Carboxylic Acid Derivatives
Synthesis of Carboxylic Acid Anhydrides
As indicated by the name, carboxylic acid anhydrides are compounds that formally consist of two condensed carboxylic acid molecules.
However, carboxylic acids do not spontaneously condense through water elimination. Instead, the synthesis of carboxylic acid anhydrides may, for instance, be achieved by treating carboxylate anions with acyl chlorides.
Much like in the Fischer esterification, the mechanism of this reaction is an addition-elimination mechanism in which the chloride anion is the leaving group. In the initial reaction step, the carboxylate anion's negatively charged oxygen nucleophilically attacks the considerably electrophilic acyl halide's carbonyl carbon. As a result, a tetrahedral intermediate is formed. In the second reaction step, chloride - a good leaving group - is eliminated from the tetrahedral intermediate.
Dehydration, which is required for the synthesis of anhydrides from carboxylic acids, may also be achieved by the application of a dehydrating agent, such as acetic anhydride, trifluoroacetic anhydride, phosphorus pentachloride, or dicyclohexylcarbodiimide DCC (compare also Formation of Carboxamides). Phthalic anhydride may easily be produced by heating phthalic acid, as the carboxyl groups of phthalic acid are optimally oriented according to each other in a fixed position.