Strong nucleophile in polar aprotic solvent is ?

The nucleophilicity of ions depends greatly on the type of solvent used. Solvents are classified into protic and aprotic categories.

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   Polar Protic Solvents (e.g., water, ethanol):

   
   
   
   
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     These solvents can form hydrogen bonds with nucleophiles.

     
     
   
   
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     Small anions such as fluoride (F⁻) are strongly solvated and become less available for nucleophilic attack.

     
     
   
   
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     In such solvents, nucleophilicity follows the trend:
     I⁻ > Br⁻ > Cl⁻ > F⁻

     
     
   
   
   
   


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   Polar Aprotic Solvents (e.g., acetone, DMSO, DMF, acetonitrile):

   
   
   
   
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     These solvents have a high dielectric constant and dissolve salts well.

     
     
   
   
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     However, they cannot form hydrogen bonds with anions.

     
     
   
   
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     This means that nucleophiles are not strongly solvated, especially small anions like F⁻.

     
     
   
   
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     In the absence of hydrogen bonding stabilization, nucleophilicity closely follows basicity.

     
     
   
   
   
   

Since fluoride ion (F⁻) is the most basic among the halides (due to its small size and high charge density), it becomes the strongest nucleophile in polar aprotic solvents. In contrast, iodide (I⁻), which is a good nucleophile in protic solvents, becomes weaker in aprotic solvents because its large size reduces its charge density and reactivity.

Key Comparisons:

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  In polar protic solvents: I⁻ is strongest due to poor solvation of large ions.

  
  


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  In polar aprotic solvents: F⁻ is strongest because there is no solvation hindrance.

  
  

This principle is crucial in SN2 reactions, where the choice of solvent directly influences the rate of nucleophilic substitution. Polar aprotic solvents are preferred in SN2 mechanisms precisely because they enhance the strength of anionic nucleophiles. ✅ Therefore, in a polar aprotic solvent, the strongest nucleophile is Fluoride ion (F⁻) — Option A.