Overall reaction
RCH₂OH → (1. (COCl)₂, DMSO, −78 °C; 2. Et₃N) RCHO + Me₂S + CO + CO₂
DMSO activated by oxalyl chloride oxidises an alcohol to an aldehyde at −78 °C.
Name reaction · mechanism
Swern Oxidation
The Swern oxidation turns primary alcohols into aldehydes and secondary alcohols into ketones without over-oxidation or heavy metals. DMSO is activated by oxalyl chloride at −78 °C (releasing CO₂, CO and chloride), the alcohol adds to sulfur, triethylamine forms a sulfur ylide, and an intramolecular elimination gives the carbonyl compound and dimethyl sulfide.
Mechanism step by step
- Activation. DMSO oxygen attacks oxalyl chloride
- Chloride leaves. C=O re-forms
- Chlorosulfonium ion. Chloride attacks sulfur
- Fragmentation. CO₂, CO and Cl⁻ are lost (gas evolution)
- Alcohol adds. O attacks S; chloride leaves
- Deprotonation. Et₃N removes the O–H proton
- Sulfur ylide. A second deprotonation, at S–CH₃
- Elimination. Five-membered transition state: C=O forms, Me₂S leaves
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
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.
For JEE and NEET
- Stops at the aldehyde (no water present).
- Needs −78 °C until the base is added.
- By-products: dimethyl sulfide (smell), CO (toxic) and CO₂.
- Alternatives: PCC, Dess–Martin periodinane.
FAQ
Why does the Swern oxidation not over-oxidise to the acid?
No water is present, so the aldehyde cannot form a hydrate that would be oxidised further.
What is the role of triethylamine?
It deprotonates the alkoxysulfonium ion to a sulfur ylide, which removes the C–H through a five-membered transition state.
Why −78 °C?
The activated DMSO species decomposes above about −60 °C.
Draw it in MolDraw
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