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
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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. They can be viewed as C-alkylated derivatives of carbon monoxide. 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.[2] Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples.[3][4]
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:
- CH3COCl + AlCl3 → CH3CO+ + AlCl−4
- 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
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 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.
- ^ 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.