Oxidation of aldehydes to carboxylic acids using Cr(VI)

by James

Description: Chromium trioxide (CrO3) and water will oxidize aldehydes to carboxylic acids.
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Notes: This can be written with a variety of other reagents such as H2CrO4, K2Cr2O7/H2SO4, and other Cr salts.

Examples:

Notes: The byproduct of each of these reactions are Cr(IV) compounds.

Mechanism: For the purposes of Org 1 the mechanism of this reaction is probably not that important. However it is shown here for completeness.

The first step involves a 1,2-addition of water (Step 1, arrows A and B) followed by proton transfer (Step 2, arrows C and D) to form a hydrate. Oxygen then attacks chromium in a 1,2-addition (Step 3, arrows E and F) followed by proton transfer to give a chromate ester (Step 4, arrows G and H). Deprotonation of the hydrogen on C1 leads elimination, formation of the C–O (PI) bond to give the carboxylic acid (Step 5, arrows I, J, K, and L) as well as the chromate ion, which is neutralized by the H3O+ formed by this reaction.

Notes: There are other reasonable ways of drawing the proton transfer steps, and other reasonable bases to use in the elimination step besides H2O. These are mere suggestions.

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