Acylium ions

In organic chemistry, acylium ions are cations with the formula RCO+, where R = alkyl or aryl.[1] They are a kind of carbocation.

## Structure, bonding, synthesis

[edit]

Structural details of the acetyl cation.[2]

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]

CH3COCl + GaCl3 → CH3CO+ \+ 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 (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.[2]

## Reactivity

[edit]

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

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

R3CCO+ \+ H2O → R3CCO2H + H+

## References

[edit]

1. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 978-0-471-72091-1
2. ^ a b c Davlieva, Milya G.; Lindeman, Sergey V.; Neretin, Ivan S.; Kochi, Jay K. (2004). "Structural effects of carbon monoxide coordination to carbon centers. σ and π Bindings in aliphatic acyl versus aromatic aroyl cations". New Journal of Chemistry. 28 (12): 1568. doi:10.1039/B407654K.
3. ^ Esselman, Brian J.; Hill, Nicholas J. (2015). "Proper Resonance Depiction of Acylium Cation: A High-Level and Student Computational Investigation". Journal of Chemical Education. 92 (4): 660–663\. doi:10.1021/ed5002152.
4. ^ Boer, F. Peter (1968). "Crystal structure of a Friedel-Crafts intermediate. Methyloxocarbonium hexafluoroantimonate". Journal of the American Chemical Society. 90 (24): 6706–6710\. doi:10.1021/ja01026a025.
5. ^ Le Carpentier, J. M.; Weiss, R. (1972). "Etude de complexes acide de Lewis–halogénure d'acide. I. Structure cristalline des hexachloroantimonate, tétrachloroaluminate et tétrachlorogallate de méthyl oxocarbonium". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 28 (5): 1421–1429\. Bibcode:1972AcCrB..28.1421L. doi:10.1107/S0567740872004406.