Oxidation of Alcohols

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 oxidized 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 oxidized to ketones but no further:

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

Cr OXIDATION OF ALCOHOLS
The mechanism is not trivial, so attention here is focused 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.

Questions:
What type of reaction could the oxidation step be decribed as ? 
What is the oxidation state of Cr in chromic acid ? 
What is the oxidation state of the Cr in the chromate ester intermediate ? 
What is the oxidation state of the Cr in the "leaving group", HCrO₃^- ? 

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

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