acyl

An acyl group (IUPAC name: alkanoyl) is a functional group derived by the removal of one or more hydroxyl groups from an oxoacid.^[1] In organic chemistry, the acyl group is usually derived from a carboxylic acid. It therefore has the formula RCO, where R represents an alkyl group that is attached to the CO group with a single bond. Although the term is almost always applied to organic compounds, acyl groups can in principle be derived from other types of acids such as sulfonic acids, phosphonic acids. Most commonly, acyl groups are attached to a larger molecular fragment, in which case the carbon and oxygen atoms are linked by a double bond.

Acyl compounds

Well known acyl compounds are the acyl chlorides, such as acetyl chloride (CH₃COCl) and benzoyl chloride (C₆H₅COCl). These compounds, which are treated as sources of acylium cations, are good reagents for attaching acyl groups to various substrates. Amides (RC(O)NR₂) and esters (RC(O)OR’) are classes of acyl compounds, as are ketones (RC(O)R) and aldehydes (RC(O)H).

Acylium cations, anions, and radicals

Acylium ions are cations of the type R-C⁺=O.^[1] The oxygen and carbon are linked by a triple bond. Such species are common reactive intermediates. For example in the Friedel-Crafts acylation. Furthermore acylium cations are characteristic fragments observed in EI-mass spectra of ketones. Acylium intermediates occur in many organic reactions such as the Hayashi rearrangement. Salts containing acylium ions are also generated when acyl halides are treated with pyridines and with [tertiary amine]]s:

RC(O)Cl + Et₃N → [RC(O)NEt₃⁺]Cl^-

Acyl anions are unknown and organolithium compounds with Li-C(O)R linkages are not well studied.

In biochemistry

In biochemistry, acyl CoAs are acyl derivatives formed via fatty acid metabolism. Acetyl CoA, the most common derivative, serves as an acyl donor in many biosynthetic transformations. Such acyl compounds are thiol esters. Many saccharides are acylated.

In organometallic chemistry and catalysis

Acyl ligands are intermediates in many carbonylation reactions, which are important in some catalytic reactions. Metal acyls usually arise via insertion of carbon monoxide into metal-alkyl bonds. Metal acyls also arise from the reactions involving acyl chlorides with low-valence metal complexes or by the reaction of organolithium compound with metal carbonyls. Metal acyls are often described by two resonance structures, one of which emphasizes the basicity of the oxygen center. O-Alkylation of metal acyls gives Fischer carbenes. complexes.^[2]

Nomenclature

The names of acyl groups are typically derived from the corresponding acid by substituting the acid ending -ic with the ending -yl as shown in the table below. Note that methyl, ethyl, propyl, butyl, etc. that end in -yl are not acyl but alkyl groups derived from alkanes. IUPAC nomenclature is recommended but rarely used.

Acyl species

In acyloxy groups the acyl group is bonded to oxygen: R-C(=O)-O-R' where R-C(=O) is the acyl group.

Acylium ions are cations of the type R-C⁺=O and play an important role as intermediates in organic reactions ^[1] for example the Hayashi rearrangement.

References

See also

• Acylation
• Functional group