Swern oxidation: practice and mechanism
The Swern oxidation is mild and water-free: primary alcohols give the aldehyde (not the acid), secondary ones the ketone. Tertiary alcohols do not react.
Example
DMSO is activated with oxalyl chloride and the alcohol binds to the sulfur. Triethylamine triggers an intramolecular elimination: the carbonyl compound and dimethyl sulfide form. Like PCC, the Swern oxidation stops at the aldehyde, but without chromium.
Mechanism step by step
- 1
Oxalyl chloride activates DMSO at −78 °C, giving a chlorosulfonium ion (CO and CO₂ escape).
- 2
The alcohol attacks the sulfur: an alkoxysulfonium ion.
- 3
Et₃N deprotonates a methyl group on S; in a cyclic step the ylide removes the H from the alcohol carbon: C=O forms and dimethyl sulfide leaves.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Swern stops at the aldehyde: there is no water to form a hydrate, and the oxidant is only good for one step. Jones (CrO₃, H₂SO₄), for example, gives the acid.
Often confused with: Strong oxidation (chromic acid) · Acid-catalysed dehydration (E1)
Frequently asked questions
What are the advantages over PCC?
No toxic chromium, very mild (−78 °C), no over-oxidation. Drawback: dimethyl sulfide smells strongly.
What other mild oxidations are there?
Dess–Martin periodinane (DMP) and TEMPO/NaOCl. All stop at the aldehyde.