Organic acid anhydride
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An organic acid anhydride is an organic compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the parent acid is a carboxylic acid, the formula of the anhydride being (RC(O))2O. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride. One or both acyl groups of an acid anhydride may also be derived from another type of organic acid, such as sulfonic acid or a phosphonic acid.
Properties
Acid anhydrides react with halogen acids, reaction which yields equal amounts of the acylated product and the carboxylic acid:
- RC(O)OC(O)R + HY → RC(O)Y + RCO2H
Acid anhydrides will react with water to form their respective carboxylic acid, though the reaction is slow.
Preparation
Acetic anhydride is industrially produced by the carbonylation of methyl acetate.
- CH3CO2CH3 + CO → (CH3CO)2O
Laboratory routes rely on the dehydration of the corresponding acids. Phosphorus pentoxide is a common dehydrating agent:
- 2 CH3COOH + P4O10 → CH3C(O)OC(O)CH3 + P4O9(OH)2
Acyl chlorides will react with the salts of carboxylic acids to yield acid anhydrides:
- CH3C(O)Cl + CH3COONa → (CH3CO)2O + NaCl
Simple and mixed anhydrides containing the acetyl group can be prepared from ethenone:
- CH3COOH + H2C=C=O → (CH3CO)2O
- RCOOH + H2C=C=O → RCO2C(O)CH3
Maleic anhydride is produced by the oxidation of benzene or butane.
