Conversion of Carboxylic Acids into Isolable Acylating Agents

2 Conversion of Carboxylic Acids into Isolable Acylating Agents 

Carboxylic acids can also be activated by converting them to their anhydrides. For this purpose they are dehydrated with concentrated sulfuric acid, phosphorus pentoxide, or 0.5 equivalents of SOCl2 (1 equivalent of SOCl2 reacts with carboxylic acids to form acid chlorides rather than anhydrides). However, carboxylic anhydrides cannot transfer more than 50% of the original carboxylic acid to a nucleophile. The other 50% is released—depending on the pH value—either as the carboxylic acid or as a carboxylate ion and is therefore lost. Consequently, in laboratory chemistry, the conversion of carboxylic acids into anhydrides is not as relevant as carboxylic acid activation. Nonetheless, acetic anhydride is an important acetylat-ing agent because it is commercially available and inexpensive. 

---