In a chemistry olympiad I had this morning, there was a question on the acidity of polysulfanes. Specifically, it asked candidates to explain the increasing acidity of polysulfanes as the number of sulfur atoms in the polysulfane increases. The standard answer would involve usage of the concepts of inductive effects of electronegative elements. However, I thought perhaps, there was more to it. Thus, in addition to writing about the increasing stabilisation of the $\ce {HS_x^-}$ conjugate base via stronger inductive effects as $x$ inceases, I also wrote an additional point about the increasing stabilisation coming from the increased ability of the anionic sulfur atom in $\ce {HS_x^-}$ to accept a hydrogen bond from the other terminal $\ce {H-S}$ as $x$ increases.

I was thinking that as the number of sulfur atoms in the chain increases, the chain becomes more "flexible" and thus, the ease of hydrogen bond formation increases. Of course, this will mean that the acidity would increase up to a certain point, where the ease of hydrogen bond formation no longer increases. After consultation with various general inorganic chemistry textbooks, I was not able to find a reason explaining this trend in acidity.

Firstly, I would like to ask if anyone knows the correct explanation. Secondly, I would like to ask if my proposal is plausible.

  • $\begingroup$ Actually H-bonding should go against acidity increase here, IMO. Important would be rather stereoelectronic effects. $\endgroup$
    – Mithoron
    Nov 19, 2018 at 15:51
  • $\begingroup$ chemistry.stackexchange.com/questions/7460/… $\endgroup$
    – Mithoron
    Nov 19, 2018 at 15:55
  • $\begingroup$ @Mithoron But wouldn't that limit it to the addition of 1 sulfur atom? Since it would only be the "alpha" atom that is responsible. And wouldn't the alpha effect suggest basicity of the conjugate base increases with increasing number of S atoms, contradicting the trend? $\endgroup$ Nov 20, 2018 at 0:24


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