MCQ Detailed View

The SN1 reaction mechanism (Substitution Nucleophilic Unimolecular) is a two-step process commonly observed in tertiary haloalkanes. It proceeds through the formation of a carbocation intermediate after the leaving group departs. The stability of this carbocation is the deciding factor in whether the reaction follows an SN1 or SN2 pathway.

Among the given options:

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  CH₃Br (methyl bromide): Hydrolysis of methyl halides always proceeds through the SN2 mechanism because a methyl carbocation (CH₃⁺) is extremely unstable and cannot exist.

  
  


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  CH₃CH₂Br (ethyl bromide): Primary haloalkanes also generally undergo SN2 reactions due to the instability of primary carbocations.

  
  


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  CH₃CH₂CH₂Br (n-propyl bromide): Similarly, this is a primary haloalkane, which favors SN2 substitution.

  
  


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  (CH₃)₃CBr (tert-butyl bromide): This is a tertiary haloalkane, and its carbocation, tert-butyl cation ((CH₃)₃C⁺), is highly stabilized by hyperconjugation and inductive effects of the three alkyl groups. This stability allows the halide ion (Br⁻) to leave first, forming a stable carbocation. In the second step, the nucleophile (such as OH⁻ in hydrolysis) attacks this carbocation to produce the alcohol.

  
  

The mechanism can be summarized:

1. 
   

   Ionization (slow step): (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻

   
   


2. 
   

   Nucleophilic attack (fast step): (CH₃)₃C⁺ + H₂O → (CH₃)₃C–OH + H⁺

   
   

This explains why (CH₃)₃CBr is hydrolyzed via the SN1 mechanism, while the others are not.

The SN1 pathway is favored by:

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  Tertiary haloalkanes (due to stable carbocations)

  
  


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  Polar protic solvents (like water, alcohols) that stabilize ions

  
  


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  Good leaving groups (like Br⁻ or I⁻)

  
  

Hence, the correct answer is (CH₃)₃CBr, which perfectly demonstrates the SN1 hydrolysis mechanism.