acyliumion

An acylium ion is a cationic species derived from an acyl group, typically represented by the formula R-CO+. It is a resonance-stabilized carbocation in which the positive charge is delocalized between the carbonyl carbon and the carbonyl oxygen, often depicted in forms such as R-CO+ and R-C≡O+. The acylium center is typically sp-hybridized, contributing to a linear or near-linear arrangement around the carbonyl unit.

Acylium ions are most commonly generated in situ under Friedel-Crafts conditions. Treatment of acyl chlorides or acid anhydrides with Lewis acids such as AlCl3 produces the acylium species, which then acts as a powerful electrophile for electrophilic aromatic substitution. This reactivity underpins many acylation reactions that furnish aryl ketones. Beyond arylations, acylium ions can participate in hydroacylation of alkenes and related processes, expanding carbonyl-containing product frameworks.

In solution, acylium ions are typically short-lived intermediates, stabilized by resonance with the carbonyl group and by coordination with counterions or solvent. They can also be observed as transient or trap-derived species in mass spectrometry, where fragments corresponding to acyl groups generate characteristic acylium ions.

Key points to remember: they are electrophilic, resonance-stabilized acyl carbocations derived from acyl derivatives, central to many carbonyl syntheses and reaction types in organic chemistry. See also Friedel-Crafts acylation, acyl cations, and hydroacylation.