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21.5 Chemistry of Acid Anhydrides

  • Page ID
    90996
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    Objectives

    After completing this section, you should be able to

    1. write an equation to illustrate the preparation of an acid anhydride from an acid halide and the sodium salt of a carboxylic acid.
    2. identify the product formed from the reaction of a given acid halide with the sodium salt of a given carboxylic acid.
    3. identify the acid halide, carboxylate salt, or both, required to prepare a given acid anhydride.
    4. write an equation to describe the reaction of an acid anhydride with each of the following: water, alcohol, ammonia, a primary or secondary amine, lithium aluminum hydride.
    5. identify the product formed when a given acid anhydride is reacted with any of the reagents listed in Objective 4, above.
    6. write a detailed mechanism for the reaction of an acid anhydride with any of the reagents listed in Objective 4, above.
    7. identify the acid anhydride, the nucleophilic reagent, or both, needed to prepare a specified carboxylic acid, ester, amide, or primary alcohol.
    Study Notes

    The reactions described in this section are, in principle, identical to those discussed in Section 21.4. Once you have understood the mechanism of nucleophilic acyl substitution, these reactions should not present you with any great difficulty, and memorization can be kept to a minimum.

    This figure provides a convenient general summary of a few of the reactions described in Section 21.5. Note that from a synthetic perspective the ester‑ and amide‑forming reactions are the most common, so they are the focus of this section.

    Diagram of acid anhydride forming different derivatives. Adding water to acid anhydride corms carboxylic acid. Adding an alcohol forms an ester. Adding hydride forms an alcohol. Adding ammonia forms an amide.

    Preparation of Acid Anhydrides

    Acid Chlorides react with carboxylic acids to form anhydrides

    Acid Anhydrides react with alcohols to form esters

    Reactions of anhydrides use Pyridine as a solvent

    Chemical structure of pyridine.

    Reaction diagram. An acid anhydride reacts with an alcohol with pyridine forming an ester and a carboxylic acid.

    Example 21.5.1

    Reaction diagram. Benzoic anhydride reacts with methanol and pyridine forming methyl benzoate and benzoic acid.

    Mechanism

    1) Nucleophilic Attack by the Alcohol

    2) Deprotonation by pyridine

    3) Leaving group removal

    4) Protonation of the carboxylate

    Acid Anhydrides react with amines to form amides

    Reaction diagram. An acid anhydride reacts with 2 amines forming an amide and RNH3+ and COOR-.

    Example 21.5.2

    Mechanism

    1) Nucleophilic Attack by the Amine

    2) Deprotonation by the amine

    3) Leaving group removal

    Exercises

    Exercise 21.5.1

    Draw out the mechanism for the following reaction.

    25-5-1Qu.png

    Answer

    25-5-1Sol.png

    Exercise 21.5.2

    Draw the product of the reaction between these two molecules.

    25-5-2Qu.png

    Answer

     25-5-2Sol.png

    Contributors and Attributions


    21.5 Chemistry of Acid Anhydrides is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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