Chapter 17 : C=O prep. by oxidation of ROH

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

Oxidation of Primary and Secondary Alcohols
(review of Chapter 15)

general scheme for the oxidation of alcohols

Reaction type: Oxidation-Reduction

Summary

The outcome of oxidation reactions of alcohols depends on the substituents on the carbinol carbon (C-OH).
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 aldehyde.

1^o alcohol aldehyde

2^o alcohol ketone

3^o alcohol

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 ?

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.