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This was a question that was asked in one of my exams, in which we had to confirm if the given order was correct or incorrect. The key says the given order is correct and the professor at my high school explained saying that along with the -M of C=S bond, due to the d-orbital resonance at 2 position by sulfur, 2 is more acidic than 1. What I don't understand is, wouldn't +M of sulfur make it less acidic compared to position 1?

I have been taught that d-orbital resonance is used as an explanation for why thiophenol is more acidic than phenol, or chloroform is more acidic than fluoroform, but I don't understand how we can neglect the +M effect in this case.

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    $\begingroup$ Apage, evil d-orbital, I banish you from poor OP's mind! $\endgroup$
    – Mithoron
    Commented Apr 30 at 21:47

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The +M effect is as you describe if it is there. But it can come only through sulfur taking up a positive formal charge, which is most favored when you have a cation to begin with. Here you have an anion instead, and if you try to make the sulfur a mesomeric electron donor you find there are too many electrons and you would have to break a sigma bond. This is not favored, and so in this case the proposed mesomeric effect is suppressed.

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    $\begingroup$ Got it, thank you so much. So it is correct to say that for example a carbanion with three NH2 or Phenyl groups is decently stable since +M is suppressed and it is stabilized by the collective -I effect? $\endgroup$
    – Meme
    Commented Apr 30 at 9:04
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    $\begingroup$ Yes that works. $\endgroup$
    – Oscar Lanzi
    Commented Apr 30 at 9:33

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