My question is regarding Swern oxidation. Why is the methyl group of the DMSO deprotonated and not directly the red hydrogen atom of the R2-CH-O bond deprotonated? Shouldn't it be that oxygen more electronegative (stronger -I effect) than sulfur and shouldn't that make that corresponding hydrogen atom more acidic? Or is it perhaps that sulfur has a positive charge and therefore acts as a stronger electrophile that attracts the electrons from carbon and therefore its hydrogen groups are more acidic?

Mechanism in Question

  • $\begingroup$ Related: ylide stability. Also, mechanisms usually show the shortest path. Try going your way and see what ends up happening. $\endgroup$ Commented Jul 17, 2019 at 14:06
  • $\begingroup$ If I go my way... I guess a carboanion is built, which is very unstable? But then the electronegative oxygen will take the electrons anyway right? Wouldnt the net effect be the same? Just that there doesnt have to be a second attack? $\endgroup$ Commented Jul 17, 2019 at 14:11
  • $\begingroup$ The oxygen is electronegative, but is it electron-deficient? $\endgroup$ Commented Jul 17, 2019 at 14:16
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    $\begingroup$ The hydrogen alpha to an oxygen is not generally very acidic at all - consider diethyl ether. The +ve charge on the sulfur makes the proton on the neighbouring carbon pretty acidic $\endgroup$
    – Waylander
    Commented Jul 17, 2019 at 14:17
  • $\begingroup$ S increases acidity more than O to begin with, compare sulfides and ethers; and the positive charge makes the proton next to S even more acidic. $\endgroup$ Commented Jul 17, 2019 at 16:49


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