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

Structure, bonding, synthesis

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. 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. Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples. The strength of the C\tO bond is indicated by the frequency of its vibration (νCO). 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.

Reactivity

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