Swern oxidation template

DMSO and oxalyl chloride give the chlorosulfonium ion (with CO₂, CO and Cl⁻); the alcohol adds, Et₃N forms the ylide and a five-membered elimination gives propanal and Me₂S.

Mechanism step by step

DMSO activated by oxalyl chloride oxidises an alcohol to an aldehyde at −78 °C

1. Activation – DMSO oxygen attacks oxalyl chloride
2. Chloride leaves – C=O re-forms
3. Chlorosulfonium ion – Chloride attacks sulfur
4. Fragmentation – CO₂, CO and Cl⁻ are lost (gas evolution)
5. Alcohol adds – O attacks S; chloride leaves
6. Deprotonation – Et₃N removes the O–H proton
7. Sulfur ylide – A second deprotonation, at S–CH₃
8. Elimination – Five-membered transition state: C=O forms, Me₂S leaves

Key points

• Primary alcohols stop at the aldehyde: no water, no over-oxidation.
• Must be kept at −78 °C: the chlorosulfonium ion decomposes on warming.
• By-products: CO (toxic), CO₂, Me₂S (strong smell) — work in a fume hood.
• No chromium or heavy metals; secondary alcohols give ketones.

Drawn with 172 atoms, 28 arrows, 12 shapes – every one editable in MolDraw.

Sources

• Mancuso, Huang & Swern, J. Org. Chem. 1978, 43, 2480 Original paper
• Omura & Swern, Tetrahedron 1978, 34, 1651 Original paper
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook