Acylium ions explained

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.^[1] 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]

Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples.^{[3] [4]}

The strength of the 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]

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:

References

1. 10.1021/ed5002152 . Proper Resonance Depiction of Acylium Cation: A High-Level and Student Computational Investigation . 2015 . Esselman . Brian J. . Hill . Nicholas J. . Journal of Chemical Education . 92 . 4 . 660–663 .
2. 10.1039/B407654K . Structural effects of carbon monoxide coordination to carbon centers. σ and π Bindings in aliphatic acyl versus aromatic aroyl cations . 2004 . Davlieva . Milya G. . Lindeman . Sergey V. . Neretin . Ivan S. . Kochi . Jay K. . New Journal of Chemistry . 28 . 12 . 1568 .
3. 10.1021/ja01026a025 . Crystal structure of a Friedel-Crafts intermediate. Methyloxocarbonium hexafluoroantimonate . 1968 . Boer . F. Peter . Journal of the American Chemical Society . 90 . 24 . 6706–6710 .
4. 10.1107/S0567740872004406 . Etude de complexes acide de Lewis–halogénure d'acide. I. Structure cristalline des hexachloroantimonate, tétrachloroaluminate et tétrachlorogallate de méthyl oxocarbonium . 1972 . Le Carpentier . J. M. . Weiss . R. . Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry . 28 . 5 . 1421–1429 . 1972AcCrB..28.1421L .