Question

Which of the following compound will be most reactive for $S_{N}1$ reactions?

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

Since the rate determining step of $S_{N}1$ reaction involves Carbocation formation.
$\text{Rate of reaction}\propto \text{Stability of carbocation}$
Also, $\text{Rate of}{S}_{N}1\propto \text{Leaving tendency of leaving group}$

Carbocation formed by (a), (b) and (c) are stabilised by formation of oxonium ion intermediate. Hence, (a), (b) and (c) are more reactive than (d).

Leaving tendency of leaving group is given by $\Rightarrow I^{\ominus }>B{r}^{\ominus }>C{l}^{\ominus }$
So, (a) is most reactive for $S_{N}1$ followed by (c) and then (b).
Hence, overall stability order will be a>c>b>d.