Organic acid anhydride
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
Industrial
Acetic anhydride is industrially produced by the carbonylation of methyl acetate, catalyzed by hydrogen iodide and dicarbonyldiiodorhodiate (I).
- CH3CO2CH3 + CO → (CH3CO)2O
Maleic anhydride is produced by the oxidation of benzene or butane.
Laboratory
Laboratory routes rely on the dehydration of the corresponding acids or their salts. A variety of methods have been developed, because anhydrides are of interest in many reactions and are often expensive or unavailable for purchase.
With phosphorus pentoxide
Phosphorus pentoxide is a common dehydrating agent:
- 2 CH3COOH + P4O10 → CH3C(O)OC(O)CH3 + P4O9(OH)2
However, tar is a common byproduct.
With acetic anhydride
Acetic anhydride will react with many carboxylic acids to convert them to their anhydrides. This is an equilibrium reaction which is driven to completion by the relatively high volatility of AcOH:
- 2 RCOOH + Ac2O + heat → (RCO)2O + 2 AcOH (g)
Alternatively, acetic anhydride can be produced by the reaction of other anhydrides with the salts of acetic acid:
- 2 AcONa + (RCO)2O → 2 RCOONa + Ac2O
From acyl chlorides
Acyl chlorides will react with the salts of carboxylic acids to yield acid anhydrides:
- CH3C(O)Cl + CH3COONa → (CH3CO)2O + NaCl
For this reason, preparations of acyl chlorides are also of interest in the preparation of acid anhydrides. When acyl chlorides are prepared from the salts of carboxylic acids, using an excess of the salt often produces the anhydride.
From ethenone
Simple and mixed anhydrides containing the acetyl group can be prepared from ethenone, also known as ketene:
- CH3COOH + H2C=C=O → (CH3CO)2O
- RCOOH + H2C=C=O → RCO2C(O)CH3
However, the application of this process is limited by the high toxicity of ethenone.
Dehydration with in situ sulfur bromide
Acetic anhydride is produced when sodium acetate is treated with sulfur dissolved in a large quantity of bromine:
- 4 AcONa + 1/8 S8 + 2 Br2 → 2 Ac2O + 4 NaBr + SO2
This method is popular due to its use of cheap reagents and avoidance of complex procedures.
Distillation of silver salts
Anhydrous silver salts of carboxylic acids decompose to silver oxide and release the anhydrides when heated to around 380-400 C in an atmosphere of argon.
- 2 AgOAc + heat → Ag2O + Ac2O (g)
However, the presence of even small traces of water or oxygen, including water vapor, catalyzes the rapid decomposition of the formed anhydride, due to the strong oxidizing power of the silver ion. It has been suggested that similar reactions are possible with salts of copper and zinc, but subsequent testing found no success. Despite the apparent simplicity of the process, this reaction is rarely used in practice.
Thermal dehydration of pre-cyclic diacids
Molecules containing two carboxyl groups separated by two or three carbon atoms can be thermally dehydrated to their anhydrides with the elimination of H2O, where the anhydride forms part of a five- or six-membered ring. Diacids having longer or shorter chains, such as malonic acid and adipic acid, give poor yields of anhydrides by this method.
- COOHCH2CH2COOH + heat → C2O3C2H4 + H2O
Diacid-catalyzed thermal dehydration
High-boiling acids such as benzoic acid are dehydrated by heating at 230-260 C for a long time with catalytic amounts of a pre-cyclic diacid such as phthalic acid. This method relies on driving the thermal equilibrium between the acid and its anhydride to the right with the loss of water according to Le Chatelier's principle.
- 2 BzOH + catalytic Ph(COOH)2 + heat → Bz2O + H2O (g)
This method is described in US patent 3513180.
