The synthesis of Trifluoromethanesulfonic anhydride is as follows:
A dry, 100-ml., round-bottomed flask is charged with 36.3 g. (0.242
mole) of trifluoromethanesulfonic acid (Note 1) and 27.3 g. (0.192 mole)
of phosphorus pentoxide (Note 2). The flask is stoppered and allowed to
stand at room temperature for at least 3 hours. During this period the
reaction mixture changes from a slurry to a solid mass. The flask is
fitted with a short-path distilling head and heated first with a stream
of hot air from a heat gun and then with the flame from a small burner.
The flask is heated until no more trifluoromethanesulfonic anhydride
distills, b.p. 82–115°, yielding 28.4–31.2 g. (83–91%) of the anhydride,
a colorless liquid. Although this product is sufficiently pure for use
in the next step, the remaining acid may be removed from the anhydride
by the following procedure. A slurry of 3.2 g. of phosphorus pentoxide
in 31.2 g. of the crude anhydride is stirred at room temperature in a
stoppered flask for 18 hours. After the reaction flask has been fitted
with a short-path distilling head, it is heated with an oil bath,
yielding 0.7 g. of forerun, b.p. 74–81°, followed by 27.9 g. of the pure
trifluoromethanesulfonic acid anhydride, b.p. 81–84° .
