Inorganic chemistry,acylium ionsare cations with the formula RCO+,where R = alkyl or aryl.[1]They are a kind ofcarbocation.
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. Atriple bondexists between C and O.[3]Acylium ions can be viewed as C-alkylated derivatives ofcarbon 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 asantimony pentachloride.
Several acylium salts have been characterized byX-ray crystallography,including thepropionylandmesitylderivatives. Because acylium cation is highly electrophilic, its salts can only be isolated withweakly coordinating anions.[2]
- CH3COCl + GaCl3→ CH3CO++ GaCl−4
Acetylhexafluoroantimonateandhexachloroantimonateare other early examples.[4][5]
The strength of theC≡Obond 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 forcarbon monoxideis 2143 cm-1.[2]
Reactivity
editAcylium ions are potentelectrophilesas evidenced by their ability to attack arenes. Acylium ions are intermediates in several reactions, such as theFriedel-Crafts acylationofarenesbyacetyl chloridein the presence ofaluminium 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 thetetrachloroaluminate.
The acylium ion derived frompivaloyl chlorideis unusual because it exists in equilibrium with the tert-butyl cation:
- (CH3)3CCO+⇌ (CH3)3C++ CO
Central to theKoch carbonylationis 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,ISBN978-0-471-72091-1
- ^abcDavlieva, 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.