Acylium ions

In organic chemistry, acylium ions are cations with the formula RCO+, where R = alkyl or aryl.

[3] Acylium ions can be viewed as C-alkylated derivatives of carbon monoxide (which also has a C-O triple bond).

Several acylium salts have been characterized by X-ray crystallography, including the propionyl and mesityl derivatives.

Because acylium cation is highly electrophilic, its salts can only be isolated with weakly coordinating anions.

Acylium ions are intermediates in several reactions, such as the Friedel-Crafts acylation of arenes by acetyl chloride in the presence of aluminium trichloride: Such depictions may be simplistic because of ion-pairing between the acetyl cation (an acylium cation) and the tetrachloroaluminate.