Friedel-Crafts acylation is a significant reaction in organic chemistry that allows for the introduction of acyl groups into aromatic compounds, ultimately leading to the formation of ketones. The process begins with an acyl halide, typically an acid chloride, which reacts with a Lewis acid catalyst to generate an electrophile known as an acylium ion. An acylium ion is characterized by a carbon atom double-bonded to an oxygen atom and attached to an R group, carrying a positive charge. This structure is resonance-stabilized, allowing for two primary resonance forms, but the first resonance structure, with the positive charge on the carbon, is most useful for visualizing the mechanism.

The mechanism of Friedel-Crafts acylation is straightforward. Initially, the chlorine atom from the acyl halide donates its electrons to the Lewis acid catalyst, forming the acylium ion and a complex with the catalyst (e.g., AlCl₄^-). This acylium ion acts as a potent electrophile, capable of reacting with the electron-rich benzene ring. Unlike alkylation, this reaction does not involve carbocation intermediates, which means there are no rearrangements to consider. The resonance stabilization of the acylium ion prevents it from shifting to the R group, allowing it to maintain its structure during the reaction.

As the acylium ion interacts with the benzene ring, a sigma complex is formed. This intermediate features a positively charged carbon atom bonded to the aromatic ring, and it is crucial to remember to include the hydrogen atom in the final structure, as it plays a role in the elimination step. The next phase involves the elimination of a proton, facilitated by the conjugate base of the Lewis acid catalyst. This beta elimination results in the formation of a ketone on the benzene ring, alongside the regeneration of the Lewis acid catalyst and the release of an acid byproduct.

In summary, Friedel-Crafts acylation is a valuable method for synthesizing ketones from aromatic compounds, characterized by its use of acylium ions as electrophiles and the absence of rearrangements, leading to predictable and efficient outcomes.