Pyridinium Chlorochromate (PCC) Oxidation
PCC oxidation is one of the selective methods for oxidizing primary alcohols to aldehydes.
Although very often the outcome of the oxidation will depend on the presence or absence of water, traditionally, the most common mild oxidizing agents are considered pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), Swern oxidation using DMSO, (COCl)2 and Et3N, and the Dess-Martin (DMP) oxidation:
This is the advantage of mild oxidizing agents since, remember, strong oxidizing agents, such as Jones’, oxidize primary alcohols all the way to carboxylic acids.
Below is a general scheme of the alcohol oxidation patterns depending on the nature of the oxidizing agent, and this is covered in a lot more detail in this post: Oxidation of Alcohols.
As shown above, mild reagents stop the oxidation once the carbonyl group is formed. And if it is a primary alcohol, the product is an aldehyde, while the oxidation of a secondary alcohol results in a ketone.
For now, let’s focus on the PCC oxidation. Like other mild oxidizing agents, such as the Swern and Dess-Martin (DMP) oxidation, it stops the oxidation of the alcohol once a carbonyl group is formed. If it is a primary alcohol, the product is an aldehyde, while the oxidation of secondary alcohols results in a ketone:
PCC belongs to the family of chromium-based oxidizing agents, most of which are CrO3, Na2Cr2O7, and chromic acid, collectively known as Jones oxidation, but unlike those, it is a mild oxidizing agent.
PCC is a Cr6+ salt formed between pyridine (C6H5N), HCl, and CrO3. It is soluble in halogenated organic solvents such as dichloromethane, which allows to carry out the reaction in the absence of water. This also explains why PPC does not oxidize the aldehyde to a carboxylic acid, as there is no water to turn the aldehyde into an aldehyde hydrate (check the strong oxidizing agents here)
The reaction starts by converting the alcohol to its corresponding chromate ester, which then undergoes a deprotonation by a base to form a C=O double bond:
In the acid-base step, either the chloride ion or the alcohol can serve as a base to remove the hydrogen. Pyridine is certainly a better candidate for deprotonation; however, it is present in low concentration as a free base in acidic conditions.
So far, all our attention was on the organic substrate, but remember, this is an oxidation-reduction reaction, and while the alcohol is being oxidized to a carbonyl, the Cr6+ is reduced to the corresponding Cr4+ species.
I like the composition of the mechanism. it is good
I think there should be explations to the mechanism as to how protons are transfered to make simple for understanding to slow learners like myself, however, the other aspects of the tution are good for me. Thanks
Hi,
Thanks for the comment. Proton transfers can be intramolecular when an atom form the same molecule abstract the proton form another atom, intermolecular when an atom from a different/another molecule does that, or it can occur by water attacking the proton.
Unfortunately, it is not always that the mechanisms for proton transfers are shown, partly to make the mechanism shorter.