 Swern oxidation is one of the selective methods for oxidizing primary alcohols to aldehydes.
Like other mild oxidizing agents, such as pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), and the Dess-Martin (DMP) oxidation, it stops 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.
Tertiary alcohols cannot be oxidized by Swern or by any other technique.
Here 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.
Â
Â
For now, let’s focus on the Swern oxidation. First, what reagents are used here?
Dimethyl sulfoxide (DMSO) and oxalyl chloride (COCl)2 are used as oxidizing agents in the Swern oxidation:
Â
Â
Â
The Mechanism of Swern Oxidation
There are quite a few reagents used for the oxidation of alcohols, but the good news is that most of them follow a similar mechanism, and we will try to identify a common trend for these reactions.
The idea here is to install a leaving group on the hydroxyl oxygen and remove the neighboring hydrogen by kicking out the leaving group to form a c=O π bond. It should remind you of the E2 mechanism:
Â
Â
Notice the importance of the ɑ-hydrogen in the elimination step. It is the removal of this hydrogen that provides the electron to form the C=O π bond.
Â
The Mechanism of Swern Oxidation
In the first step of Swern oxidation, DMSO and oxalyl chloride react to form a chlorodimethylsulfonium salt, which is a Lewis acid and reacts with the alcohol, thus installing the good leaving group necessary for the elimination step:
Â
Â
The E2 elimination can be achieved by amines as organic bases, and triethylamine (TEA) is used most often for this purpose.
Â
Â
