Formation of anhydrides from acid halides and carboxylates
Description: Addition of a carboxylic acid to an acid chloride results in the anhydride, displacing the chloride ion. [private_ReactionGuide]
Notes: The reaction can work with the neutral carboxylic acid, but it is more effective to use the conjugate base of the carboxylic acid (“carboxylate”) which is more nucleophilic. In the example above the Na(+) that is shown is merely a spectator ion.
This reaction works because Cl(–) is a better leaving group (i.e. weaker base) than the carboxylate ion.
Notes: Note that oftentimes the counter-ion will not be shown. “OAc” is the acetate ion, CH3COO–
Mechanism: Addition of the carboxylate to the acid chloride (Step 1, arrows A and B) results in a tetrahedral intermediate, which then eliminates chloride ion (Step 2, arrows C and D) to give the anhydride.
Notes: Note that the salt is the byproduct of this reaction. For this example it’s unimportant that Na is shown, it’s just a spectator ion.