Chemistry of Carboxylic Anhydrides

Carboxylic anhydrides, RCOCR, are named by simply adding the term anhydride to the acid name (or names, with regard to mixed anhydrides). This method also applies to cyclic derivatives. 

The reactions of carboxyhc anhydrides with nucleophiles, although less vigorous, are completely analogous to those of the acyl hahdes. The leaving group is a carboxylate instead of a haUde ion. 

In every addition-elimination reaction except hydrolysis, the carboxylic acid side product is usually undesired and is removed by work-up with aqueous base. Cyclic anhydrides undergo similar nucleophilic addition-elimination reactions that lead to ring opening. 

Treatment of butanedioic (succinic) anhydride with ammonia at elevated temperatures leads to a compound C4H5NO2. What is its structure (Hint Consult Section 19-10.) 

Formulate the mechanism for the reaction of acetic anhydride with methanol in the presence of sulfuric acid. 

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Although the chemistry of carboxylic anhydrides is very similar to that of acyl halides, anhydrides have some practical advantages. Acyl halides are so reactive that they are difficult to store for extended periods without some hydrolysis occurring due to exposure to atmospheric moisture. As a result, chemists usually prepare acyl hahdes immediately before they are to be used. Anhydrides, being slightly less reactive toward nucleophiles, are more stable, and several (including all the examples illustrated in this section) are commercially available. Consequently, carboxylic anhydrides are often the preferred reagents for the preparation of many carboxylic acid derivatives. 

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