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While reading on order of nucleophilicity of $\ce{RS-}$ and $\ce{RO-}$, I came across this:nucleophilicity

Source: Page 355, Organic Chemistry by Jonathan Clayden, Nick Greeves, and Stuart Warren

However, my teacher has said that in polar protic medium, nucleophilicity of $\ce{RS-}$ dominates over $\ce{RO-}$ due to the heavy solvation of oxygen and the relatively low solvation of sulfur. And, in polar aprotic medium, due to the high potential energy of the $\ce{RO-}$ anion, it becomes a better nucleophile than $\ce{RS-}$ anion. (Just like the order of nucleophilicity halide anions).

Although they have given data for reaction in polar protic ($\ce{EtOH}$) medium in the book, I feel like nucleophilicity of $\ce{RS-}$ should still be higher than $\ce{RO-}$, independent of the medium, because of the reason of HOMO-LUMO interaction mentioned in the book (which should be independent of the solvent) HOMO-LUMO

Also, electrostatic interactions must be a less dominating factor than HOMO-LUMO interactions for $S_N 2$ reactions as the C-X bond is not that polar.

In the above data of reaction with $\ce{MeBr}$, the difference in rate is in the order of $10^4$. I feel like that must account for something more than the effect of solvent.

However, I could not find data for the rate of $S_N 2$ in polar aprotic medium. So, in polar aprotic medium, which of $\ce{RO-}$ or $\ce{RS-}$ is more nucleophilic and why?

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The statement in the textbook may not be all that general. And nucleophilicities may reverse for polar aprotic and, especially, nonpolar aprotic solvents, more so at lower concentrations (trying my best to simulate gas-phase like conditions here).

Although I was not able to find a study performed in polar aprotic or nonpolar aprotic solvents, I did find one performed in gas phase where $\ce{HO-}$ is a much better nucleophile than $\ce{HS-}$.$^\text{1}$ I have attached some values of bimolecular rate coefficients for the following reaction:

$$ \ce{RX(g) + Nu-(g) ->RNu(g) + X-(g)} $$

Alkyl Halide $\ce{HO-}$ $\ce{HS-}$
$\ce{CH3Cl}$ 20 0.12
$\ce{CH3Br}$ 22 3.2
$\ce{C2H5Cl}$ 25 <0.001
$\ce{C2H5Br}$ 26 2.0

I know this doesn't exactly answer your question; however, it helps to know that sulfur is not plainly a better nucleophile than oxygen for saturated carbon.

Reference

  1. DePuy C. H., Gronert S., Mullin A., and Bierbaum V. M. (1990). Gas-phase SN2 and E2 reactions of alkyl halides. J. Am. Chem. Soc., 112, 8650-8655.
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    $\begingroup$ That does seem a bit convincing, atleast to know that in gas phase the order reverses, even if by a slight amount. Thanks for the answer! I will keep this question open if someone does turn up with data for polar aprotic medium. $\endgroup$
    – omega
    Jun 10, 2023 at 18:57

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