Question

Among the following pair of halides, in which pair the second halide is more reactive than the first?

• A. $S_{N}2$ reaction:
• B. $S_{N}1$ reaction:
• C. Both (a) and (b)
• D. None of the above

Answer 1

The correct option is D None of the above

In $S_{N}2$ reaction, the nucleophile will attack the substrate from the back.
More the bulky substituents present around the carbon, more difficulty for the nucleophile to approach the carbon having leaving group.

$\text{Rate of}{S}_{N}2\text{reaction}\propto \frac{1}{\text{Steric crowding}}$
Therefore, in (a) first alkyl halide is more reactive than the second.

In $S_{N}1$ reaction, the rate determining step is the formation of carbocation.

$\text{Rate of}{S}_{N}1\text{reaction}\propto \text{Stability of carbocation}$

According to Bredt's rule, a bridge head carbocation is unstable.
Hence, in (b) first alkyl halide is more reactive than the second.

Thus, correct answer is (d)