Question

Compare the given compounds for their reactivity in $S_{N}1$ reaction.

• A. III < I < IV < II
• B. I < IV < III < II
• C. II < III < IV < I
• D. None of these

Answer 1

The correct option is B I < IV < III < II

The increasing order for reactivity towards $S_{N}1$ reaction is
I < IV< III < II
The rate determining step (RDS) is the slowest step in a reaction on which the rate of the reaction depends. Formation of carbocation governs the rate of $S_{N}1$ reaction and is rate determining step.
$\text{Rate of reaction}\propto \text{stability of carbocation}$

The carbocation formed from I is primary carbocation and not resonance stabilized. Hence, it is least stable and least likely to be formed.
The carbocation formed from IV is secondary carbocation and not resonance stabilized hence, it is less stable and less likely to be formed but more stable than I.

The carbocation formed from III is tertiary carbocation and not resonance stabilized. Hence, it is more stable than I and IV and more likely to be formed.

The carbocation formed from II is tertiary carbocation and resonance stablized. Hence, it is most stable than other three and most likely to be formed.

Correct stability order of carbocation is II>III>IV>I
Reactivity order toward $S_{N}1$ reaction is II>III>IV>I.