Dmso Swern Oxidation

Alcohol Reactions: Swern Oxidation using DMSO

Alcohol Oxidation: Swern (DMSO/(COCl)₂, Et₃N, CH₂Cl₂)

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. Triethylamine then promotes β-elimination, expelling dimethyl sulfide (Me₂S), carbon monoxide, and carbon dioxide while delivering the carbonyl product. Tertiary alcohols remain unchanged because they lack an α-hydrogen for the elimination step.

Review the chromium-based alternative 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 → base-induced β-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 — Base-promoted β-elimination to the carbonyl (0 °C → rt)

Triethylamine removes the α-hydrogen; electrons form the C=O while Me₂S departs.
Et₃N removes the α-H; electrons form the C=O as Me₂S leaves in an E2-like collapse.

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
Branched primary alcohol substrate Swern reagent card Aldehyde product

The branched primary alcohol oxidises one step to the corresponding aldehyde without over-oxidation.

Secondary Alcohol → Ketone

Swern conditions
Cyclic secondary alcohol substrate Swern reagent card Ketone product

Cyclohexanol delivers cyclohexanone; the stereocentre at C‑1 is lost (sp³ → sp²).

Tertiary Alcohol (No Reaction)

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

No β-hydrogen is available, so the tertiary alcohol remains unchanged under Swern conditions.


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) mediates β-elimination and generates 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.
  • α-Chiral secondary alcohols may epimerise during the base step; DIPEA and colder additions minimise this.
  • 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.
  • Use ~1.3–1.5 equiv of both DMSO and oxalyl chloride; 2–3 equiv of Et₃N is typical.
  • Quench at or below 0 °C before warming to ambient temperature, limiting side reactions.
  • 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.
  • 3° alcohols do not react; no α-H means no β-elimination.
  • Byproducts: CO, CO₂, and Me₂S; ventilation required.
  • Mechanism: activated DMSO → alkoxysulfonium → base-promoted elimination → carbonyl.


Interactive Toolbox


  • Mechanism Solver — load alcohol_swern_dmso to view the five Swern frames with arrows.
  • Reaction Solver — test custom alcohol substrates under Swern conditions to predict aldehyde/ketone vs. no-reaction outcomes.
  • IUPAC Namer — confirm systematic names for the aldehydes and ketones produced by the Swern oxidation.