Oxidation of Alcohols to Aldehydes

Oxidation of Alcohols

Reaction type: Oxidation-Reduction

Summary

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.

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

Study Tip:
If you see Cr reagents, you are probably looking at an oxidation reaction.
Most likely the oxidation of an alcohol or an aldehyde