Dmso Swern Oxidation

Alcohol Reactions: Swern Oxidation using DMSO

The Swern oxidation converts primary alcohols to aldehydes and secondary alcohols to ketones under anhydrous, low-temperature conditions. DMSO is activated with oxalyl chloride at −78 °C, forming a chlorodimethylsulfonium electrophile that the alcohol intercepts. CO and CO₂ are released during activation. After alcohol addition, triethylamine forms a sulfur ylide, which undergoes intramolecular syn elimination to give the carbonyl product and dimethyl sulfide (Me₂S). Simple tertiary alcohols do not undergo the ordinary carbonyl-forming oxidation because they lack a hydrogen on the alcohol-bearing carbon.

Review alternative oxidation methods in the DMP & PCC oxidation guide.


Quick Summary


  • Reagents & conditions: DMSO, oxalyl chloride, and CH₂Cl₂ at −78 °C followed by Et₃N warming to 0 °C → rt.
  • Outcomes: primary alcohols → aldehydes; secondary alcohols → ketones; tertiary alcohols → no oxidation (no α-H).
  • Mechanistic spine: activated DMSO (chlorodimethylsulfonium) → alkoxysulfonium → sulfur ylide → intramolecular syn elimination → carbonyl + Me₂S.
  • Byproducts: CO, CO₂, and Me₂S (ventilation/scrubbing essential).
  • Stereochemistry: the oxidised carbon becomes sp²; configuration at that centre is lost.


Mechanism — Swern (5 Frames)


Step 1 — Activation of DMSO with oxalyl chloride (−78 °C)

DMSO oxygen attacks oxalyl chloride, releasing CO and CO₂ to begin Swern activation.
DMSO engages oxalyl chloride, venting CO/CO₂ and priming the sulfoxide for electrophilic behaviour.

Step 2 — Chlorodimethylsulfonium chloride formation (−78 °C)

Electrophilic chlorodimethylsulfonium chloride is generated with chloride as the counterion.
Collapse delivers chlorodimethylsulfonium chloride (Me₂SCl⁺ Cl⁻), the electrophile that activates the alcohol.

Step 3 — Alcohol attack on chlorodimethylsulfonium (−78 → −60 °C)

The alcohol oxygen attacks sulfur while chloride assists in proton transfer, forming the alkoxysulfonium salt.
The alcohol oxygen attacks sulfur; chloride mediates proton transfer to give the alkoxysulfonium salt.

Step 4 — Sulfur-ylide formation and syn elimination (base, then warming)

Net carbonyl formation with Et3N via sulfur-ylide formation and intramolecular syn elimination; the C8 skeleton is retained.
Et₃N first removes a sulfur-methyl proton to form a sulfur ylide. Intramolecular syn elimination then transfers the alcohol-carbon H and forms C=O, releasing Me₂S. The figure shows the net organic change; counterions and salts are omitted.

Step 5 — Quench and separation

Carbonyl product is isolated while volatile byproducts (CO, CO₂, Me₂S) are scrubbed or vented.
The carbonyl product is isolated; volatile byproducts are vented or oxidatively scrubbed.


Worked Examples


Primary Alcohol → Aldehyde

Swern conditions
2-Methylpropan-1-ol substrate Swern reagent card 2-Methylpropanal product

2-Methylpropan-1-ol gives 2-methylpropanal, retaining all four carbons.

Secondary Alcohol → Ketone

Swern conditions
Cyclic secondary alcohol substrate Swern reagent card Ketone product

Cyclohexanol gives cyclohexanone (sp³ → sp² at C1). The starting alcohol is achiral; its two ring paths are equivalent.

Tertiary Alcohol (No Reaction)

Swern conditions
Tertiary alcohol substrate Swern reagent card No oxidation outcome illustration

No hydrogen is present on the alcohol-bearing carbon, so this tertiary alcohol does not undergo the ordinary Swern carbonyl-forming oxidation.


Mechanistic Checklist


  • α-H required on the alcohol carbon; tertiary alcohols do not oxidise.
  • Keep the activation mixture below −60 °C before base addition to avoid side reactions.
  • Chlorodimethylsulfonium chloride is the electrophile; the alcohol forms an alkoxysulfonium salt prior to elimination.
  • Triethylamine (or DIPEA) forms a sulfur ylide before intramolecular syn elimination generates the carbonyl and Me₂S.
  • Anhydrous medium prevents aldehyde hydration, so over-oxidation to acids does not occur.


Edge Cases & Exam Traps


  • Moisture quenches the activated sulfonium species and stalls the oxidation.
  • Warming during activation can generate byproducts; add oxalyl chloride slowly at −78 °C.
  • A stereocenter on the oxidized carbon is lost on carbonyl formation. A surviving stereocenter alpha to the new carbonyl may epimerise; sensitive substrates need a suitable base, temperature and workup.
  • Benzylic and allylic substrates oxidise rapidly; ensure controlled quench to avoid over-exposure to base.
  • The pungent Me₂S byproduct demands good ventilation and often an oxidative scrub (e.g., dilute NaOCl).


Practical Tips


  • Prepare the activation mixture (DMSO + (COCl)₂ in CH₂Cl₂) first, then add the alcohol solution, and finally Et₃N.
  • Typical checked procedures use about 1.1–1.6 equiv of oxalyl chloride, 2–3.2 equiv of DMSO and about 5 equiv of amine base; use the substrate-specific procedure rather than treating these as fixed ratios.
  • Add base cold, then follow the specified warming and aqueous-workup sequence. Many procedures warm before aqueous workup; sensitive products may require a colder quench.
  • Vent Me₂S through a bleach trap or ozonolysis scrubber to tame the odour.
  • Keep glassware dry and use inert atmosphere where possible.


Exam-Style Summary


  • Swern oxidation (DMSO/(COCl)₂, Et₃N, CH₂Cl₂, −78 °C → rt) converts 1° alcohols to aldehydes and 2° alcohols to ketones.
  • Simple 3° alcohols lack an alcohol-carbon H and do not follow the ordinary carbonyl-forming pathway.
  • Byproducts: CO, CO₂, and Me₂S; ventilation required.
  • Mechanism: activated DMSO → alkoxysulfonium → sulfur ylide → intramolecular syn elimination → carbonyl.


Interactive Toolbox


  • Mechanism Solver — this general link does not preload the Swern mechanism; support for the illustrated sequence has not been verified.
  • Reaction Solver — open for supported practice. Automatic handling of these Swern examples has not been verified.
  • IUPAC Namer — open for supported naming tasks; this general link does not preload the illustrated products.