||Chapter 15: Alcohols, Diols
Oxidation of Alcohols
Reaction type: Oxidation-Reduction
- 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
- 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
- Secondary alcohols can
be oxidised to ketones but no further:
- Tertiary alcohols cannot
be oxidised (no carbinol C-H)
|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.
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-
If you see Cr reagents, you are probably looking at an oxidation reaction.
Most likely the oxidation of an alcohol or an aldehyde
||© Dr. Ian Hunt, Department of Chemistry, University