Acylium ions

(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 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: : Likewise 1-adamantylcarbonylium decomposes to 1-adamantanium and carbon monoxide. Central to the Koch carbonylation is the hydrolysis of acylium ions to carboxylic acids: : ==References==