Acylium ions
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Through Organic chemistry, Carbon monoxide and Electrophile
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.
- CH3COCl + GaCl3 → CH3CO+ + GaCl−4
Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples.
The strength of the C≡O 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.
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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:
- 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
Likewise 1-adamantylcarbonylium decomposes to 1-adamantanium and carbon monoxide.
Central to the Koch carbonylation is the hydrolysis of acylium ions to carboxylic acids:
- R3CCO+ + H2O → R3CCO2H + H+