Question

Identify the compound that undergoes SN1 mechanism most fastly

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is D

Solution:

The SN1 mechanism is a substitution reaction where the rate-determining step is the formation of a carbocation. The rate of the reaction depends on the stability of the carbocation formed, and the more stable the carbocation, the faster the reaction proceeds.

Stability of Carbocations:

• Tertiary carbocations: These are the most stable due to the +I effect of three alkyl groups, which stabilize the positive charge through both inductive and hyperconjugative effects.
• Secondary carbocations: These are less stable than tertiary carbocations but more stable than primary carbocations.
• Primary carbocations: These are the least stable and least likely to form in an SN1 reaction.

Step 1: Analyze the Structure of the Compounds

Option 1: Alcohol reacts with HBr in presence of NaBr, H₂SO₄: This is a reaction of alcohol with HBr, leading to the formation of alkyl bromides. The reaction proceeds via an SN1 mechanism for tertiary alcohols (due to the formation of a stable carbocation) and an SN2 mechanism for primary alcohols. For tertiary alcohols, this reaction will proceed the fastest due to the stability of the tertiary carbocation.

Option 2: Alcohol reacts with halogen in presence of sunlight: This is a free radical substitution reaction, not an SN1 reaction. Sunlight provides energy that promotes the formation of free radicals, and the reaction proceeds via a free radical mechanism, not via the formation of carbocations, which is essential for the SN1 mechanism.

Option 3: Alcohol reacts with HI in presence of NaI/H₃PO₄: This is an example of the reaction of alcohol with HI to form alkyl iodides. This reaction follows the SN1 mechanism for tertiary alcohols (due to the formation of a stable carbocation) but will follow the SN2 mechanism for primary alcohols (due to the steric hindrance of the primary carbocation). Tertiary alcohols will undergo the SN1 mechanism the fastest due to the stability of the tertiary carbocation.

Option 4: Alcohol reacts with HCl in presence of anhydrous ZnCl₂: This is an example of the Lucas reaction, which leads to the formation of alkyl chlorides through the SN1 mechanism. Alcohols with tertiary carbons (like tert-butyl alcohol) undergo the reaction faster than those with primary or secondary carbons. So, if the alcohol is tertiary, it will react faster, but if it is secondary or primary, the reaction will be slower.

Conclusion:

From the analysis of all the options, the compounds that undergo the SN1 mechanism fastest are those that form stable carbocations, which are typically tertiary carbocations. Therefore, the correct answer is:

Option 4: Alcohol reacts with HCl in presence of anhydrous ZnCl₂, assuming the alcohol is tertiary, as tertiary carbocations are the most stable and will undergo the SN1 mechanism the fastest.