HF-addition to haloacetyl fluorides in superacidic media

Abstract

The reactions of difluoroacetyl fluoride and trifluoroacetyl fluoride were investigated in the binary superacid HF/SbF₅ by low-temperature NMR spectroscopy. Whereas both haloacetyl fluorides form oxonium species after the addition of HF, the protonated acyl fluorides were not observed. Protonated 1,1,2,2-tetrafluoroethanol was isolated as a solid and represents an example of a protonated α-fluoroalcohol. The salt was characterized by low-temperature vibrational spectroscopy and single-crystal X-ray diffraction. [CHF₂CF₂OH₂][SbF₆] crystallizes in the triclinic space group P with two formula units per unit cell. Protonated perfluoroethanol is only stable in solution. The reactivity of both haloacyl fluorides is discussed based on quantum chemical calculations at the MP2/aug-cc-pVTZ-level of theory.