MCQ Detailed View

The mechanism of nucleophilic substitution in alkyl halides depends strongly on the structure of the carbon bonded to the halogen. Two main mechanisms are observed: SN1 (unimolecular nucleophilic substitution) and SN2 (bimolecular nucleophilic substitution).

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  SN1 mechanism: This is favored by tertiary alkyl halides. In this pathway, the rate-determining step is the formation of a carbocation after the leaving group departs. A stable carbocation (like tertiary) makes this mechanism feasible.

  
  


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  SN2 mechanism: This is favored by primary alkyl halides. The nucleophile directly attacks the carbon at the same time as the leaving group departs, in a single concerted step. Steric hindrance is low in primary halides, so the reaction proceeds easily.

  
  

Now let us analyze the given options:

1. 
   

   CH₃–X (Methyl halide): Only follows SN2 because carbocation stability is too poor for SN1.

   
   


2. 
   

   (CH₃)₃C–CH₂–X (Neo-pentyl halide): Very hindered, so SN2 is practically impossible; SN1 is also very slow due to poor carbocation stability.

   
   


3. 
   

   (CH₃)₂CH–X (Isopropyl halide): This is a secondary alkyl halide. Secondary halides can undergo both SN1 and SN2 reactions, depending on reaction conditions. In polar protic solvents and weak nucleophiles, SN1 dominates; in polar aprotic solvents with strong nucleophiles, SN2 is favored.

   
   


4. 
   

   (CH₃)₃C–X (Tert-butyl halide): Follows SN1 only, because it easily forms a stable tertiary carbocation.

   
   

Thus, the alkyl halide that may undergo both SN1 and SN2 mechanisms is (CH₃)₂CH–X, a secondary halide.