MCAT Organic Chemistry Review

Aldehydes and Ketones I: Electrophilicity and Oxidation—Reduction

6.3 Oxidation–Reduction Reactions

Aldehydes occupy the middle of the oxidation–reduction spectrum; they are more oxidized than alcohols but less oxidized than carboxylic acids. Ketones, on the other hand, are as oxidized as secondary carbons can get.

OXIDATION OF ALDEHYDES

When aldehydes are further oxidized, they form carboxylic acids. Any oxidizing agent stronger than PCC can perform this reaction. Some examples include potassium permanganate (KMnO4), chromium trioxide (CrO3), silver(I) oxide (Ag2O), and hydrogen peroxide (H2O2). This is shown in Figure 6.11.

Figure 6.11. Aldehyde Oxidation Most oxidizing agents will turn aldehydes into carboxylic acids; PCC, however, is anhydrous and is not strong enough to oxidize past the point of an aldehyde.

REDUCTION BY HYDRIDE REAGENTS

Aldehydes and ketones can also undergo reduction to form alcohols. This is often performed with hydride reagents. The most common of these seen on the MCAT are lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4), which is often used when milder conditions are needed. This reaction is shown in Figure 6.12.

Figure 6.12. Ketone Reduction Ketones are easily reduced to their respective alcohols using hydride reagents.

MCAT Concept Check 6.3:

Before you move on, assess your understanding of the material with these questions.

1.    What functional group is formed when an aldehyde is oxidized? What are some common oxidizing agents that assist this reaction?

·        Functional group:

·        Oxidizing agents:

2.    What functional group is formed when aldehydes and ketones are reduced? What are some common reducing agents that assist this reaction?

·        Functional group:

·        Reducing agents: