Metaanhydrides

Metaanhydrides are a class of organic compounds characterized by the presence of two acyl groups derived from the same or different carboxylic acids, joined by an oxygen atom. They are formally derived from carboxylic acids by the removal of a molecule of water. The general formula for a metaanhydride is R-CO-O-CO-R', where R and R' can be the same or different alkyl or aryl groups. When R and R' are identical, the compound is called a symmetrical metaanhydride. If R and R' are different, it is an unsymmetrical metaanhydride.

Metaanhydrides are typically prepared by the reaction of a carboxylic acid with an acid anhydride or a carbodiimide, or by the dehydration of carboxylic acids using strong dehydrating agents. They are reactive compounds, susceptible to nucleophilic attack, particularly by water, alcohols, and amines. Hydrolysis regenerates the parent carboxylic acids. Reaction with alcohols yields esters and carboxylic acids, while reaction with amines produces amides and carboxylic acids.

Due to their reactivity, metaanhydrides serve as useful acylating agents in organic synthesis. They are employed in the introduction of acyl groups onto various substrates, facilitating the formation of esters, amides, and other derivatives. Acetic anhydride, derived from acetic acid, is a common and widely used example of a metaanhydride. Metaanhydrides are generally less reactive than acid halides but more reactive than esters.