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2.9: Acid Anhydrides for Ester Synthesis

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    270982
  • Synthesis of Acid Anhydrides

    Acid chlorides react with carboxylic acids to form anhydrides as shown in the reaction below. Acid anhydrides often form when one equivalent of water is removed from two equivalents of an organic acid in a dehydration reaction:

    Sny1.jpg

    Some cyclic anhydrides can be synthesized from the corresponding dicarboxylic acid with gentle heating. The example below shows the reaction of glutaric acid to form a cyclic anhydride.

    Example: Acid Anhydride Synthesis

    ch 22 sect 5 example.png

    Ester Synthesis using acid anhydrides

    Acid anhydrides react with alcohols to produce esters as shown in the reaction below. The reactions of anhydrides frequently use pyridine as a solvent. As in the case of using acid halides, the synthesis of ester using acid anhydrides instead of carboxylic acids results in an irreversible reaction, which improves the product yield: 

    2a.jpg

    The presence of pyridine facilitates proton transfers during the reaction. A carboxylic acid is also produced, but is not considered a synthetic product. The ester is considered the "product of interest".

    The synthesis of methyl benzoate from benzoic anhydride and methanol is shown in the example.

    Example: Ester Sythesis

    3a.jpg

     

    Acid Anhydride Hydrolysis

    Acid anhydrides readily hydrolyze to carboxylic acids. In many cases, this reaction is an unwanted side reaction and steps will be taken in the lab to keep the system "dry" (aka water-free). The presence of pyridine facilitates proton transfers during the reaction. The hydrolysis reaction for benzoic anhydride is shown below.

    ch 22 sect 5 anhydride hydrolysis updated.png

     

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