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
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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
editAcylium 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- ^ 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
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.