Process for the preparation of perfluoropropionyl fluoride

A process for the preparation of perfluoropropionyl fluoride (II) by the isomerization of hexafluoropropene oxide (I):

• ##STR6## wherein the reaction medium used is a liquid complex ammonium hydrofluoride of the general formula (III):
• ›R.sup.1 R.sup.2 R.sup.3 NH!.sup.+ ›H.sub.(n-1) F.sub.n !.sup.-III
• in which n is an integer or fraction .ltoreq.3 and the radicals R.sup.1, R.sup.2 and R.sup.3 are identical or different and each of these radicals is
• an alkyl radical having 1 to 20 carbon atoms,
• a cycloalkyl radical having 5 to 7 carbon atoms,
• an aralkyl radical having 7 to 10 carbon atoms or
• an aryl radical having 6 to 10 carbon atoms which can also be substituted by C.sub.1 - to C.sub.3 -alkyl or C.sub.1 to C.sub.3 -alkoxy groups,
• or in which two of the radicals R.sup.1 to R.sup.3, together with the N atom which carries them, form a 5- to 7-membered ring which can contain an O atom or another N atom,
• or in which the radicals R.sup.1 to R.sup.3, together with the N atom which carries them, form two or three 5- to 7-membered saturated rings which can contain further N atoms,
• or in which the radicals R.sup.1 to R.sup.3 together form a 6-membered heterocyclic ring which can contain one or two N atoms and can also be benzo-fused.