MCQ Detailed View

The SN2 reaction (bimolecular nucleophilic substitution) is a single-step reaction in which a nucleophile attacks the carbon atom bearing the leaving group while the leaving group departs simultaneously. The rate of the reaction depends on both the nucleophile and the substrate, expressed as:

Rate = k [substrate] [nucleophile]

Because the nucleophile attacks from the backside of the carbon, steric hindrance around the carbon center strongly influences the reactivity.

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   Primary alkyl halides:

   
   
   
   
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     Best substrates for SN2 reactions.

     
     
   
   
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     The carbon bearing the leaving group is attached to only one alkyl group, so steric hindrance is minimal.

     
     
   
   
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     Nucleophile can easily attack the carbon from the opposite side of the leaving group.

     
     
   
   
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     Example: CH₃CH₂Cl reacts quickly in SN2 reactions.

     
     
   
   
   
   


2. 
   

   Secondary alkyl halides:

   
   
   
   
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     React slower in SN2 because steric hindrance increases.

     
     
   
   
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     Compete between SN1 and SN2 pathways depending on conditions.

     
     
   
   
   
   


3. 
   

   Tertiary alkyl halides:

   
   
   
   
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     Very poor substrates for SN2.

     
     
   
   
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     The central carbon is surrounded by three bulky alkyl groups, blocking the backside attack of the nucleophile.

     
     
   
   
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     They generally follow the SN1 mechanism instead.

     
     
   
   
   
   


4. 
   

   Methyl halides (CH₃X):

   
   
   
   
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     Even faster than primary halides in SN2 reactions because they have no steric hindrance at all.

     
     
   
   
   
   

Thus, the SN2 mechanism works best with primary alkyl halides (and methyl halides) because steric hindrance is low and the nucleophile can attack effectively.

Correct answer: Primary alkyl halide.