Acylium ions
In organic chemistry, acylium ions are cations with the formula RCO+, where R = alkyl or aryl.{{March6th}} 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.{{cite journal |doi=10.1021/ed5002152 |title=Proper Resonance Depiction of Acylium Cation: A High-Level and Student Computational Investigation |date=2015 |last1=Esselman |first1=Brian J. |last2=Hill |first2=Nicholas J. |journal=Journal of Chemical Education |volume=92 |issue=4 |pages=660–663 }} 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.{{cite journal |doi=10.1039/B407654K |title=Structural effects of carbon monoxide coordination to carbon centers. σ and π Bindings in aliphatic acyl versus aromatic aroyl cations |date=2004 |last1=Davlieva |first1=Milya G. |last2=Lindeman |first2=Sergey V. |last3=Neretin |first3=Ivan S. |last4=Kochi |first4=Jay K. |journal=New Journal of Chemistry |volume=28 |issue=12 |page=1568 }}
:{{chem2|CH3COCl + GaCl3 -> CH3CO+ + GaCl4-}}
Acetyl hexafluoroantimonate and hexachloroantimonate are other early examples.{{cite journal |doi=10.1021/ja01026a025 |title=Crystal structure of a Friedel-Crafts intermediate. Methyloxocarbonium hexafluoroantimonate |date=1968 |last1=Boer |first1=F. Peter |journal=Journal of the American Chemical Society |volume=90 |issue=24 |pages=6706–6710 }}{{cite journal |doi=10.1107/S0567740872004406 |title=Etude de complexes acide de Lewis–halogénure d'acide. I. Structure cristalline des hexachloroantimonate, tétrachloroaluminate et tétrachlorogallate de méthyl oxocarbonium |date=1972 |last1=Le Carpentier |first1=J. M. |last2=Weiss |first2=R. |journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry |volume=28 |issue=5 |pages=1421–1429 |bibcode=1972AcCrB..28.1421L }}
The strength of the {{chem2|C\tO}} 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.
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:
:{{chem2|C6H5R + CH3CO+ + AlCl4- -> 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:
:{{chem2|(CH3)3CCO+ <-> (CH3)3C+ + CO}}
Central to the Koch carbonylation is the hydrolysis of acylium ions to carboxylic acids:
:{{chem2|R3CCO+ + H2O -> R3CCO2H + H+}}