MCQ Detailed View

The stereochemistry of nucleophilic substitution depends on the mechanism involved. The two main substitution pathways are SN1 and SN2, and they show different effects on molecular configuration.

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  SN2 mechanism: This reaction occurs in one step, where the nucleophile directly attacks the carbon atom opposite to the leaving group. This always results in complete inversion of configuration (Walden inversion).

  
  


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  SN1 mechanism: This reaction occurs in two steps. In the first step, the leaving group departs, forming a carbocation intermediate. This carbocation is planar because the central carbon is sp² hybridized. Since the intermediate is flat, the nucleophile can attack from either side with equal probability.

  
  

As a result:

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  Half of the product molecules retain the original configuration (no inversion).

  
  


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  Half undergo inversion of configuration.

  
  

This leads to a racemic mixture, i.e., 50% inversion and 50% retention, giving an overall loss of optical activity if the starting compound is optically active.

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  E1 reaction: Involves elimination with carbocation intermediate but not relevant to inversion of configuration.

  
  


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  E2 reaction: A single-step elimination, no racemization.

  
  


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  SN2 reaction: Always 100% inversion, not 50%.

  
  


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  SN1 reaction: Shows 50% inversion + 50% retention, i.e., racemization.

  
  

Thus, the correct answer is SN1 reaction.