Acylium cation from acyl chlorides

The electrophile in a Friedel-Crafts acylation reaction is an acyl cation (also referred to as an acylium ion) Acyl cations are stabilized by resonance The acyl cation derived from propanoyl chloride is represented by the two resonance forms... 

A Friedel-Crafts acylation is a synthetic method that avoids the problem of rearrangement of the cation. Figure 7-10 illustrates the generation of the electrophile (the acylium ion) from an acid chloride. The presence of resonance stabilizes the acylium ion, and that reduces the possibility of rearrangement. 

The AlCl3 mediated electrophilic substitution of alkynylsilanes with acid chlorides provides a convenient route to alkynyl ketones. Transfer of the acetylenic moiety from silicon to the acyl chloride proceeds via the acyl cation (acylium ion) formed by reaction of the acyl chloride with AlClg. " ... 

The acyl cation 2a or acylium ion 2b is a familiar intermediate in the Friedel-Crafts reaction. It is easy to make (acid chloride + Lewis acid 1) and it can be observed by NMR as it expresses the natural reactivity pattern of the acyl group. The acyl anion by contrast has umpolung or reverse polarity.1 One might imagine making it from an aldehyde by deprotonation 3 and that it would be trigonal 4a or possibly an oxy-carbene 4b. Such species are (probably) unknown and their rarity as well as their potential in synthesis has led to many synthetic equivalents for this elusive synthon. The acyl anion, the d1 synthon, is the parent of all synthons with umpolung2 and should perhaps have been treated before the homoenolates dealt with in the previous chapter. 

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