Question

Consider the following bromides and compare them for their reactivity in $S_{N}1$ reaction.

• A. (II) > (III) > (I)
• B. (II) > (I) > (III)
• C. (III) > (II) > (I)
• D. (I) > (II) > (III)

Answer 1

The correct option is A (II) > (III) > (I)

The reactivity of $S_{N}1$ reaction depends upon the stability of the intermediate, carbocation formed during these reactions. The stability order of the carbocation formed from the given species is Allylic carbocation stablises through resonance due to conjugation and is more stable than other two carbocation. $2^o$ carbocation stablises by hyperconjugation is more stable than $1^o$ carbocation due to more $\alpha$-hydrogen atom than $1^o.$
Hence, the reactivity order of the given bromide towards $S_{N}1$ reaction is II > III > I