Carbocations from acyl halides

In a similar manner, acyl carbocations formed from acyl halides act as an electrophile in Friedel-Crafts acylation reactions (Scheme 2.3). 

The acylium ion (RCO ), which may be generated from an acyl halide and a Lewis acid catalyst, also achieves some stabilization from oxygen lone pairs, and the carbocation remains localized on the carbonyl group. Consequently, Friedel-Crafts acylation is not accompanied by the rearrangements that affect alkylations (Scheme 4.7). 

My first contact with Meerwein was in 1954 when I received, while still working in isolated Hungary, a letter from him. He had read a paper of ours, and offered useful comments, even pointing out that we had missed a relevant reference. We subsequently kept up correspondence. He continued to be interested in our early efforts to obtain long lived carbocations from alkyl (and acyl) fluorides in liquid boron trifluoride (and other Lewis acid halides) at low temperature, as well as via metathetic reactions of organic halides with silver tetrafluoroborate (a reagent we introduced in 1955 for the ionization of bromocyclohexadienes to form benzenium ions and which Meerwein developed to a wide utility akylation method). 

The first step is not a free-radical process, and its actual mechanism is not known.451 25 is an acyl hypohalite and is presumed to be an intermediate, though it has never been isolated from the reaction mixture. Among the evidence for the mechanism is that optical activity at R is lost (except when a neighboring bromine atom is present, see p. 682) if R is neopentyl, there is no rearrangement, which would certainly happen with a carbocation and the side products, notably RR, are consistent with a free-radical mechanism. There is evidence that the Simonini reaction involves the same mechanism as the Hunsdiecker reaction but that the alkyl halide formed then reacts with excess RCOOAg (0-24) to give the ester.452 See also 9-13. 

The electrophile, an acyl cation, is generated in a manner similar to that outlined in Figure 17.4 for the generation of the carbocation electrophile from an alkyl halide. First the Lewis acid, aluminum trichloride, complexes with the chlorine of the acyl chloride. Then A1C14 leaves, generating an acyl cation. The acyl cation is actually more stable than most other carbocations that we have encountered because it has a resonance structure that has the octet rule satisfied for all of the atoms ... 

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