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]

CHCOCl + GaCl → CHCO⁺ + 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). 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:

CHR + CHCO⁺ + AlCl−4 → CHCOCHR + HCl + AlCl

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:

(CH)CCO⁺ ⇌ (CH)C⁺ + CO

Likewise 1-adamantylcarbonylium decomposes to 1-adamantanium and carbon monoxide.^[6]

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

RCCO⁺ + HO → RCCOH + H⁺