# 19.12 Biological Reductions

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).

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.

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,

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:

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

## Contributors

• John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."