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21.5 Chemistry of Acid Anhydrides

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  • Page ID
    90996
  • 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.

    scheme of acid anhydride forming different carboxylic acid derivatives

    Preparation of Acid Anhydrides

    Acid Chlorides react with carboxylic acids to form anhydrides

    Sny1.jpg

    Acid Anhydrides react with alcohols to form esters

    Reactions of anhydrides use Pyridine as a solvent

    1a.jpg

    2a.jpg

    Example 21.5.1

    3a.jpg

    Mechanism

    1)   Nucleophilic Attack by the Alcohol

    4a.jpg

    2)  Deprotonation by pyridine

    5a.jpg

    3) Leaving group removal

    6a.jpg

     4)  Protonation of the carboxylate

    7a.jpg

    Acid Anhydrides react with amines to form amides

    1.jpg

    Example 21.5.2

    2.jpg

    Mechanism

    1)   Nucleophilic Attack by the Amine

    3.jpg

    2)  Deprotonation by the amine

    4.jpg

    3) Leaving group removal

    5.jpg

    Exercises

    Questions

    Q21.5.1

    Draw out the mechanism for the following reaction.

    Q21.5.2

    Draw the product of the reaction between these two molecules.

    Solutions

    S21.5.1

    S21.5.1

    Contributors