Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

Oxidation of Aldehydes

Summary

• Aldehydes, RCHO, can be oxidised to carboxylic acids, RCO₂H.
• Ketones are not oxidised under these conditions as they lack the critical H for the elimination to occur (see mechanism below).
• The reactive species in the oxidation is the hydrate formed when the aldehyde reacts with the water.
• Typical reagents are aqueous Cr (VI) species:

• Oxidation of Alcohols

OXIDATION OF ALDEHYDES
Part 1: Formation of the hydrate (mechanism) occurs first.
Part 2: Now we essentially have an alcohol which reacts with the chromium species to form a chromate ester.
Part 3: A base (here a water molecule) abstracts a proton from the chromate ester, the C=O forms and a Cr species leaves. This is really an E2 elimination reaction. Note the importance of the original aldehyde H... if its' missing, no oxidation can occur.

© Dr. Ian Hunt, Department of Chemistry