The Swern oxidation is the oxidation of primary and secondary alcohol to the aldehyde and ketone in the presence of oxalyl chloride, an organic base such as triethyl amine, and dimethyl sulphoxide. This reaction is named after Daniel Swern.
However, the advantage of this oxidation is that we do not use toxic metals. Hence, only mild conditions are required for this oxidation.
Mechanism of Swern oxidation
The Swern oxidation has the following mechanism:
Step 1: Nucleophilic substitution reaction
In the first step, weakly basic DMSO attacks the electrophilic carbon of the oxalyl chloride. Hence, chloride ion leaves the molecule. Thus, it forms an intermediate that has a sigma bond between the two species. However, this bond is broken down. When the chloride ion is attacked by the sulfur atom. Hence, a new bond is formed between DMSO and chloride ions. It results in the formation of dimethylchlorosulphonium chloride.
Step 2: Addition of alcohol
This step involves the addition of alcohol to the dimethylchlorosulphonium chloride. However, this gives the intermediate which is the alkoxy sulphonium ion.
Step 3: Elimination reaction
The last step involves the deprotonation of the intermediate by the base. However, this will give a sulfur ylide. Further, this ylide undergoes decomposition and forms carbonyl compound and dimethyl sulfide.
Variation of Swern oxidation
When we use oxalyl chloride, the reaction occurs at -60 οC. However, when the reaction is done in the presence of trifluoroacetic anhydride, the reaction is done under the -30 οC.
Disadvantage of the Swern oxidation
The by-products of the Swern oxidation like carbon mono oxide and dimethyl sulfide are very harmful Like, carbon monoxide is very toxic. This is because it reduces the binding ability of hemoglobin to oxygen. Dimethyl sulfide has also some harmful health effects. Such as, it may cause some respiratory problems.
Concepts Berg
First, add DMSO in a flask. Then, it is dissolved in dichloromethane. However, the solution is cooled down to -78 degree centigrade. Later, in this solution add oxalyl chloride with the help of a syringe. After that, alcohol is added to this mixture. Hence, the base triethylamine is added and stirred to the solution for 10 minutes. The solution is diluted with water. The organic component is washed with water and then aqueous NaCl. In the last, the solution is dried with anhydrous magnesium sulfate.
The Swern is only a weak oxidizing agent. However, it only oxidized alcohol to aldehyde.
The Swern oxidation is the oxidation of alcohol into aldehyde and ketone by using oxalyl chloride, DMSO, and triethyl amine.
The DMSO and oxalyl chloride react and form dimethylchlorosulphonium ion which acts as an oxidizing agent.
The starting materials of Swern and Parikh-Doering oxidation are different. However, the Swern oxidation is done under low temperatures. Whereas the Parikh-Doering oxidation occurs at room temperature.
Reference Books
- Oxidation of alcohols and aldehyde and ketone by Gabriel Tojo and Marcos Fernandes
Reference Links
- Swern oxidation (wikipedia.org)
