Nitronium tetrafluoroborate can be prepared by adding a mixture of anhydrous hydrogen fluoride gas and boron trifluoride to a solution of either
highly concentrated nitric acid or nitrogen pentoxide dissolved in nitromethane.
WARNING: Rubber gloves, an apron, and a plastic face mask are strongly recommended. All operations should be carried out in a hood.
If hydrogen fluoride comes in contact with the skin, the contacted area should be thoroughly washed with water and then immersed in ice water while
the patient is taken to rushed to the emergency department. Burns caused by hydrogen fluoride may not be noticed for several hours, by which time
serious tissue damage may have occurred.
Note: Any operations involving liquid hydrogen fluoride must be carried out with equipment resisting hydrogen fluoride, such as fused
silica, polyolefin, monel steel, or teflon. Kel-F grease is recommended for ground-glass joints. Nitronium tetrafluoroborate slowly attacks silicone
stopcock grease, causing air to enter the flask. After completion of the reaction, all equipment should be washed with plenty of water.
A 1-L three-necked polyolefin flask is provided with a short inlet tube for nitrogen, a long inlet tube for gaseous boron trifluoride, a drying tube,
and a magnetic stirring bar. The flask is immersed in an ice-salt bath and flushed with dry nitrogen. Under a gentle stream of nitrogen and with
stirring, the flask is charged with 400 ml. of methylene chloride, 41 ml. (65.5 g., 1.00 mole) of red fuming nitric acid (95%), and 22 ml. (22 g.,
1.10 moles) of cold, liquid, anhydrous hydrogen fluoride. 5. It is convenient to condense anhydrous hydrogen fluoride, b.p. 19.5°, from a cylinder
into a small calibrated polyolefin flask immersed in a mixture of dry ice and acetone. As hydrogen fluoride is very hygroscopic, it should be
carefully protected from atmospheric moisture, preferably by maintaining an atmosphere of dry nitrogen over it, otherwise by means of a drying tube.
The hydrogen fluoride is then simply poured into the reaction flask.
Gaseous boron trifluoride (136 g, 2.00 moles) from a cylinder mounted on a scale is bubbled into the stirred, cooled reaction mixture. (The
temperature of the reaction is not critical, but the reaction is slower at higher temperatures because of the lower solubility of boron trifluoride in
the solvent). The first mole is passed in rather quickly (in about 10 minutes). When approximately 1 mole has been absorbed, copious white fumes begin
to appear at the exit, and the rate of flow is diminished so that it takes about 1 hour to pass in the second mole; even at this slow rate, there is
considerable fuming at the exit. After all the boron trifluoride has been introduced, the mixture is allowed to stand in the cooling bath under a slow
stream of nitrogen for 1.5 hours. The mixture is swirled, and the suspended product is separated from the supernatant liquid by means of a
medium-porosity, sintered-glass Buchner funnel. Note that since free hydrogen fluoride is no longer present, filtration can be carried out with glass
or porcelain equipment.
The gooey solid remaining in the flask is transferred to the funnel with the aid of two 50-ml. portions of nitromethane. The solid on the funnel,
nitronium tetrafluoroborate, is washed successively with two 100-ml. portions of nitromethane and two 100-ml. portions of methylene chloride. In order
to protect the salt from atmospheric moisture during the washing procedure, suction is always stopped while the salt is still moist. The moist salt is
transferred to a round-bottomed flask and dried by evaporating the solvent. At the end of the procedure the flask can be gently heated to 40–50°C
(Nitronium tetrafluoroborate is thermally stable up to 170°. Above this temperature it starts to dissociate into nitryl fluoride and boron
trifluoride.) The yield of colorless nitronium tetrafluoroborate is 85–106 g. (64–80%) It is stored in a wide-mouthed polyolefin bottle with a
screw cap. The edge of the cap is sealed with paraffin wax after it is screwed on. Nitronium tetrafluoroborate is very hygroscopic. It is stable as
long as it is anhydrous, but it is decomposed by moisture, and all transfers should be in a dry box.
Nitronium tetrafluoroborate slowly attacks polyethylene and polypropylene, but apparatus made of these materials will last for several preparations of
the salt.
The last part of the procedure can be used to purify nitronium tetrafluoroborate that has picked up water on standing. The impure salt is washed twice
with nitromethane, twice with methylene chloride, and is dried under reduced pressure.
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