Question

Which one of the two lodine atoms will be more reactive in the $S_{N}1$ and $S_{N}2$ reaction?

• A. A will be faster in $S_{N}1$ reaction but slower in $S_{N}2$
• B. A will be faster both in $S_{N}1$ and $S_{N}2$ reaction
• C. A and B will be equally reactive.
• D. B will be faster in both $S_{N}1$ and $S_{N}2$ reaction

Answer 1

Answer: (B)

Allylic halide has greater reactivity in both $S_{N}1$ and $S_{N}2$ reactions. The transition state of allylic halide is stabilised by conjugation with an allylic double bond in $S_{N}2$ mechanism. Also, the allylic carbocation formed in $S_{N}1$ mechanism is stabilised by resonance.