Acylium

An acylium ion is a cationic reactive intermediate in organic chemistry, typically written as R-CO+. It is a resonance-stabilized species derived from acyl derivatives such as acyl chlorides, anhydrides, or ketenes under strongly Lewis acidic conditions. The carbonyl fragment carries the positive charge, and the ion can be represented by two major resonance forms: a carbonyl form (R-CO+) and a form in which a C≡O bond is involved in delocalization (R-C≡O+). This delocalization gives the acylium ion a relatively planar, electrophilic character centered on the carbonyl carbon.

Formation of acylium ions commonly occurs when an acyl chloride (R-CO-Cl) reacts with a Lewis acid such as AlCl3 or FeCl3, generating the active electrophile R-CO+ paired with a counterion (e.g., AlCl4−). They can also arise from other acyl derivatives under suitable catalytic conditions. In practice, acylium ions are best known for their role in Friedel–Crafts acylation, where they acylate arenes to form aryl ketones, with the reaction proceeding through the acylium intermediate before workup to the neutral ketone product.

Reactivity and stability of acylium ions depend on substituents and the solvent environment; resonance stabilization and coordination to counterions or solvents help moderate their high electrophilicity. Beyond Friedel–Crafts acylation, acylium ions participate in intramolecular cyclizations and related electrophilic carbonyl additions in various synthetic contexts.