Acyl

An acyl group is a functional group derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids.

In organic chemistry, the acyl group is usually derived from a carboxylic acid (IUPAC name: alkanoyl). Therefore, it 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. In the most common arrangement, 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 formula RCO+. The oxygen and carbon are linked by a triple bond. Such species are common reactive intermediates, for example, in the Friedel-Crafts acylations also in many other organic reactions such as the Hayashi rearrangement. Salts containing acylium ions can be generated by removal of the halide acyl halides:
 * RC(O)Cl +  SbCl5   →   [RCO]SbCl6

The C-O distance in these cations is near 1.1 angstrom, even shorter than that in carbon monoxide. Acylium cations are characteristic fragments observed in EI-mass spectra of ketones.

Acyl anions and acyl radicals are very rare. Organolithium compounds with Li-C(O)R linkages are not well studied.

In biochemistry
In biochemistry there are many instances of acyl groups, in all major categories of biochemical molecules.

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.

Names of acyl groups of amino acids are formed by the replacement of the ending -ine by the ending -yl. For example the acyl group of glycine is glycyl-, and of lysine is lysyl-.

Names of acyl groups of ribonucleoside monophosphates such as AMP (5'-adenylic acid), GMP (5'-guanylic acid), CMP (5'-cytidylic acid), and UMP (5'-uridylic acid) are adenylyl-, guanylyl-, cytidylyl-, and uridylyl- respectively.

In phospholipids, the acyl group of phosphatidic acid is called phosphatidyl-.

Finally, 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 arise usually 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.

Nomenclature
The names of acyl groups are derived typically 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 for example the Hayashi rearrangement.