MCQ Detailed View

The choice of solvent plays a critical role in determining the rate of nucleophilic substitution reactions. For SN2 (bimolecular nucleophilic substitution) reactions, the nucleophile must directly attack the electrophilic carbon in a single concerted step. Therefore, the reactivity of the nucleophile is crucial, and solvent effects strongly influence this reactivity.

Solvents can be broadly divided into protic and aprotic types:

1. 
   

   Protic solvents (e.g., water, ethanol):
   These solvents have hydrogen atoms bonded to electronegative atoms (O–H or N–H). They can form hydrogen bonds and strongly solvate anions (nucleophiles). While this stabilization seems favorable, it actually decreases nucleophilicity because the nucleophile is "trapped" in a shell of solvent molecules. As a result, SN2 reactions are slowed down in protic solvents.

   
   


2. 
   

   Aprotic solvents (e.g., hexane):
   Nonpolar aprotic solvents such as hexane cannot stabilize ions effectively. Since nucleophiles and leaving groups may not dissolve well in these media, the reaction also proceeds poorly.

   
   


3. 
   

   Polar aprotic solvents (e.g., dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone):
   These solvents are polar, so they can stabilize cations (like Na⁺ or K⁺), but they lack O–H or N–H bonds, so they do not strongly solvate anions. This leaves the nucleophile "free" and highly reactive. In such solvents, nucleophiles can easily attack the electrophilic carbon, making SN2 reactions proceed rapidly.

   
   

Among the given options:

• 
  

  Water and ethanol are protic solvents → poor for SN2.

  
  


• 
  

  Hexane is nonpolar and nonpolar aprotic → unsuitable for ionic species.

  
  


• 
  

  Dimethylformamide (DMF) is a polar aprotic solvent, which strongly favors SN2 reactions.

  
  

Thus, the solvent that best supports an SN2 substitution reaction is Dimethylformamide (DMF).

This principle is essential for understanding organic reaction mechanisms because solvent choice often determines whether a reaction follows an SN1 or SN2 pathway.