Friedel-Crafts reaction

A Friedel-Crafts reaction introduces an alkyl or acyl group onto an aromatic ring by electrophilic aromatic substitution, commonly with activation by a Lewis acid. Alkylation and acylation are related reaction families but have different intermediate chemistry.

An alkylation pathway can involve carbocation-like rearrangements and can promote repeated substitution. An acylation transfers an acyl group through an activated electrophile, often described using an acylium ion, and its carbonyl substituent generally deactivates the product toward further substitution. These differences explain why the two variants are not interchangeable methods of adding a carbon substituent.

electrophilic aromatic substitution that installs an alkyl group on an aromatic ring.

Friedel-Crafts acylation attaches an acyl group to an aromatic carbon through electrophilic substitution. Lewis-acid activation of an acyl chloride is often represented by an acylium ion or strongly polarized contact pair.

An acylium ion is a positively charged acyl species, commonly written $\mathrm{RC}{\mathrm{O}}^{+}$, with charge and bonding described by resonance forms $\mathrm{R}-\mathrm{C}\equiv {\mathrm{O}}^{+}$ and $\mathrm{R}-{\mathrm{C}}^{+}=\mathrm{O}$. Its carbon is strongly electrophilic.

Acylium-like species explain carbonyl-group transfer in Friedel-Crafts acylation. Unlike an ordinary alkyl carbocation, the acylium ion is stabilized by oxygen donation and usually does not undergo the same skeletal rearrangements. The precise reactive species can include a counterion or Lewis-acid complex, so the isolated ion drawing is a mechanistic model.