Everything about The Swern Oxidation totally explained
The
Swern oxidation, named after
Daniel Swern, is a
chemical reaction whereby a primary or secondary
alcohol is
oxidized to an
aldehyde or
ketone using
oxalyl chloride,
dimethyl sulfoxide (DMSO) and an organic base, such as
triethylamine. The reaction is known for its mild character and wide tolerance of
functional groups.
The by-products are
dimethyl sulfide (Me
2S),
carbon monoxide (CO),
carbon dioxide (CO
2) and — when triethylamine is used as base —
triethylammonium chloride (Et
3NHCl). Two of the by-products,
dimethyl sulfide and
carbon monoxide, are very toxic volatile compounds, so the reaction and the work-up needs to be performed in a fume hood.
Dimethyl sulfide is a volatile liquid (B.P. 37 °C) with an unpleasant odour. A convenient way to reduce the stench of dimethyl sulfide is to treat the crude product mixture with
oxone, whereupon the sulfide is converted to dimethyl sulfoxide, which has no odor.
Several reviews have been published.
Mechanism
The first step of the
Swern oxidation is the low-temperature reaction of
dimethyl sulfoxide (DMSO),
1a &
1b, with
oxalyl chloride,
2. The first intermediate,
3, quickly decomposes giving off
CO2 and
CO and producing dimethylchlorosulfonium chloride,
4.
After addition of the alcohol
5, the dimethylchlorosulfonium chloride
4 reacts with the alcohol to give the key alkoxysulfonium ion intermediate,
6. The addition of at least 2 equivalents of base — typically
triethylamine — will
deprotonate the alkoxysulfonium ion to give the sulfur
ylide 7. In a five-membered ring
transition state, the sulfur ylide
7 decomposes to give
dimethyl sulfide and the desired ketone (or aldehyde)
8.
Variations
When using oxalyl chloride as the
dehydration agent, the reaction must be kept colder than -60 °C to avoid side reactions. With
trifluoroacetic anhydride instead of oxalyl chloride, the reaction can be warmed to -30 °C without side reactions.
In some cases, the use of
triethylamine as the base can lead to epimerisation at the carbon alpha to the newly formed carbonyl. Using the bulkier base
diisopropylethylamine (i-Pr
2NEt, Hünig's base) can mitigate this side reaction.
Considerations
Dimethyl sulfide, a byproduct of the Swern oxidation, is one of the most foul odors known in organic chemistry. Human olfactory glands can detect this compound in concentrations as low as parts per billion. A simple remedy for this bothersome problem is to rinse used glassware with
bleach (usually containing
sodium hypochlorite), which will oxidize the dimethyl sulfide, eliminating the smell.
Further Information
Get more info on 'Swern Oxidation'.
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