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NAMED ORGANIC REACTIONS

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SWERN OXIDATION

In Swern oxidation, the primary or secondary alcohols are oxidized to aldehydes or ketones respectively by treating them with dimethyl sulfoxide (DMSO) activated by oxalyl chloride at low temperatures (-78 oC to -60 oC) and then with an organic base like triethyl amine.

general representation of swern oxidation

* Further oxidation of aldehydes to carboxylic acids is not possible under Swern oxidation conditions.

* The Swern oxidation is the best alternative to the use of carcinogenic chromium based oxidizing agents.

* The reaction must be performed below -60 oC to avoid the formation of side products like mixed thioacetals. However the use of trifluoroacetic anhydride instead of oxalyl chloride, (COCl)2 allows the reaction to be warmed up to -30 oC.

* The dimethyl sulfide (DMS) is formed as a byproduct. It has very unpleasant smell. Hence the glassware is usually rinsed with sodium hypochlorite to oxidize it. 

To avoid not only the formation of volatile and malodorous dimethyl sulfide but also to facilitate the workup, the alternative sulfoxides like dodecyl methyl sulfoxide or sulfoxides bound to polymers can be used. 

 

MECHANISM OF SWERN OXIDATION

* The DMSO reacts with oxalyl chloride and gets converted to a reactive species, dimethylchlorosulfonium ion.

 generation of dimethylchlorosulfonium ion

* The dimethylchlorosulfonium ion, thus formed, reacts with the alcohol to give an alkoxysulfonium ion.

generation of alkoxysulfonium ion

* The deprotonation of the alkoxysulfonium ion using a base furnishes sulfur ylide, which undergoes β-elimination through a five membered cyclic transition state to give a carbonyl compound and dimethyl sulfide.

 formation and decomposition of sulfur ylide

 

ILLUSTRATIONS

1) The hexan-1-ol is conveniently oxidized to hexanal in Swern oxidation.

oxidation of hexan-1-ol to hexanal

 

2) The oxidation of alcohols occurs selectively in presence of disulfide in Swern conditions. The sulfurs are not oxidized.

disulfide bond is not oxidized

3) In some cases, the side reactions like α-epimerization and migration of double bonds into conjugation with newly formed carbonyl group are possible with the use of triethylamine as the base. To mitigate these reactions, a bulkier base like diisopropylethylamine (iPr2NEt) also known as, H√ľnig's base can be employed.

E.g.

use of hunigs base

However the double bond can be migrated when triethylamine is used.

migration of double bond

 

4) Some times the use of N-ethylpiperidine as a base may improve the yields in Swern oxidation.

use of N-ethylpiperidine

 

 

 

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Author: Aditya vardhan Vutturi Google+ profile