Nucleophilicity of Halogens in aprotic solvent

  • #1

Summary:

reversal of nucleophilicity of halogens in .
I know that in protic solvents I- is a strong nucleophile and F- is a poor nucleophile due to solvolysis. But why should I- become a poor nucleophile in an aprotic solvent and F- become a strong nucleophile in aprotic solvent? Why this reversal of nucleophilicity i.e., F->,Cl->,Br->,I-> ?
 

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  • #2
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In other words - what does "solvolysis" mean here, and how does it affect nucleophilicity.
In an aprotic solvent, F- is the strongest nucleophile, and I- the weakest.
But in a protic solvent, solvolysis means that protons are either completely transferred to the nucleophile, forming HF and HI, or at least partially transferred in form of a hydrogen bond bridge.
Proton makes a big change to F, and HF is much weaker nucleophile than F-. H2F+ cation, or HF bonded to hydrogen bonds, is even weaker. Whereas the effects of proton on I are weaker. The result is that HI is a stronger nucleophile than HF.
 
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  • #3
Sorry, I meant solvation. But your answer does not clear the point why there is reversal of nucleophilicity of halogens in aprotic solvents?
 
  • #4
TeethWhitener
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Sorry, I meant solvation. But your answer does not clear the point why there is reversal of nucleophilicity of halogens in aprotic solvents?
Simple (anthropomorphized) explanation: nucleophiles are attracted to nuclei; protons are nuclei. A really strong nucleophile like fluoride will be attracted to protons, and will also attract protons. If your solvent is protic, your fluoride nucleophile will be too busy wooing the solvent protons to actually perform the reaction you want it to perform. If your solvent is aprotic, then the fluoride won’t be distracted by solvent protons, and can focus on the desired reaction. In contrast, a nucleophile like iodide is a little less attracted/attractive to nuclei. So it can spend more time in a protic solvent doing its job as a nucleophile reagent, and it won’t be as easily distracted by those sweet sweet solvent protons as fluoride will. In an aprotic solvent, however, there are no protons to distract, so in the contest between iodide and fluoride for the nucleofuge’s heart, the dashing fluoride wins over the awkward iodide.
 
  • #5
Thank you very much for your nice explanation. However it is a bit difficult to understand the language. Could you please elaborate? BY the bye what is the meaning of the words anthropomorphize and nucleofuge?
 
  • #6
TeethWhitener
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https://www.merriam-webster.com/dictionary/anthropomorphize

Webster's doesn't have "nucleofuge," but it's what a nucleophile is attracted to. So in a nucleophilic substitution of a fluoride ion on an alkyl iodide, the nucleophile is the fluoride and the nucleofuge is the carbon to which the iodine is attached.
 
  • #7
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Less anthropomorphically, note that besides classifying nucleophiles and electrophiles as "weak" or "strong", both are also classified as "hard" or "soft".
Fluoride is a hard nucleophile, because it is concentrated negative charge. Iodide is a soft nucleophile because it is spread out. Proton is a hard electrophile.
In protic solvents, fluoride ion is efficiently neutralized by the protons that solvate it. The small hydrogen atoms can approach close to fluoride and efficiently neutralize its charge. Whereas iodide ion is poorly solvated - a small hydrogen atom cannot get very close to iodine, and much of the polarizable electron clouds remain available for other reactions. Therefore the partially neutralized but mostly unneutralized iodide is stronger nucleophile than the mostly neutralized fluoride.

In polar aprotic solvents, there is no such a good way to neutralize fluoride. Therefore fluoride, as a concentrated negative charge, is the stronger nucleophile.
 
  • #8
Thank you very much for your nice reply.
 
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