Donor–acceptor interactions in Friedel–Crafts systems. The CH3COCl•AlCl3 addition compound
The infrared spectra of CH3COCl and CH3COCl•AlCl3 have been studied from 4000 to 300 cm−1 with a Perkin–Elmer 621 grating spectrophotometer. The results indicate that the addition compound CH3COCl•AlCl3 exists under three structures, two ionic and one dative structure. The distinct ionic structures are the carbonium type structure [Formula: see text] with a carbonyl band at 2200 cm−1 (type 1) and a structure visualized as a hybrid between resonating structures[Formula: see text]where the mesomeric limit form corresponding to the hyperconjugation is preponderant, with a carbonyl band at 2300 cm−1 (type 2). Vibrational assignments have been made for the two ions and supported by normal coordinate analysis. Two sets of force constants have been derived on the basis of the generalized force field for the two ionic structures to provide a correspondence between observed and calculated frequencies. Evidence would seem to show that the active species in the Friedel–Crafts ketone synthesis is the oxocarbonium type 1 acetylium ion.