In organic chemistry, acylium ions are cations with the formula RCO⁺, where R = alkyl or aryl.^[1] They are a kind of carbocation.

In acylium ions, the C-C-O linkage is linear. The oxygen and the central carbon can be described as sp hybridized. A triple bond exists between C and O.^[3] Acylium ions can be viewed as C-alkylated derivatives of carbon monoxide (which also has a C-O triple bond). They are typically prepared by removal of chloride from acyl chlorides using strong Lewis acids such as antimony pentachloride.

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.^[2]

CH₃COCl + GaCl₃ → CH₃CO⁺ + GaCl−4

Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples.^[4][5]

The strength of the C≡O bond is indicated by the frequency of its vibration (ν_CO). These values are 2300 and 2200 cm⁻¹, respectively for the aryl and alkyl acylium ions. For comparison, the same vibration for carbon monoxide is 2143 cm⁻¹.^[2]

Acylium ions are potent electrophiles as evidenced by their ability to attack arenes. Acylium ions are intermediates in several reactions, such as the Friedel-Crafts acylation of arenes by acetyl chloride in the presence of aluminium trichloride:

C₆H₅R + CH₃CO⁺ + AlCl−4 → CH₃COC₆H₄R + HCl + AlCl₃

Such depictions may be simplistic because of ion-pairing between the acetyl cation (an acylium cation) and the tetrachloroaluminate.

The acylium ion derived from pivaloyl chloride is unusual because it exists in equilibrium with the tert-butyl cation:

(CH₃)₃CCO⁺ ⇌ (CH₃)₃C⁺ + CO

Central to the Koch carbonylation is the hydrolysis of acylium ions to carboxylic acids:

R₃CCO⁺ + H₂O → R₃CCO₂H + H⁺