Question

Reactivity of the following on the basis of \( S_\text{N}1 \) mechanism.

• A. IV>III>I>II
• B. II>IV>III>I
• C. III>IV>I>II
• D. IV>III>II>I

Hint:

In \( S_\text{N}1 \) reactions, the stability of the carbocation intermediates determines the reactivity. More stable carbocations lead to faster reactions.

Answer 1

The Correct Option is A

The reactiveness of compounds according to the \(S_\text{N}1\) mechanism depends primarily on the stability of the carbocation formed in the reaction. More stable carbocations lead to increased reaction rates under \(S_\text{N}1\) conditions.

Let us consider the stability factors that affect carbocations:

1. Order of Carbocation Stability: Tertiary carbocations are generally more stable than secondary carbocations, which, in turn, are more stable than primary carbocations. This is due to the inductive effect and hyperconjugation offered by the adjacent carbon atoms.
2. Resonance: Carbocations stabilized by resonance are more stable than those that are not. This is critical for benzylic and allylic carbocations, where pi-electron delocalization occurs.
3. Hyperconjugation: This effect further stabilizes carbocations by dispersing the positive charge over several atoms.

To determine the order of reactivity in the given options, consider these factors for the possible carbocations formed:

1. Option IV: If this includes a tertiary carbocation with resonance stabilization, it will exhibit the highest reactivity.
2. Option III: A secondary carbocation might form, possibly with some additional stabilization via resonance or hyperconjugation.
3. Option I: If this involves a secondary or primary carbocation without additional stabilization, it will be less reactive compared to those with resonance.
4. Option II: Forms a primary carbocation with no extra stabilization, resulting in the lowest reactivity.

Based on the stability considerations, the correct order of reactivity according to the \(S_\text{N}1\) mechanism is:

1. IV: Most reactive (beneficial tertiary carbocation with possible resonance)
2. III: Next reactive (could be secondary with hyperconjugation)
3. I: Less reactive (primary or secondary without extra benefits)
4. II: Least reactive (primary carbocation)

Thus, the correct order of reactivity according to the \(S_\text{N}1\) reaction is IV > III > I > II.

Therefore, the correct answer is: IV>III>I>II