Chapter 15: Alcohols, Diols and Thiols

Oxidation of Alcohols

Reaction type: Oxidation-Reduction

Summary

• Common reagents:

• The outcome of oxidation reactions of alcohols depends on the substituents on the carbinol carbon.
• In order for each oxidation step to occur, there must be H on the carbinol carbon.
• Primary alcohols can be oxidised to aldehydes or further to carboxylic acids.
• In aqueous media, the carboxylic acid is usually the major product.
• PCC or PDC, which are used in dichloromethane, allow the oxidation to be stopped at the intermediate aldehyde.

• Secondary alcohols can be oxidised to ketones but no further:

• Tertiary alcohols cannot be oxidised (no carbinol C-H)

Cr OXIDATION OF ALCOHOLS
The mechanism is not trivial, so attention here is focussed on the actual oxidation step. Prior to this, the alcohol reacts to form a chromate ester (shown). A base (here a water molecule) abstracts a proton from the chromate ester, the C=O forms and a Cr species leaves.  This demonstrates the importance of the carbinol H to this mechanism.

Study Tip: If you see Cr reagents, you are probably looking at an oxidation reaction.