organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, acylium ions are cations with the formula RCO⁺, where R = alkyl or aryl. They are a kind of

carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...

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 A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

exists between C and O. Acylium ions can be viewed as C-alkylated derivatives of

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

(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 Antimony pentachloride is a chemical compound with the formula SbCl5. It is a colourless oil, but typical samples are yellowish due to dissolved chlorine. Owing to its tendency to hydrolyse to hydrochloric acid, SbCl5 is a highly corrosive subs ...

. Several acylium salts have been characterized by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

, including the propionyl and mesityl derivatives. Because acylium cation is highly electrophilic, its salts can only be isolated with

weakly coordinating anion Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...

s. : Acetyl hexafluoroantimonate and

hexachloroantimonate Antimony pentachloride is a chemical compound with the formula SbCl5. It is a colourless oil, but typical samples are yellowish due to dissolved chlorine. Owing to its tendency to hydrolyse to hydrochloric acid, SbCl5 is a highly corrosive subst ...

are other early examples. The strength of the bond is indicated by the frequency of its vibration (ν_CO). These values are 2300 and 2200 cm⁻¹, respectively for the aryl and alkyl acylium ions. For comparison, the same vibration for

is 2143 cm⁻¹.

Reactivity

Acylium ions are potent

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively Electric charge, charged, have an ...

s as evidenced by their ability to attack arenes. Acylium ions are intermediates in several reactions, such as the Friedel-Crafts acylation of

arene Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

s by

acetyl chloride Acetyl chloride () is an acyl chloride derived from acetic acid (). It belongs to the class of organic compounds called acid halides. It is a colorless, corrosive, volatile liquid. Its formula is commonly abbreviated to AcCl. Synthesis On an ...

in the presence of

aluminium trichloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms a hexahydrate with the formula , containing six water molecules of hydration. Both the anhydrous form and the hexahydrate are col ...

: : 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: : Central to the Koch carbonylation is the hydrolysis of acylium ions to carboxylic acids: :{{chem2, R3CCO+ + H2O -> R3CCO2H + H+

References

Cations Reactive intermediates