What are the steps in a Friedel-Crafts acylation?

Friedel-Crafts acylation involves the conversion of an aromatic ring into an aryl ketone through the use of an acyl chloride and a Lewis acid catalyst.

The Friedel-Crafts acylation is a type of electrophilic aromatic substitution reaction that introduces an acyl group onto an aromatic ring. The reaction is named after the French chemists Charles Friedel and James Crafts who discovered it in the late 19th century. The process involves three main steps: the generation of an acylium ion, the electrophilic attack on the aromatic ring, and the deprotonation to restore aromaticity.

In the first step, an acyl chloride (RCOCl) reacts with a Lewis acid catalyst, typically aluminium chloride (AlCl3), to form an acylium ion (RCO+) and an AlCl4- ion. The Lewis acid catalyst accepts a pair of electrons from the chlorine atom in the acyl chloride, breaking the R-CO-Cl bond and generating the highly reactive acylium ion.

The second step involves the electrophilic attack on the aromatic ring by the acylium ion. The acylium ion acts as an electrophile, or electron-seeking species, and reacts with the aromatic ring, which is rich in electron density. This results in the formation of a resonance-stabilised carbocation, breaking the aromaticity of the ring.

In the final step, a base (usually the AlCl4- ion generated in the first step) removes a proton from the carbocation, restoring the aromaticity of the ring and forming an aryl ketone. The Lewis acid catalyst is also regenerated in this step, allowing it to participate in further reactions.

The Friedel-Crafts acylation is a useful reaction in organic chemistry because it allows for the synthesis of a wide range of aryl ketones, which are important intermediates in the synthesis of many other organic compounds. However, it's important to note that this reaction is not suitable for strongly deactivated aromatic rings, as the high reactivity of the acylium ion can lead to over-acylation.