
stainless-steel cylinder of 100–300-mL capacity are attached via vacuum tubing to the manifold system, cooled in
liquid nitrogen baths, and evacuated to 0.5–1 mm. The system is closed and the trap is removed from its cold bath and is slowly thawed. The volatile material in the trap is transferred to and condensed in the cylinder at such a rate that no positive pressure builds up in the closed system.
11.
The submitters report collecting
45–50 g of product (98% pure or better by GLPC on a
10-ft × 1/8-in. Porapak P column) in the cold trap attached to reaction vessel. The checkers found that the trap contained relatively nonvolatile material, principally
dimethylformamide, in addition to the desired product.
12.
The checkers used a
30-cm jacketed, low-temperature spinning band column for this distillation. The IR spectrum of the distilled product is identical to that of an authentic sample; IR (vapor) cm
−1: 1806 (C=O).
3. Discussion
Earlier methods of preparing
2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithietane (hexafluorothioacetone dimer, HFTA dimer) include the reaction of
hexafluoropropene (HFP) and
sulfur over a
carbon bed at 425°C
3 and the reaction of HFP and
sulfur in
tetramethylene sulfone at 120°C in the presence of
potassium fluoride (
autoclave).
4 Dimethylformamide appears to be a far superior solvent for this reaction, permitting the use of atmospheric pressure and modest temperatures, as well as affording a cleaner product.
The generation of
hexafluoroacetone (HFA) from HFTA dimer has been accomplished by the hot-tube oxidation with
nitric oxide at 650°C (high temperature converts dimer into monomer).
5 The present method uses the more convenient interconversion of dimer to monomer effected by
potassium fluoride in
dimethylformamide. This permits many reactions to be conducted on the very reactive monomer without actually isolating it.
For occasional laboratory synthesis of HFA, the present method offers distinct advantages of convenience (cost, workup, standard equipment) over other known methods. These include the epoxidation of HFP followed by isomerization of the epoxide to HFA,
6 7 the high-temperature halogen exchange of
hexachloroacetone with
Cr3+ /HF,
8 and permanganate oxidation of the extraordinarily toxic
perfluoroisobutylene.
9Hexafluoroacetone is a reactive electrophile. It reacts with activated aromatic compounds (e.g.,
phenol), and can be condensed with olefins, dienes, ketenes, and acetylenes. It forms adducts with many compounds containing active
hydrogen (e.g., H
2O or HCN). Reduction of HFA with
NaBH4 or LiAlH4 affords the useful solvent
hexafluoroisopropyl alcohol. The industrial importance of HFA arises largely from its use in polymers and as an intermediate in monomer synthesis.
10
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
P2O5
KIO3
2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithietane (hexafluorothioacetone dimer, HFTA dimer)
Cr3+ /HF
hydrogen (1333-74-0)
phenol (108-95-2)
nitrogen (7727-37-9)
sulfur (7704-34-9)
carbon (7782-42-5)
nitric oxide
potassium fluoride (7789-23-3)
LiAlH4 (16853-85-3)
dimethylformamide (68-12-2)
NaBH4 (16940-66-2)
tetramethylene sulfone (126-33-0)
perfluoroisobutylene (382-21-8)
hexachloroacetone (116-16-5)
Hexafluoroacetone,
2-Propanone, 1,1,1,3,3,3-hexafluoro- (684-16-2)
potassium iodate (7758-05-6)
hexafluoropropene (116-15-4)
2,2,4,4-Tetrakis(trifluoromethyl)-1,3-dithietane (791-50-4)
hexafluoroisopropyl alcohol (920-66-1)
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