Question

Reactivity order for $S_{N}1$ is:

• A. i > ii > iii > iv
• B. ii > i > iii > iv
• C. iii > i > ii > iv
• D. iv > iii > ii > i

Answer 1

The correct option is C iii > i > ii > iv

Reactivity of $S_{N}1$ reaction depends on the stability of the carbocation formed. Higher the stability of carbocation, higher will be its reactivity.
Stability of carbocation follows the order,
Tertiary > Secondary > Primary
Here, compound (iv) forms primary carbocation which is least stable than (i) and (ii) which form secondary carbocation.
now between (i) and (ii), carbocation formed by (i) will be more stable than (ii) since Ph will have -I effect. So overall stability order between i, ii and iv is i>ii>iv.
Stability of the carbocation also depends +M/-M effect of the group attached to the carbocation.
In (iii), +M group $-OC{H}_3$ increases the stability of the compound and make it most stable than (i) and (ii) and (iv). Hence correct reactivity order is iii > i> ii > iv