Acylium

An acyl group (/ˈæsɪl/ or /ˈeɪsaɪl/) is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids. It contains a double bonded oxygen atom and an alkyl group.

In organic chemistry, the acyl group (IUPAC name: alkanoyl) is usually derived from a carboxylic acid. Therefore, it has the formula RCO–, where R represents an alkyl group that is linked to the carbon atom of the group by 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.

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).

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