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19.12 Biological Reductions

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    Objectives

    After completing this section, you should be able to

    1. determine whether or not a given aldehyde will undergo the Cannizzaro reaction.
    2. write an equation to illustrate the Cannizzaro reaction.
    3. identify the products formed when a given aldehyde undergoes a Cannizzaro reaction.
    4. write the detailed mechanism of the Cannizzaro reaction.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • Cannizzaro reaction

    Study Notes

    In 1853 Stanislao Cannizzaro discovered that the base‑induced disproportionation of an aldehyde resulted in a carboxylic acid (oxidation product) and an alcohol (reduction product).

    reaction of sodium hydroxide with benzaldehyde to form sodium benzoate and phenylmethanol

    Note that in the mechanism shown in the reading has an intermediate generated from methanal (formaldehyde) that effectively becomes a hydride (H−) donor, which reduces a second molecule of methanal to methanol.

    methanediol anion losing a hydride anion to produce formic acid

    Carbonyl reductions are a part of important biological pathways in living organisms. Recall in Section 17.4 that NADH can donate H− to reduce aldehydes and ketones. You may wish to review this section.

    Cannizzaro Reaction

    A characteristic reaction of aldehydes without \(\alpha\) hydrogens is the self oxidation-reduction they undergo in the presence of strong base. With methanal as an example,

    Roberts and Caserio Screenshot 16-4-47.png

    If the aldehyde has \(\alpha\) hydrogens, other reactions usually occur more rapidly.

    The mechanism of this reaction, usually called the Cannizzaro reaction,\(^4\) combines many features of other processes studied in this chapter. The first step is reversible addition of hydroxide ion to the carbonyl group:

    Roberts and Caserio Screenshot 16-4-49.png

    A hydrogen can be transferred as hydride ion to methanal from the hydroxyalkoxide ion, thereby reducing the methanal to methanol:

    Roberts and Caserio Screenshot 16-4-50.png

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