MCQ Detailed View

Organic reaction mechanisms such as substitution (SN1, SN2) and elimination (E1, E2) differ based on their reaction pathway. Among them, E1 and SN1 reactions share the same first step.

In both E1 (unimolecular elimination) and SN1 (unimolecular nucleophilic substitution), the first step is ionization of the substrate, where the leaving group departs and a carbocation intermediate is formed.

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  Step 1 (common to both E1 and SN1):
  R–LG → R⁺ + LG⁻
  (Here, LG is the leaving group, usually a halide or a sulfonate ion.)

  
  

Once the carbocation is formed, the pathway determines the final product:

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   E1 mechanism (elimination):

   
   
   
   
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     After carbocation formation, a base abstracts a proton (H⁺) from a β-carbon.

     
     
   
   
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     This leads to the formation of a double bond (alkene).

     
     
   
   
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     Example: Dehydration of alcohols in acidic conditions often proceeds via E1.

     
     
   
   
   
   


2. 
   

   SN1 mechanism (substitution):

   
   
   
   
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     After carbocation formation, a nucleophile attacks the carbocation.

     
     
   
   
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     This results in a substituted product.

     
     
   
   
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     Example: Hydrolysis of tert-butyl halides follows the SN1 mechanism.

     
     
   
   
   
   

In contrast:

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  E2 is a single-step elimination with no carbocation intermediate.

  
  


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  SN2 is a single-step substitution where the nucleophile attacks as the leaving group departs.

  
  

Therefore, only E1 and SN1 share the same first step: carbocation formation through leaving group departure.

Correct answer: E1 and SN1.