Acyl group

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.

Carboxylate ions are essentially unreactive towards nucleophilic substitution, since they possess no leaving group.

The reactivity of these five classes of compounds covers a broad range; the relative reaction rates of acid chlorides and amides differ by a factor of 1013.

[2] A major factor in determining the reactivity of acyl derivatives is leaving group ability, which is related to acidity.

The reactivity of acyl compounds towards nucleophiles decreases as the basicity of the leaving group increases, as the table shows.

[2] Once a nucleophile attacks and a tetrahedral intermediate is formed, the energetically favorable resonance effect is lost.

These compounds, which are treated as sources of acylium cations, are good reagents for attaching acyl groups to various substrates.

Acylium ions are common reactive intermediates, for example in the Friedel–Crafts acylation and many other organic reactions such as the Hayashi rearrangement.

However, they undergo rapid decarbonylation to afford the alkyl radical:[8] Acyl anions are almost always unstable—usually too unstable to be exploited synthetically.

The tetrahedral intermediate of an acyl compound contains a substituent attached to the central carbon that can act as a leaving group.

After the tetrahedral intermediate forms, it collapses, recreating the carbonyl C=O bond and ejecting the leaving group in an elimination reaction.

Under acidic conditions, the carbonyl group of the acyl compound 1 is protonated, which activates it towards nucleophilic attack.

Because the last step involves the loss of a proton, nucleophilic acyl substitution reactions are considered catalytic in acid.

Under basic conditions, a nucleophile (Nuc) attacks the carbonyl group of the acyl compound 1 to give tetrahedral alkoxide intermediate 2.

Unlike acid-catalyzed processes, both the nucleophile and the leaving group exist as anions under basic conditions.